Biology of Humans



Accommodation A change in the shape of the lens of the eye brought about by contraction of the smooth muscle of the ciliary body that changes the degree to which light rays are bent so that an image can be focused on the retina.

Acetylcholine A neurotransmitter found in both the central nervous system and the peripheral nervous system. It is the neurotransmitter released at neuromuscular junctions that causes muscle contraction.

Acetylcholinesterase An enzyme that breaks the neurotransmitter acetylcholine into its inactive components, acetate and choline. Acetylcholinesterase stops the action of acetylcholine at a synapse.

Acid Any substance that increases the concentration of hydrogen ions in solution.

Acinar cells Exocrine cells of the pancreas that secrete digestive enzymes into ducts that empty into the small intestine.

Acquired immune deficiency syndrome See AIDS.

Acromegaly A condition characterized by enlarged soft tissues and thickened bones of the extremities. It is caused by overproduction of growth hormone in adulthood.

Acrosome A membranous sac on the head of a sperm cell that contains enzymes that facilitate sperm penetration into the egg during fertilization.

ACTH See adrenocorticotropic hormone.

Actin The contractile protein that makes up the major portion of the thin filaments in muscle cells. An actin (thin) filament is composed of actin, troponin, and tropomyosin. In muscle cells, contraction occurs when actin interacts with another protein called myosin.

Actin filaments The thin filaments in muscle cells composed primarily of the protein actin and essential to muscle contraction. In addition to actin, thin filaments contain two other proteins important in the regulation of muscle contraction: tropomyosin and troponin.

Action potential A nerve impulse. An electrochemical signal conducted along an axon. A wave of depolarization caused by the inward flow of sodium ions followed by repolarization caused by the outward flow of potassium ions.

Active immunity Immune resistance in which the body actively participates by producing memory B cells and memory T cells after exposure to an antigen, either naturally or through vaccination.

Active site A specific location on an enzyme where the substrate binds.

Active transport The movement of molecules across the plasma membrane, usually against a concentration gradient (from a region of lower concentration to one of higher concentration) with the aid of a carrier protein and energy (usually in the form of adenosine triphosphate, or ATP) supplied by the cell.

Acute renal failure An abrupt, complete or nearly complete, cessation of kidney function.

Adaptation The process by which populations become better attuned to their particular environments as a result of natural selection.

Adaptive immune responses Body defense responses that are acquired by exposure to cells that do not belong in the body. It involves antibody-mediated responses and cell-mediated responses. Adaptive responses have memory for the pathogen that triggered them.

Adaptive trait A characteristic (structure, function, or behavior) of an organism that makes an individual better able to survive and reproduce in its natural environment. Adaptive traits arise through natural selection.

Addison's disease An autoimmune disorder characterized by fatigue, loss of appetite, low blood pressure, and increased skin pigmentation resulting from undersecretion of glucocorticoids and aldosterone.

Adenosine triphosphate (ATP) A nucleotide that consists of the sugar ribose, the base adenine, and three phosphate groups. ATP is the energy currency of all living cells.

ADH See antidiuretic hormone.

Adhesion junction A specialized junction between cells in which the plasma membranes of adjacent cells are held together by protein filaments; a desmosome.

Adipose tissue A type of loose connective tissue that contains cells specialized for storing fat.

Adolescence The stage in postnatal development that begins with puberty. It is a period of rapid physical and sexual maturation during which the ability to reproduce is achieved. Adolescence ends with the cessation of growth in the late teens or early twenties.

Adrenal cortex The outer region of the adrenal gland that secretes glucocorticoids, mineralocorticoids, and gonadocorticoids.

Adrenal glands The body's two adrenal glands are located on top of the kidneys. The outer region of each adrenal gland secretes glucocorticoids, mineralocorticoids, and gonadocorticoids, and the inner region secretes epinephrine and norepinephrine.

Adrenal medulla The inner region of the adrenal gland that secretes epinephrine and norepinephrine.

Adrenaline See epinephrine.

Adrenocorticotropic hormone (ACTH) The anterior pituitary hormone that controls the synthesis and secretion of glucocorticoid hormones from the cortex of the adrenal glands.

Adulthood The stage in postnatal development that is generally reached somewhere between 18 and 21 years of age and during which bodily changes continue as part of the growth and aging process.

Afferent (sensory) neuron A nerve cell specialized to conduct nerve impulses from the sensory receptors toward the central nervous system.

Age structure Of a population, the number of males and females of each age in a population. The ages are often grouped into prereproductive, reproductive, and postreproductive categories. Generally, only individuals of reproductive age add to the size of the population.

Agglutinate To clump together.

Aging The normal and progressive alteration in the structure and function of the body. Aging is possibly caused by declines in critical body systems, disruption of cell processes by free radicals, slowing or cessation of cell division, and decline in the ability to repair damaged DNA.

Agranulocytes The white blood cells without granules or with very small granules in their cytoplasm, including monocytes and lymphocytes.

AIDS Acquired immune deficiency syndrome. A diagnosis of AIDS is made when an HIV-positive person develops one of the following conditions: (1) a helper T cell count below 200/mm3 of blood; (2) one of 26 opportunistic infections, the most common of which are Pneumocystis jiroveci pneumonia and Kaposi's sarcoma, a cancer of connective tissue that affects primarily the skin; (3) a loss of more than 10% of body weight (wasting syndrome); or (4) dementia (mental incapacity such as forgetfulness or inability to concentrate).

Albinism A genetic inability to produce the brown pigment melanin that normally gives color to the eyes, hair, and skin.

Aldosterone A hormone (the primary mineralocorticoid) released by the adrenal cortex that stimulates the reabsorption of sodium within kidney nephrons.

Allantois The extraembryonic membrane whose blood vessels become part of the umbilical cord, the ropelike connection between the embryo and the placenta.

Allele An alternative form of a gene. One of two or more slightly different versions of a gene that code for different forms of the same trait.

Allergen An antigen that stimulates an allergic response.

Allergy A strong immune response to an antigen (an allergen) that is not usually harmful to the body.

Allometric growth The change in the relative rates of growth of various parts of the body. Such growth helps shape developing humans and other organisms.

Alveolus (plural, alveoli) A thin-walled rounded chamber. In the lungs, the alveoli are the surfaces for gas exchange. They form clusters at the end of each bronchiole that are surrounded by a vast network of capillaries. The alveoli greatly increase the surface area for gas exchange.

Amino acid The building blocks of proteins consisting of a central carbon atom bound to a hydrogen atom, an amino group (NH2), a carboxyl group (COOH), and a side chain designated by the letter R. There are 20 amino acids important to human life; some can be synthesized by our bodies (nonessential amino acids), whereas others cannot be synthesized and must be obtained from the foods we eat (essential amino acids).

Amniocentesis A method of prenatal testing for genetic problems in a fetus in which amniotic fluid is withdrawn through a needle so that the fluid can be tested biochemically and the cells can be cultured and examined for genetic abnormalities.

Amnion The extraembryonic membrane that encloses the embryo in a fluid-filled space called the amniotic cavity. Amniotic fluid forms a protective cushion around the embryo that later can be examined as part of prenatal testing in a procedure known as amniocentesis.

Ampulla A wider region in a canal or duct. In the inner ear, an ampulla is found at the base of each semicircular canal.

Anabolic steroids Synthetic hormones that mimic testosterone and stimulate the body to build muscle and increase strength. Steroid abuse can have many dangerous side effects.

Anabolism The building (synthetic) chemical reactions within living cells, as when cells build complicated molecules from simple ones. Compare with catabolism.

Anal canal The canal between the rectum and the anus. Feces pass through the anal canal.

Analgesic A substance, such as Demerol, that relieves pain.

Analogous structure A structure of one organism that is similar to that of another organism because of convergent evolution and not because the organisms share a common ancestry. Compare with homologous structures.

Anaphase In mitosis, the phase when the chromatids of each chromosome begin to separate, splitting at the centromere. Now separate entities, the chromatids are considered chromosomes, and they move toward opposite poles of the cell.

Anaphylactic shock An extreme allergic reaction that occurs within minutes after exposure to a substance to which a person is allergic. It can cause pooling of blood in capillaries, which causes dizziness, nausea, and sometimes unconsciousness and extreme difficulty in breathing. Anaphylactic shock can lead to death.

Androgen A steroid sex hormone secreted by the testes in males and produced in small quantities by the adrenal cortex in both sexes.

Anemia A condition in which the blood's ability to carry oxygen is reduced. It can result from too little hemoglobin, too few red blood cells, or both.

Anencephaly A neural tube defect that involves incomplete formation of the brain and results in stillbirth or death shortly after birth.

Anesthesia The drug-induced loss of the sensation of pain. It may be general, regional, or local.

Aneurysm A blood-filled sac in the wall of an artery caused by a weak area in the artery wall.

Angina pectoris Choking or strangling chest pain, usually experienced in the center of the chest or slightly to the left, that is caused by a temporary insufficiency of blood flow to the heart. It begins during physical exertion or emotional stress, when the demands on the heart are increased and the blood flow to the heart muscle can no longer meet the needs.

Angioplasty A procedure that widens the channel of an artery obstructed because of atherosclerosis. It involves inflating a tough, plastic balloon inside the artery.

Angiotensin I Renin converts angiotensinogen into this protein.

Angiotensin II A protein that stimulates the adrenal gland to release aldosterone.

Anorexia nervosa An eating disorder characterized by deliberate self-starvation, a distorted body image, and low body weight.

ANP See atrial natriuretic peptide.

Antagonistic pairs Muscles arranged in pairs so that the actions of the members of the pair are opposite to one another. This arrangement is characteristic of most skeletal muscles.

Antibody A Y-shaped protein produced by plasma cells during an adaptive immune response that recognizes and binds to a specific antigen because of the shape of the molecule. Antibodies defend against invaders in a variety of ways, including neutralization, agglutination and precipitation, or activation of the complement system.

Antibody-mediated immune responses Immune system responses conducted by B cells that produce antibodies and that defend primarily against enemies that are free in body fluids, including toxins or extracellular pathogens, such as bacteria or free viruses.

Anticodon A three-base sequence on transfer RNA (tRNA) that binds to the complementary base pairs of a codon on the mRNA.

Antidiuretic hormone (ADH) A hormone manufactured by the hypothalamus but stored in and released from the posterior pituitary. It regulates the amount of water reabsorbed by the distal convoluted tubules and collecting ducts of nephrons. ADH causes water retention at the kidneys and elevates blood pressure. It is also called vasopressin.

Antigen A substance that is recognized as foreign by the immune system. Antigens trigger an immune response.

Antigen-presenting cell (APC) A cell that presents an antigen to a helper T cell, initiating an immune response toward that antigen. An important type of antigen-presenting cell is a macrophage.

Aorta The body's main artery that conducts blood from the left ventricle toward the cells of the body. The aorta arches over the top of the heart and gives rise to the smaller arteries that feed the capillary beds of the body tissues.

Apoptosis A series of predictable physical changes in a cell that is undergoing programmed cell death. Apoptosis is sometimes used as a synonym for programmed cell death.

Appendicular skeleton The part of the skeleton that includes the pectoral girdle (shoulders), the pelvic girdle (pelvis), and the limbs (arms and legs).

Appendix A slender closed pouch that extends from the large intestine near the juncture with the small intestine.

Aqueous humor The fluid within the anterior chamber of the eye. It supplies nutrients and oxygen to the cornea and lens and carries away their metabolic wastes.

Arachnoid The middle layer of the meninges (the connective tissue layers that protect the central nervous system).

Areolar connective tissue A type of loose connective tissue composed of cells in a gelatinous matrix. It serves as a universal packing material between organs and anchors skin to underlying tissues and organs.

Arrector pili The tiny, smooth muscles attached to the hair follicles in the dermis.

Arteriole A small blood vessel located between an artery and a capillary. Arterioles serve to regulate blood flow through capillary beds to various regions of the body. They also regulate blood pressure. Arterioles are barely visible to the unaided eye.

Artery A large-diameter muscular tube (blood vessel) that transports blood away from the heart toward the cells of body tissues. Arteries conduct blood low in oxygen to the lungs and blood high in oxygen to the body tissues. Arteries typically have thick muscular and elastic walls that dampen the blood pressure pulsations caused by heart contractions.

Arthritis An inflammation of a joint.

Artificial insemination A treatment for infertility in which sperm are deposited in the woman's cervix or vagina at about the time of ovulation.

ASDs See autism spectrum disorders.

Asperger syndrome A neurodevelopmental disorder characterized by impaired social interaction and unusual patterns of behavior, including restricted, repetitive, and

stereotyped motor patterns; also called Asperger's disorder. Asperger syndrome differs from autism in having no clinically significant general delay in language.

Association neuron An interneuron. These neurons are located within the central nervous system between sensory and motor neurons and serve to integrate information.

Asthma A condition marked by spasms of the muscles of bronchioles, making air flow difficult. It is often triggered by allergy.

Astigmatism Irregularities in the curvature of the cornea or lens that cause distortions of a visual image because the irregularities cause light rays to converge unevenly.

Atherosclerosis A narrowing of the arteries caused by thickening of the arterial walls and a buildup of lipid (primarily cholesterol) deposits. Atherosclerosis reduces blood flow through the vessel, choking off the vital supply of oxygen and nutrients to the tissues served by that vessel.

Atom A unit of matter that cannot be further broken down by chemical means; it is composed of subatomic particles, which include protons (positively charged particles), neutrons (with no charge), and electrons (with negative charges).

Atomic number The number of protons in the nucleus of an atom.

ATP See adenosine triphosphate.

Atrial fibrillation Rapid, ineffective contractions of the atria of the heart.

Atrial natriuretic peptide (ANP) The hormone released by cells in the right atrium of the heart in response to stretching of the heart caused by increased blood volume and pressure. ANP decreases water and sodium reabsorption by the kidneys, resulting in the production of large amounts of urine.

Atrioventricular (AV) bundle A tract of specialized cardiac muscle cells that runs along the wall between the ventricles of the heart and conducts an electrical impulse that originated in the sinoatrial (SA) node and was conducted to the AV node to the ventricles. The bundle forks into right and left branches and then divides into many other specialized cardiac muscle cells, called Purkinje fibers, that penetrate the walls of the ventricles.

Atrioventricular (AV) node A region of specialized cardiac muscle cells located in the partition between the two atria. It receives an electrical signal that spreads through the atrial walls from the sinoatrial node and relays the stimulus to the ventricles by means of a bundle of specialized muscle fibers, called the atrioventricular bundle, that runs along the wall between the ventricles.

Atrioventricular (AV) valves Heart valves located between the atria and the ventricles that keep blood flowing in only one direction, from the atria to the ventricles. The right AV valve consists of three flaps of tissue and is also called the tricuspid valve. The left AV valve consists of two flaps of tissue and is also called the bicuspid or the mitral valve.

Atrium (plural, atria) An upper chamber of the heart that receives blood from veins and pumps it to a ventricle.

Auditory tubes Small tubes that join the upper region of the pharynx (throat) with the middle ear. They help to equalize the air pressure between the middle ear and the atmosphere. Also called Eustachian tubes.

Autism A neurodevelopmental disorder characterized by impaired communication and social interaction, and the presence of unusual patterns of behavior, including restricted, repetitive, and stereotyped motor patterns; also called autistic disorder.

Autism spectrum disorders (ASDs) A group of neurodevelopmental disorders that includes autism, Asperger syndrome, and pervasive developmental disorder not otherwise specified.

Autoimmune disorder An immune response misdirected against the body's own tissues.

Autonomic nervous system The part of the peripheral nervous system that governs the involuntary, unconscious activities that maintain a relatively stable internal environment. The autonomic nervous system has two branches: the sympathetic and the parasympathetic.

Autosomes The 22 pairs of chromosomes (excluding the pair of sex chromosomes) that determine the expression of most of the inherited characteristics of a person.

Autotroph An organism that makes its own food (organic compounds) from inorganic substances. The autotrophs include photoautotrophs, which use the energy of light, and chemoautotrophs, which use the energy in chemicals.

Axial skeleton The part of the skeleton that includes the skull, the vertebral column, the breastbone (sternum), and the rib cage.

Axon A long extension from the cell body of a neuron that carries an electrochemical message away from the cell body toward another neuron or effector (muscle or gland). The tips of the axon release a chemical called a neurotransmitter that can affect the activity of the receiving cell. Typically, there is one long axon on a neuron.

Axon terminal The tip of a branch of an axon that releases a chemical (neurotransmitter) that alters the activity of the target cell. A synaptic knob.


B cell See B lymphocyte.

B lymphocyte B cell. A type of white blood cell important in antibody-mediated immune responses that can transform into a plasma cell and produce antibodies.

Balanced polymorphism A phenomenon in which natural selection maintains two or more alleles for a trait in a population from one generation to the next. It occurs when the environment changes frequently or when the heterozygous condition is favored over either homozygous condition.

Ball-and-socket joint A joint, such as the shoulder and hip joints, that allows motion in all directions.

Barr body A structure formed by a condensed, inactivated X chromosome in the body cells of female mammals.

Basal body The structure that anchors the microtubules of a cilium or flagellum to a cell. It contains nine triplets of microtubules arranged in a ring.

Basal cell carcinoma The most common type of skin cancer, occurring in the rapidly dividing cells of the basal layer of the epidermis.

Basal metabolic rate (BMR) A measure of the minimum energy required to keep an awake, resting body alive. It generally represents between 60% and 75% of the body's energy needs.

Base Any substance that reduces the concentration of hydrogen ions in solution.

Basement membrane A noncellular layer beneath epithelial tissue that binds the epithelial cells to underlying connective tissue. It helps epithelial tissue resist stretching and forms a boundary.

Basilar membrane The floor of the central canal in the cochlea of the inner ear that supports the spiral organ (of Corti), which is the true site of hearing; when the basilar membrane vibrates in response to sound, hair cells on the spiral organ are bent, generating electrochemical messages that are interpreted as sound.

Basophil A white blood cell that releases histamine, a chemical that both attracts other white blood cells to the site and causes blood vessels to widen during an inflammatory response.

Benign tumor An abnormal mass of tissue that usually remains at the site where it forms.

Bicuspid valve A heart valve located between the left atrium and ventricle. It is also called the mitral valve or the left atrioventricular (AV) valve.

Bile A mixture of water, ions, cholesterol, bile pigments, and bile salts that emulsifies fat (keeps fat as small globules), facilitating digestion by lipase. Bile is produced by the liver, stored in the gallbladder, and acts in the small intestine.

Bilirubin A yellow pigment produced from the breakdown of the heme portion of hemoglobin by liver cells. It is excreted by the liver in bile.

Binary fission A type of asexual reproduction in which the genetic information is replicated and then a cell, a bacterium for example, or organism divides into two equal parts.

Biodiversity The number and variety of all living things in a given area. It includes genetic diversity, species diversity, and ecological diversity.

Biofeedback The use of artificial signals to provide feedback about unconscious visceral and motor activity, particularly that associated with stress.

Biogeochemical cycle The recurring process by which materials (for example, carbon, water, nitrogen, and phosphorus) cycle between living and nonliving systems and back again.

Biogeography The study of the geographic distribution of organisms. New distributions of organisms occur when organisms move to new areas (dispersal) and when areas occupied by the organisms move or are subdivided.

Biological magnification The tendency of a nondegradable chemical to become more concentrated in organisms as it passes along a food chain.

Biomass In ecosystems, the dry weight of the body mass of a group of organisms in a particular habitat.

Biopsy The removal and examination, usually microscopic, of a piece of tissue to diagnose a disease, usually cancer.

Biosphere The part of Earth in which life is found. It encompasses all of Earth's living organisms.

Biotechnology The industrial or commercial use or alteration of living organisms, cells, or molecules to achieve specific useful goals.

Bioterrorism The use of biological agents, such as viruses, parasites, bacteria and their toxins, to intimidate or attack societies or governments. There is concern that biological agents could be intentionally introduced to food or water supplies, for example.

Bipedalism Walking on two feet. This trait evolved early in hominin evolution and set the stage for the evolution of other characteristics, such as increases in brain size.

Bipolar neuron A neuron that has only two processes. The axon and the dendrite extend from opposite sides of the cell

body. Bipolar neurons are receptor cells found only in some of the special sensory organs, such as in the retina of the eye and in the olfactory membrane of the nose.

Birth defects Developmental defects present at birth. Such defects involve structure, function, behavior, or metabolism and may or may not be hereditary.

Birth rate The number of births per a specified number of individuals in the population during a specific length of time.

Bladder A muscular saclike organ that receives urine from the two ureters and temporarily stores it until release into the urethra.

Blastocyst The stage of development consisting of a hollow ball of cells. It contains the inner cell mass, a group of cells that will become the embryo, and the trophoblast, a thin layer of cells that will give rise to part of the placenta.

Blind spot The region of the retina where the optic nerve leaves the eye and on which there are no photoreceptors. Objects focused on the blind spot cannot be seen.

Blood Connective tissue that consists of cells and platelets suspended in plasma, a liquid matrix.

Blood-brain barrier A mechanism that protects the central nervous system by selecting the substances permitted to enter the cerebrospinal fluid from the blood. The barrier results from the relative impermeability of the capillaries in the brain and spinal cord.

Blood pressure The force exerted by the blood against the walls of the blood vessels. It is caused by the contraction of the ventricles and is influenced by vasoconstriction.

Blood type A characteristic of a person's red blood cells determined on the basis of large molecules on the surface of the plasma membrane.

Blue babies Newborns whose foramen ovale, the fetal opening between the right and left atria of the heart, fails to close. As a result, much of their blood still bypasses the lungs and is low in oxygen. The condition can be corrected with surgery.

BMR See basal metabolic rate.

Bolus A small, soft ball of food mixed with saliva.

Bone Strong connective tissue with specialized cells in a hard matrix composed of collagen fibers and mineral salts.

Bone marrow The soft material filling the cavities in bones. Yellow bone marrow serves as a fat-storage site. Red bone marrow is the site where blood cells are produced.

Bone remodeling The ongoing process of bone deposition and absorption in response to hormonal and mechanical factors.

Bottleneck effect The genetic drift associated with dramatic, unselective reductions in population size such that the genetic makeup of survivors is not representative of the original population.

Brain The organ composed of neurons and glial cells that receives sensory input and integrates, stores, and retrieves information and directs motor output.

Brain waves The patterns recorded in an EEG (electroencephalogram) that reflect the electrical activity of the brain and are correlated with the person's state of alertness.

Breast The front of the chest, especially either of the two protuberant glandular organs (mammary glands) that in human females and other female mammals produce milk to nourish newborns.

Breathing center A region in the medulla of the brain that controls the basic breathing rhythm.

Breech birth Delivery in which the baby is born buttocks first rather than head first. It is associated with difficult labors and umbilical cord accidents.

Bronchi (singular, bronchus) The respiratory passageways between the trachea and the bronchioles that conduct air into the lungs.

Bronchial tree The term given to the air tubules in the respiratory system because their repeated branching resembles the branches of a tree.

Bronchioles A series of small tubules branching from the smallest bronchi inside each lung.

Bronchitis Inflammation of the mucous membranes of the bronchi, causing excess mucus and a deep cough.

Brush border A fuzzy border of microvilli on the surface of absorptive epithelial cells of the small intestine.

Buffer A substance that prevents dramatic changes in pH by removing excess hydrogen ions from solution when concentrations increase and adding hydrogen ions when concentrations decrease.

Bulbourethral glands Cowper's glands. Male accessory reproductive glands that release a clear slippery liquid immediately before ejaculation.

Bulimia nervosa An eating disorder characterized by binge eating followed by purging by means of enemas, laxatives, diuretics, or self-induced vomiting.

Bursa (plural, bursae) A flattened sac containing a thin film of synovial fluid that surrounds and cushions certain synovial joints. Bursae are common in locations where ligaments, muscles, skin, or tendons rub against bone.

Bursitis Inflammation of a bursa (a sac in a synovial joint that acts as a cushion). Bursitis causes fluid to build up within the bursa, resulting in intense pain that becomes worse when the joint is moved and cannot be relieved by resting in any position.


C-section See cesarean section.

Calcitonin (CT) A hormone secreted by the thyroid gland when blood calcium levels are high. It stimulates the removal of calcium from the blood and inhibits the breakdown of bone.

Callus A mass of repair tissue formed by collagen fibers secreted from fibroblasts or woven bone that forms around and links the ends of a broken bone.

Capillary A microscopic blood vessel between arterioles and venules with walls only one cell layer thick. It is the site where the exchange of materials between the blood and the tissues occurs.

Capillary bed A network of true capillaries servicing a particular area. Precapillary sphincters regulate blood flow through the capillary bed. When the sphincters relax, blood fills the capillary bed and materials can be exchanged between the blood and the tissues. When the sphincters contract, blood flows directly from an arteriole to a venule, bypassing the capillaries.

Carbaminohemoglobin The compound formed when carbon dioxide binds to hemoglobin.

Carbohydrate An organic molecule that provides fuel for the human body. Carbohydrates, which we know as sugars and starches, can be classified by size into the monosaccharides, disaccharides, and polysaccharides.

Carbon footprint A measure of the amount of carbon dioxide that enters the atmosphere due to the activities of a person or population; it includes personal activities and the activities needed for production and waste removal of the products consumed.

Carbonic anhydrase An enzyme in the red blood cells that catalyzes the conversion of unbound carbon dioxide to carbonic acid.

Carcinogen A substance that causes cancer.

Carcinoma in situ A tumor that has not spread; "cancer in place."

Cardiac cycle The events associated with the flow of blood through the heart during a single heartbeat. It consists of systole (contraction) and diastole (relaxation) of the atria and then of the ventricles of the heart.

Cardiac muscle A contractile tissue that makes up the bulk of the walls of the heart. Cardiac muscle cells are cylindrical and have branching interconnections between them. Cardiac muscle cells are striped (striated) and have a single nucleus. Contraction of cardiac muscle is involuntary.

Cardiovascular Pertaining to the heart and blood vessels.

Cardiovascular system The organ system composed of the heart and blood vessels. The cardiovascular system distributes blood, delivers nutrients, and removes wastes.

Carnivore An animal that obtains energy by eating other animals. A secondary consumer.

Carpal tunnel syndrome A condition of the wrist and hand whose symptoms may include numbness or tingling in the affected hand, along with pain in the wrist, hand, and fingers that is caused by repeated motion in the hand or wrist, causing the tendons to become inflamed and press against the nerve.

Carrier An individual who displays the dominant phenotype but is heterozygous for a trait and can therefore pass the recessive allele to descendants.

Carrying capacity The number of individuals of a given species that a particular environment can support for a prolonged time period. The carrying capacity of the environment is determined by such factors as availability of resources, including food, water, and space; ability to clean away wastes; and predation pressure.

Cartilage A type of specialized connective tissue with a firm gelatinous matrix containing protein fibers for strength. The cartilage cells (chondrocytes) lie in small spaces (lacunae) within the matrix.

Catabolism Chemical reactions within living cells that break down complex molecules into simpler ones, releasing energy from chemical bonds. Compare with anabolism.

Cataract A cloudy or opaque lens in the eye. Cataracts reduce visual acuity and may be caused by glucose accumulation associated with type 1 and type 2 diabetes mellitus, excessive exposure to sunlight, and exposure to cigarette smoke.

CD4 cell See helper T cell.

Cecum A pouch that hangs below the junction of the small and large intestines; the appendix extends from the cecum.

Cell The smallest structure that shows all the characteristics of life.

Cell adhesion molecule (CAM) A molecule that pokes through the plasma membranes of most cells and helps hold cells together to form tissues and organs.

Cell body The part of a neuron that contains the organelles and nucleus needed to maintain the cells.

Cell cycle The entire sequence of events that a cell goes through from its origin in the division of a parent cell through its own division into two daughter cells. The cell cycle consists of two major phases: interphase and cell division.

Cell differentiation The process by which cells become specialized with respect to structure and function.

Cell-mediated immune responses Immune system responses conducted by T cells that protect against cellular threats, including body cells that have become infected with viruses or other pathogens and cancer cells.

Cell theory This fundamental organizing principle of biology states that (1) a cell is the smallest unit of life; (2) cells make up all living things, from unicellular to multicellular organisms; and (3) new cells arise from pre-existing cells.

Cellular respiration The oxygen-requiring pathway by which glucose is broken down by cells to yield carbon dioxide, water, and energy.

Cellulose A structural polysaccharide found in the cell walls of plants. Humans lack the enzymes necessary to digest cellulose, so it passes unchanged through our digestive tract. Although cellulose has no value as a nutrient, it is an important form of dietary fiber known to facilitate the passage of feces through the large intestines.

Cementum A calcified but sensitive part of a tooth that covers the root.

Central nervous system (CNS) The brain and the spinal cord.

Centriole A structure, found in pairs, within a centrosome.

Each centriole is composed of nine sets of triplet microtubules arranged in a ring.

Centromere The region of a replicated chromosome at which sister chromatids are held together until they separate during cell division.

Centrosome The region near the nucleus that contains centrioles. It forms the mitotic spindle during prophase.

Cerebellum A region of the brain important in sensory-motor coordination. It is largely responsible for posture and smooth body movements.

Cerebral cortex The extensive area of gray matter covering the surfaces of the cerebrum. It is often referred to as the conscious part of the brain. The cerebral cortex has sensory, motor, and association areas.

Cerebral white matter A region of the cerebrum beneath the cortex consisting primarily of myelinated axons that are grouped into tracts that allow various regions of the brain to communicate with one another.

Cerebrospinal fluid The fluid bathing the internal and external surfaces of the central nervous system. It serves as a shock absorber, supports the brain, nourishes the brain, delivers chemical messengers, and removes waste products.

Cerebrovascular accident See stroke.

Cerebrum The largest and most prominent part of the brain, composed of the cerebral hemispheres. It is responsible for thinking, sensory perception, originating most conscious motor activity, personality, and memory.

Cervical cap A barrier means of contraception consisting of a small rubber dome that fits snugly over the cervix and is held in place partly by suction. It prevents sperm from reaching the egg.

Cervix The narrow neck of the uterus that projects into the vagina whose opening provides a passageway for materials moving between the vagina and the body of the uterus.

Cesarean section A procedure by which the baby and placenta are removed from the uterus through an incision in the abdominal wall and uterus. The term is often shortened to C-section.

Chancre A painless bump that forms during the first stage of syphilis at the site of contact, usually within 2 to 8 weeks of the initial contact.

Chemical digestion A part of the digestive process that involves breaking chemical bonds so that complex molecules are broken into their component subunits. Chemical digestion produces molecules that can be absorbed into the bloodstream and used by the cells.

Chemical evolution The sequence of events by which life evolved from chemicals slowly increasing in complexity over perhaps 300 million years.

Chemistry The branch of science concerned with the composition and properties of material substances, including their abilities to change into other substances.

Chemoreceptor A sensory receptor specialized to respond to chemicals. We describe the input from the chemoreceptors of the mouth as taste (gustation) and from those of the nose as smell (olfaction). Other chemoreceptors monitor levels of chemicals, such as carbon dioxide, oxygen, and glucose, in body fluids.

Childhood The stage in postnatal development that runs from about 13 months to 12 or 13 years of age. It is a time of continued growth during which gross and fine motor skills improve and coping skills develop. With the exception of the reproductive system, organ systems become fully functional.

Chitin A structural polysaccharide found in the exoskeletons (hard outer coverings) of animals such as insects, spiders, and crustaceans.

Chlamydia A genus of bacteria. In this text, it is an infection (usually sexually transmitted) caused by Chlamydia trachomatis, commonly causing urethritis and pelvic inflammatory disease.

Cholecystokinin A hormone secreted by the small intestine that stimulates the pancreas to release its digestive enzymes and the gallbladder to contract and release bile.

Chordae tendineae Strings of connective tissue that anchor the atrioventricular valves to the wall of the heart, preventing the backflow of blood.

Chorion The extraembryonic membrane that becomes the embryo's major contribution to the placenta.

Chorionic villi Fingerlike projections of the chorion of the embryo that grow into the uterine lining of the mother during formation of the placenta and become part of the placenta.

Chorionic villi sampling (CVS) A procedure for screening for genetic defects of a fetus by removing a piece of chorionic villi and examining the cells for genetic abnormalities.

Choroid The pigmented middle layer of the eyeball that contains blood vessels.

Chromatid One of the two identical replicates of a duplicated chromosome. The two chromatids that make up a chromosome are held together by a centromere and are referred to as sister chromatids. During cell division, the two strands separate and each becomes a chromosome in one of the two daughter cells.

Chromatin DNA and associated proteins in a dispersed, rather than condensed, state.

Chromosomal mutation A change in DNA in which a section of a chromosome becomes rearranged, duplicated, or deleted.

Chromosome DNA (which contains the genetic information of a cell) and specialized proteins, primarily histones.

Chronic renal failure A progressive and often irreversible decline in the rate of glomerular filtration.

Chylomicron A particle formed when proteins coat the surface of the products of lipid digestion, making lipids soluble in water and allowing them to be transported throughout the body.

Chyme The semifluid mass created during digestion once the food has been churned and mixed with the gastric juices of the stomach.

Cilia Extensions of the plasma membrane found on some cells, such as those lining the respiratory tract, that move in a back- and-forth motion. They are usually shorter and much more numerous than flagella but have the same 9 + 2 arrangement of microtubules at their core.

Ciliary body A portion of the middle coat of the eyeball near the lens that consists of smooth muscle and ligaments. Contractions of the smooth muscle of the ciliary body change the shape of the lens, which then focuses images on the retina.

Circulatory system An organ system composed of the cardiovascular system (heart and blood vessels) and the lymphatic system (lymphatic vessels and lymphoid tissues and organs).

Circumcision The surgical removal of the foreskin of the penis, usually performed when the male is an infant.

Cirrhosis A chronic disease of the liver in which the liver becomes fatty and the liver cells are gradually replaced with scar tissue.

Citric acid cycle The cyclic series of chemical reactions that follows the transition reaction and yields two molecules of adenosine triphosphate (one from each acetyl CoA that enters the cycle) and several molecules of nicotine adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2), carriers of high-energy electrons that enter the electron transport chain. This phase of cellular respiration occurs inside the mitochondrion and is sometimes called the Krebs cycle.

Cleavage A rapid series of mitotic cell divisions in which the zygote first divides into two cells, and then four cells, and then eight cells, and so on. Cleavage usually begins about one day after fertilization as the zygote moves along the oviduct toward the uterus.

Climax community The relatively stable community that eventually forms at the end of ecological succession and remains if no disturbances occur.

Clitoris A small, erectile body in the female that plays a role in sexual stimulation. It develops from the same embryological structure from which the glans penis develops in the male.

Clonal selection The hypothesis that, by binding to a receptor on a lymphocyte surface, an antigen selectively activates only those lymphocytes able to recognize that antigen and programs that lymphocyte to divide, forming an army of cells specialized to attack the stimulating antigen.

Clone A population of identical cells descended from a single ancestor.

CNS See central nervous system.

Cochlea The snail-shaped portion of the inner ear that contains the actual organ of hearing, the spiral organ (of Corti).

Codominance The condition in which the effects of both alleles are separately expressed in a heterozygote.

Codon A three-base sequence on messenger RNA (mRNA) that specifies one of the 20 common amino acids or the beginning or end of the protein chain.

Coenzyme An organic molecule such as a vitamin that functions as a cofactor and helps enzymes convert substrate to product.

Cofactor A nonprotein substance such as zinc, iron, and vitamins that helps enzymes convert substrate to product. It may permanently reside at the active site of the enzyme or may bind to the active site at the same time as the substrate.

Collagen fibers Strong insoluble protein fibers common in many connective tissues.

Collecting ducts Within the kidneys, the tubes that receive filtrate from the distal convoluted tubules of many nephrons and that eventually drain into the renal pelvis. Some tubular secretion occurs along collecting ducts.

Colon The division of the large intestine composed of the ascending colon, the transverse colon, and the descending colon.

Colostrum A cloudy yellowish fluid produced by the breasts in the interval after birth when milk is not yet available. Its composition is different from that of milk.

Columnar epithelium A type of epithelial tissue composed of tall, rectangular cells that are specialized for secretion and absorption.

Coma An unconscious state caused by trauma to neurons in regions of the brain responsible for stimulating the cerebrum, particularly those in the reticular activating system or thalamus. Coma can be caused by mechanical shock, such as might be caused by a blow to the head, tumors, infections, drug overdose (from barbiturates, alcohol, opiates, or aspirin), or failure of the liver or kidneys.

Combination birth control pill A means of hormonal contraception that consists of a series of pills with synthetic forms of estrogen and progesterone. The hormones in the pills mimic the effects of natural hormones ordinarily produced by the ovaries and inhibit FSH and LH secretion by the anterior pituitary gland and, therefore, prevent the development of an egg and ovulation.

Common cold An upper respiratory infection caused by one of the adenoviruses.

Community An assortment of organisms of various species interacting in a defined habitat.

Compact bone Very dense, hard bone, containing internal spaces of microscopic size and narrow channels that contain blood vessels and nerves. It makes up the shafts of long bones and the outer surfaces of all bones.

Complement system A group of about 20 proteins that enhances the body's defense mechanisms. The complement system destroys cellular pathogens by creating holes in the plasma membrane, making the cell leaky, enhancing phagocytosis, and stimulating inflammation.

Complementary base pairing The process by which specific bases are matched: adenine with thymine (in DNA) or with uracil (in RNA) and cytosine with guanine. Each base pair is held together by weak hydrogen bonds.

Complementary proteins A selection of foods, each containing incomplete proteins, that provides ample amounts of all essential amino acids when combined.

Complete dominance In genetic inheritance, the dominant allele in a heterozygote completely masks the effect of the recessive allele. Complete dominance often occurs because the dominant allele produces a functional protein and the recessive allele produces a less functional protein or none at all.

Complete protein A protein that contains ample amounts of all the essential amino acids. Animal sources of protein are generally complete proteins.

Compound A molecule that contains two or more different elements.

Computed tomography (CT scanning) A method of visualizing body structures, including the brain, using an x-ray source that moves in an arc around the body part to be imaged, thereby providing different views of the structure.

Concentration gradient A difference in the number of molecules or ions between two adjacent regions. Molecules or ions tend to move away from an area where they are more concentrated to an area where they are less concentrated. Each type of molecule or ion moves in response to its own concentration gradient.

Conclusion An interpretation of the data collected in an experiment.

Condom, female A barrier means of contraception used by a female that consists of a loose sac of polyurethane held in place by flexible rings (one at each end). It is used to prevent sperm from entering the female reproductive tract, and it also reduces the risk of spreading sexually transmitted infections.

Condom, male A barrier means of contraception consisting of a thin sheath of latex or animal intestines that is rolled onto an erect penis, where it prevents sperm from entering the vagina. A latex condom also helps prevent the spread of sexually transmitted diseases.

Cones Photoreceptors in the eye responsible for color vision. There are three types of cone cells: blue, green, and red.

Confounding variable In a controlled experiment, it is a second factor that differs between the control group and the experimental group.

Conjoined twins Individuals that develop from a single fertilized ovum that fails to completely split in two at an early stage of cleavage. Such twins have nearly identical genetic material and thus are always the same gender. They may be surgically separated after birth.

Connective tissue Tissue that binds together and supports other tissues of the body. All connective tissues contain cells in an extracellular matrix, which consists of protein fibers and a noncellular ground substance.

Constipation Infrequent, difficult bowel movements of hard feces. Constipation occurs when feces move through the large intestine too slowly and too much water is reabsorbed.

Consumer In ecosystems, an organism that obtains energy and raw materials by eating the tissues of other organisms.

Contact inhibition The phenomenon whereby cells placed in a dish in the presence of growth factors stop dividing once they have formed a monolayer. This may result from competition among the cells for growth factors and nutrients. Cancer cells do not display density-dependent contact inhibition and continue to divide, piling up on one another until nutrients run out.

Continuous ambulatory peritoneal dialysis (CAPD) A method of hemodialysis whereby the peritoneum, one of the body's own selectively permeable membranes, is used as the dialyzing membrane. It is an alternative to the artificial kidney machine during kidney failure.

Contraceptive sponge A barrier means of contraception consisting of a sponge containing spermicide.

Control group In a controlled scientific experiment, the control group is the one in which the variable is unaltered for comparison to the experimental group.

Controlled experiment An experiment in which the subjects are divided into two groups, usually called the control group and the experimental group. Ideally, the groups differ in only the factor(s) of interest.

Convergence In vision, the process by which the eyes are directed toward the midline of the body as an object moves closer. Convergence is necessary to keep the image focused on the fovea of the retina.

Convergent evolution The process by which two species become more alike because they have similar ecological roles and selection pressures. For example, birds and insects independently evolved wings.

Core temperature The temperature in body structures below the skin and subcutaneous layers.

Cornea A clear, transparent dome located in the front and center of the eye that both provides the window through which light enters the eye and helps bend light rays so that they focus on the retina.

Coronary arteries The arteries that deliver blood to cardiac muscle.

Coronary artery bypass A technique for bypassing a blocked coronary blood vessel to restore blood flow to the heart muscle. Typically, a segment of a leg vein is removed and grafted so that it provides a shunt between the aorta and a coronary artery past the point of obstruction.

Coronary artery disease (CAD) A condition in which fatty deposits associated with atherosclerosis form on the inside of coronary arteries, obstructing the flow of blood. It is the underlying cause of most heart attacks.

Coronary circulation The system of blood vessels that services the tissues of the heart itself.

Coronary sinus A vessel that returns deoxygenated blood collected from the heart muscle to the right atrium of the heart. The coronary sinus is formed from the merging of cardiac veins.

Corpus callosum A band of myelinated axons (white matter) that connects the two cerebral hemispheres so they can communicate with one another.

Corpus luteum A structure in the ovary that forms from the follicle cells remaining in the ovary after ovulation. The corpus luteum functions as an endocrine structure that secretes estrogen and progesterone.

Covalent bond A chemical bond formed when outer shell electrons are shared between atoms.

Cowper's glands See bulbourethral glands.

Cranial nerves Twelve pairs of nerves that arise from the brain and service the structures of the head and certain body parts such as the heart and diaphragm. Cranial nerves can be sensory, motor, or mixed.

Cranium The portion of the skull that forms the cranial (brain) case. It is formed from eight (sometimes more) flattened bones including the frontal bone, two parietal bones, the occipital bone, two temporal bones, the sphenoid bone, and the ethmoid bone.

Creatine phosphate A compound stored in muscle tissue that serves as an alternative energy source for muscle contraction.

Cretinism A condition characterized by dwarfism, mental retardation, and slowed sexual development. It is caused by undersecretion of thyroid hormone during fetal development or infancy.

Cristae Infoldings of the inner membrane of a mitochondrion.

Cross-bridges Myosin heads. Club-shaped ends of a myosin molecule that bind to actin filaments and can swivel, causing actin filaments to slide past the myosin filaments, which causes muscle contraction.

Cross-tolerance The development of tolerance for a drug that is not used, caused by the development of tolerance to another, usually similar, drug.

Crossing over The breaking and rejoining of nonsister chromatids of homologous pairs of chromosomes during meiosis (specifically at prophase I when homologous chromosomes pair up side by side). Crossing over results in the exchange of corresponding segments of chromatids and increases genetic variability in populations.

Crown The part of a tooth that is visible above the gum line. It is covered with enamel, a nonliving material that is hardened with calcium salts.

CT scanning See computed tomography.

Cuboidal epithelium A type of epithelial tissue composed of cube-shaped cells that are specialized for secretion and absorption.

Culture Social influences that produce an integrated pattern of knowledge, belief, and behavior.

Cupula A pliable gelatinous mass covering the hair cells within the ampulla of the semicircular canals of the inner ear and whose movement bends hair cells, triggering the generation of nerve impulses that are interpreted by the brain as movement of the head.

Cushing's syndrome A condition characterized by accumulation of fluid in the face and redistribution of body fat caused by prolonged exposure to cortisol.

Cutaneous membrane The skin. It is thick, relatively waterproof, and dry.

Cystitis Inflammation of the urinary bladder caused by bacteria.

Cytokinesis The division of the cytoplasm and organelles into two daughter cells during cell division. Cytokinesis usually occurs during telophase.

Cytoplasm The part of a cell that includes the aqueous fluid within the cell and all the organelles with the exception of the nucleus.

Cytoskeleton A complex network of protein filaments within the cytoplasm that gives the cell its shape, anchors organelles in place, and functions in the movement of entire cells or certain organelles or vesicles within cells. The cytoskeleton includes microtubules, microfilaments, and intermediate filaments.

Cytotoxic T cell A type of T lymphocyte that directly attacks infected body cells and tumor cells by releasing chemicals called perforins that cause the target cells to burst.


Darwinian fitness See fitness.

Decomposer An organism that obtains energy by consuming the remains or wastes of other organisms. Decomposers release inorganic materials that can then be used by producers. Bacteria and fungi are important decomposers.

Deductive reasoning A logical progression of thought proceeding from the general to the specific. It involves making specific deductions based on a larger generalization or premise. The statement is usually in the form of an "if ... then" premise.

Deforestation Removing trees from an area without replacing them.

Dehydration synthesis The process by which polymers are formed. Monomers are linked together through the removal of a water molecule.

Deletion Pertaining to chromosomes, the loss of a nucleotide or segment of a chromosome.

Denaturation The process by which changes in the environment of a protein, such as increased heat or changes in pH, cause it to unravel and lose its three-dimensional shape. Change in the shape of a protein results in loss of function.

Dendrite A process of a neuron specialized to pick up messages and transmit them toward the cell body. There are typically many short branching dendrites on a neuron.

Dense connective tissue Connective tissue that contains many tightly woven fibers and is found in ligaments, tendons, and the dermis.

Density-dependent regulating factor One of many factors that have a greater impact on the population size as conditions become more crowded. Such factors include disease and starvation.

Density-independent regulating factor One of many factors that regulate population size by causing deaths that are not related to the density of individuals in a population. Such factors include natural disasters.

Dentin A hard, bonelike substance that forms the main substance of teeth. It is covered by enamel on the crown and by cementum on the root.

Deoxyribonucleic acid (DNA) The molecular basis of genetic inheritance in all cells and some viruses. A category of nucleic acids that usually consists of a double helix of two nucleotide strands. The sequence of nucleotides carries the instructions for assembling proteins.

Depolarization A change in the difference in electrical charge across a membrane that moves it from a negative value toward 0 mV. During a nerve impulse (action potential), depolarization is caused by the inward flow of positively charged sodium ions.

Dermis The layer of the skin that lies just below the epidermis and is composed of connective tissue. The dermis contains blood vessels, oil glands, sensory structures, and nerve endings. The dermis does not wear away.

Desertification The process by which overfarming and overgrazing transform marginal farmlands and rangelands to deserts.

Desmosome A type of junction between cells that anchors adjacent cells together.

Detrital food web Energy flow begins with detritus (organic material from the remains of dead organisms) that is eaten by a primary consumer.

Detritivore An organism that obtains energy by consuming the remains or wastes of other organisms. Detritivores release inorganic materials that can then be used by producers.

Detrusor muscle A layer of smooth muscle within the walls of the urinary bladder. It plays a role in urination.

Diabetes insipidus A condition characterized by excessive urine production caused by inadequate antidiuretic hormone (ADH) production.

Diabetes mellitus A group of diseases characterized by excessive urine production, an abnormally high blood glucose level, and the presence of glucose in the urine. Caused by deficient production of insulin (type 1) or increased resistance to insulin (type 2 and gestational diabetes).

Diaphragm A broad sheet of muscle that separates the abdominal and thoracic cavities. When the diaphragm contracts, inhalation occurs.

Diaphragm, contraceptive A barrier means of contraception that consists of a dome-shaped soft rubber cup on a flexible ring. A diaphragm is used in conjunction with a spermicide to prevent sperm from reaching the egg.

Diarrhea Abnormally frequent, loose bowel movements. Diarrhea occurs when feces pass through the large intestine too quickly and too little water is reabsorbed. Diarrhea sometimes leads to dehydration.

Diastole Relaxation of the heart. Atrial diastole is the relaxation of the atria. Ventricular diastole is the relaxation of the ventricles.

Diastolic pressure The lowest blood pressure in an artery during the relaxation of the heart. In a typical, healthy adult, the diastolic pressure is about 80 mm Hg.

Digestive system The organ system that breaks down and absorbs food. The digestive system includes the mouth, esophagus, stomach, small intestine, and large intestine. Associated structures include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Dilation stage The first stage of true labor. It begins with the onset of contractions and ends when the cervix has fully dilated to 10 cm (4 in.).

Diploid The condition of having two sets of chromosomes in each cell. Somatic (body) cells are diploid.

Disaccharide A molecule formed when two monosaccharides covalently bond to each other through dehydration synthesis. It is known as a double sugar.

Distal convoluted tubule The section of the renal tubule where reabsorption and secretion occur.

Diuretic A substance that promotes urine production. Alcohol is a diuretic.

DNA (deoxyribonucleic acid) The molecular basis of genetic inheritance in all cells and some viruses. A category of nucleic acids that usually consists of a double helix of two nucleotide strands. The sequence of nucleotides carries the instructions for assembling proteins.

DNA fingerprint The pattern of DNA fragments that have been cut by a restriction enzyme and sorted by size. Each person has a characteristic, individual DNA fingerprint.

DNA library A large collection of cloned recombinant DNA fragments containing the entire genome of an organism.

DNA ligase An enzyme that catalyzes the formation of bonds between the sugar and the phosphate molecules that form the sides of the DNA ladder during replication, repair, or the creation of recombinant DNA.

DNA polymerase Any one of the enzymes that catalyze the synthesis of DNA from free nucleotides using one strand of DNA as a template.

Dominant allele The allele that is fully expressed in the phenotype of an individual who is heterozygous for that gene. The dominant allele usually produces a functional protein, whereas the recessive allele does not.

Dopamine A neurotransmitter in the central nervous system thought to be involved in regulating emotions and in the brain pathways that control complex movements.

Dorsal nerve root The portion of a spinal nerve that arises from the back (posterior) side of the spinal cord and contains axons of sensory neurons. It joins with the ventral nerve root to form a single spinal nerve, which passes through the opening between the vertebrae.

Doubling time The number of years required for a population to double in size at a given, constant growth rate.

Down syndrome A collection of characteristics that tend to occur when an individual has three copies of chromosome 21. It is also known as trisomy 21.

Drug cocktail A combination of drugs used to treat people who are HIV positive. The combination usually includes a drug that blocks reverse transcription and a protease inhibitor.

Ductus arteriosus A small vessel in the fetus that connects the pulmonary artery to the aorta. It diverts blood away from the lungs.

Ductus venosus A small vessel in the fetus through which most blood from the placenta flows, bypassing the liver.

Duodenum The first region of the small intestine. The duodenum receives chyme from the stomach and digestive juices from the pancreas and liver.

Duplication Pertaining to chromosomes, the duplication of a region of a chromosome that often results from fusion of a fragment from a homologous chromosome.

Dura mater The tough, leathery outer layer of the meninges that protects the central nervous system. Around the brain, the dura mater has two layers that are separated by a fluid- filled space containing blood vessels.

Dysplasia The changes in shape, nuclei, and organization of adult cells. It is typical of precancerous cells.


Eardrum See tympanic membrane.

ECG See electrocardiogram.

Ecological footprint A measure of the amount of productive land and water used by a person or population to produce products consumed and to remove the waste of products consumed.

Ecological pyramid A diagram in which blocks represent each tropic (feeding) level.

Ecological succession The sequence of changes in the species making up a community over time.

Ecology The study of the interactions among organisms and between organisms and their environment.

Ecosystem All the organisms living in a certain area that can potentially interact, together with their physical environment.

Ectoderm The primary germ layer that forms the nervous system, epidermis, and epidermal derivatives such as hair, nails, and mammary glands.

Ectopic pregnancy A pregnancy in which the embryo (blastocyst) implants and begins development in a location other than the uterus, most commonly in an oviduct (a tubal pregnancy).

Edema Swelling caused by the accumulation of interstitial fluid.

EEG See electroencephalogram.

Effector A muscle or a gland that brings about a response to a stimulus.

Effector cells Lymphocytes that are responsible for the attack on cells or substances not recognized as belonging in the body.

Efferent (motor) neuron A neuron specialized to carry information away from the central nervous system to an effector, either a muscle or a gland.

Egg A mature female gamete. An ovum. The egg contains nutrients and the mother's genetic contribution to the next generation.

EKG See electrocardiogram.

Elastic cartilage The most flexible type of cartilage because of an abundance of wavy elastic fibers in its matrix.

Elastic fibers Coiled proteins found in connective tissues that allow the connective tissue to be stretched and recoil. Elastic fibers are common in tissues that require elasticity.

Electrocardiogram (ECG or EKG) A graphical record of the electrical activities of the heart.

Electroencephalogram (EEG) A graphical record of the electrical activity of the brain.

Electron transport chain A series of carrier proteins embedded in the inner membrane of the mitochondrion that receives electrons from the molecules of nicotine adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2) produced by glycolysis and the citric acid cycle. During the transfer of electrons from one molecule to the next, energy is released and this energy is then used to make adenosine triphosphate (ATP). Oxygen is the final electron acceptor in the chain.

Element Any substance that cannot be broken down into simpler substances by ordinary chemical means.

Embolus A blood clot that drifts through the circulatory system and can lodge in a small blood vessel and block blood flow.

Embryo The developing human from week 3 through week 8 of gestation (the embryonic period).

Embryonic disk The flattened platelike structure that will become the embryo proper. It develops from the inner cell mass.

Embryonic period The period of prenatal development that extends from week 3 through week 8 of gestation. It is when tissues and organs form.

Emigration The departure of individuals from a population for some other area.

Emphysema A condition in which the alveolar walls break down, thicken, and form larger air spaces, making gas exchange difficult. This change results in less surface area for gas exchange and an increase in the volume of residual air in the lungs.

Enamel A nonliving material that is hardened with calcium salts and covers the crown of a tooth.

Encephalitis An inflammation of the meninges around the brain.

Endocardium A thin layer that lines the cavities of the heart.

Endocrine gland A gland that lacks ducts and releases its products (hormones) into the fluid just outside the cells.

Endocrine system The organ system that, along with the nervous system, functions in internal communication. It consists of endocrine glands, such as the pituitary gland and thyroid gland, and of organs, such as the kidneys and pancreas, that contain some endocrine tissue but have functions in addition to hormone secretion.

Endocytosis The process by which large molecules and single- celled organisms such as bacteria enter cells. It occurs when a region of the plasma membrane surrounds the substance to be ingested, then pinches off from the rest of the membrane, enclosing the substance in a saclike structure called a vesicle that is released into the cell. Two types of endocytosis are phagocytosis ("cell eating") and pinocytosis ("cell drinking").

Endoderm The primary germ layer that forms some organs and glands (for example, the pancreas, liver, thyroid gland, and parathyroid glands) and the epithelial lining of the urinary, respiratory, and gastrointestinal tracts.

Endometriosis A painful condition in which tissue from the lining of the uterus (the endometrium) is found outside the uterine cavity.

Endometrium The inner layer of the uterus consisting of connective tissue, glands, and blood vessels. The endometrium thickens and develops with each menstrual (uterine) cycle and is then lost as menstrual flow. It is the site of embryo implantation during pregnancy.

Endoplasmic reticulum The network of internal membranes within eukaryotic cells. Whereas rough endoplasmic reticulum (RER) has ribosomes attached to its surface, smooth endoplasmic reticulum (SER) lacks ribosomes and functions in the production of phospholipids for incorporation into cell membranes.

Endorphin A chemical released by nerve cells that binds to the so-called opiate receptors on the pain-transmitting neurons and quells the pain. The term is short for endogenous morphine-like substance.

Endosymbiont hypothesis The hypothesis that organelles such as mitochondria were once free-living prokaryotic organisms that either invaded or were engulfed by primitive eukaryotic cells with which they established a symbiotic (mutually beneficial) relationship.

Endothelium The lining of the heart, blood vessels, and lymphatic vessels. It is composed of flattened, tight-fitting cells. The endothelium forms a smooth surface that minimizes friction and allows the blood or lymph to flow over it easily.

Enkephalin A chemical released by nerve cells that binds to the so-called opiate receptors on the pain-transmitting neurons and quells the pain.

Enzyme A substance (usually a protein, but sometimes an RNA molecule) that speeds up chemical reactions without being consumed in the process.

Enzyme-substrate complex Complex formed when a substrate binds to an enzyme at the active site.

Eosinophil The type of white blood cell important in the body's defense against parasitic worms. It releases chemicals that help counteract certain inflammatory chemicals released during an allergic response.

Epidermis The outermost layer of the skin, composed of epithelial cells.

Epididymis A long tube coiled on the surface of each testis that serves as the site of sperm cell maturation and storage.

Epiglottis A part of the larynx that forms a movable lid of cartilage covering the opening into the trachea (the glottis).

Epinephrine Adrenaline. A hormone secreted by the adrenal medulla, along with norepinephrine, in response to stress. They initiate the physiological "fight-or-flight" reaction.

Epiphyseal plate A plate of cartilage that separates the head of the bone from the shaft, permitting the bone to grow. In late adolescence, the epiphyseal plate is replaced by bone, and growth stops. The epiphyseal plate is commonly called the growth plate.

Episiotomy An incision made to enlarge the vaginal opening, just before passage of the baby's head at the end of the second stage of labor.

Epithelial tissue One of the four primary tissue types. The tissue that covers body surfaces, lines body cavities and organs, and forms glands.

Equatorial plate A plane at the midline of a cell where chromosomes line up during mitosis or meiosis.

Erythrocyte A red blood cell. A nucleus-free biconcave cell in the blood that is specialized for transporting oxygen to cells and assists in transporting carbon dioxide away from cells.

Erythropoietin A hormone released by the kidneys when the oxygen content of the blood declines that stimulates red blood cell production.

Esophagus A muscular tube that conducts food from the pharynx to the stomach using peristalsis.

Essential amino acid Any of the eight amino acids that the body cannot synthesize and, therefore, must be supplied in the diet.

Estrogen A steroid sex hormone produced by the follicle cells and the corpus luteum in the ovary. Estrogen helps oocytes mature, stimulates cell division in the endometrium and the breast with each uterine cycle, and maintains secondary sex characteristics. The adrenal cortex also secretes estrogen.

Eukaryotic cell A cell with a nucleus and extensive internal membranes that divide it into many compartments and enclose organelles. Eukaryotes include cells in plants, animals, and all other organisms except bacteria and archaea.

Eustachian tubes Auditory tubes. Small tubes that join the upper region of the pharynx (throat) with the middle ear. They help to equalize the air pressure between the middle ear and the atmosphere.

Eutrophication The enrichment of water in a lake or pond by nutrients. Eutrophication is often caused by nitrogen or phosphate that washes into bodies of water.

Evolution Descent with modification from a common ancestor. It is the process by which life forms on the earth have changed over time.

Excitatory synapse A synapse in which the response of the receptors for that neurotransmitter on the postsynaptic membrane increases the likelihood that an action potential will be generated in the postsynaptic neuron. The postsynaptic cell is excited because it becomes less negative than usual (slightly depolarized), usually because of the inflow of sodium ions.

Excretion The elimination of wastes and excess substances from the body.

Exhalation Breathing out (expiration) involves the movement of air out of the respiratory system into the atmosphere.

Exhaustion phase The final phase of the general adaptation syndrome that occurs in response to extreme and prolonged stress. During the exhaustion phase, the heart, blood vessels, and adrenal glands begin to fail after the taxing metabolic and endocrine demands of the resistance phase.

Exocrine glands Glands that secrete their product through ducts onto body surfaces, into the spaces within organs, or into a body cavity. Examples include the salivary glands of the mouth and the oil and sweat glands of the skin.

Exocytosis The process by which large molecules leave cells. It occurs when products packaged by cells in membrane-bound vesicles move toward the plasma membrane. Upon reaching the plasma membrane, the membrane of the vesicle fuses with it, spilling its contents outside the cell.

Exon The nucleotide sequences of a newly synthesized messenger RNA (mRNA) that are spliced together to form the mature mRNA that is ultimately translated into protein.

Exophthalmos A condition characterized by protruding eyes that is caused by the accumulation of interstitial fluid due to oversecretion of thyroid hormone.

Experimental group In a controlled scientific experiment, the experimental group is the one in which the variable is altered.

Expiration The process by which air is moved out of the respiratory system into the atmosphere. It is also called exhalation.

Expiratory reserve volume The additional volume of air that can be forcefully expelled from the lungs after normal exhalation.

Expulsion stage The second stage of true labor. It begins with full dilation of the cervix and ends with delivery of the baby.

External auditory canal The canal leading from the pinna of the ear to the eardrum (tympanic membrane).

External respiration The exchange of oxygen and carbon dioxide between the lungs and the blood.

External urethral sphincter A sphincter made of skeletal muscle that surrounds the urethra. This voluntary sphincter helps stop the flow of urine down the urethra when we wish to postpone urination.

Exteroceptor A sensory receptor that is located near the surface of the body and that responds to changes in the environment.

Extracellular fluid The watery solution outside cells. It is also called interstitial fluid.

Extraembryonic membranes Membranes that lie outside the embryo, where they protect and nourish the embryo and later the fetus. They include the amnion, yolk sac, chorion, and allantois.


Facial bones The bones that form the face. They include 14 bones that support several sensory structures and serve as attachments for most muscles of the face.

Facilitated diffusion The movement of a substance from a region of higher concentration to a region of lower concentration with the aid of a membrane protein that either transports the substance from one side of the membrane to the other or forms a channel through which it can move.

Farsightedness A condition in which distant objects are seen more clearly than near ones. Farsightedness occurs either because the eyeball is too short or the lens is too thin, causing the image to be focused behind the retina.

Fascicle A bundle of skeletal muscle fibers (cells) that forms a part of a muscle. Each fascicle is wrapped in its own connective tissue sheath.

Fast-twitch muscle cells Muscle fibers that contract rapidly and powerfully, with little endurance. They have few mitochondria and large glycogen reserves. They depend on anaerobic pathways to generate adenosine triphosphate (ATP) during muscle contraction.

Fatigue A state in which a muscle is physiologically unable to contract despite continued stimulation. Muscle fatigue results from a relative deficit of adenosine triphosphate (ATP).

Fat-soluble vitamin A vitamin that does not dissolve in water and is stored in fat. Examples are vitamins A, D, E, and K.

Fatty acid Chains of carbon atoms bonded to hydrogen atoms with an acidic group (COOH) at one end. Three fatty acids bond to a molecule of glycerol to form a triglyceride (fat).

Feces Waste material discharged from the large intestine during defecation. Feces consist primarily of undigested food, sloughed-off epithelial cells, water, and millions of bacteria.

Fermentation A pathway by which cells can harvest energy in the absence of oxygen. It nets only 2 molecules of ATP as compared with the approximately 36 molecules produced by cellular respiration.

Fertilization The union between an egg (technically a secondary oocyte) and a sperm. It takes about 24 hours from start to finish and usually occurs in a widened portion of the oviduct, not far from the ovary.

Fetal alcohol syndrome A group of characteristics in children born to mothers who consumed alcohol during pregnancy. It can include mental retardation, slow growth, and certain facial features such as an eye fold near the bridge of the nose.

Fetal period The period of prenatal development that extends from week 9 of gestation until birth. It is when rapid growth occurs.

Fetus The developing human from week 9 of gestation until birth (the fetal period).

Fever An abnormally elevated body temperature. Fever helps the body fight disease-causing invaders in a number of ways.

Fibrillation Rapid, ineffective contractions of the heart.

Fibrin A protein formed from fibrinogen by thrombin. It forms a web that traps blood cells, forming a blood clot.

Fibrinogen A plasma protein produced by the liver that is important in blood clotting. It is converted to fibrin by thrombin.

Fibroblasts Cells in connective tissue that secrete the protein fibers that are found in the matrix of the connective tissue. Fibroblasts also secrete collagen fibers for the repair of body tissues.

Fibrocartilage Cartilage with a matrix containing many collagen fibers. Fibrocartilage is found around the edges of joints and the intervertebral disks.

Fibrous joints Joints that are held together by connective tissue and lack a joint cavity. Most fibrous joints do not permit movement.

Fight-or-flight response The body's reaction to stress or threatening situations by the stimulation of the sympathetic division of the autonomic nervous system. Epinephrine and norepinephrine, hormones produced by the adrenal medulla, augment and prolong the response.

First messenger A water-soluble hormone that binds to a receptor on the plasma membrane of a target cell. This binding activates a molecule within the cell, called the second messenger, which influences enzyme activity there.

Fitness The average number of reproductively viable offspring left by an individual. It is sometimes called Darwinian fitness.

Flagellum A whiplike appendage of a cell that moves in an undulating manner. It is composed of an extension of the plasma membrane containing microtubules in a 9 + 2 array. In humans, it is found on sperm cells.

Floating ribs The last two ribs that do not attach directly to the sternum (breastbone).

Fluid mosaic A term used to describe the structure of the plasma membrane. Proteins interspersed throughout the lipid molecules give the membrane its mosaic quality. The ability of some proteins to move sideways gives the membrane its fluid quality.

Follicle A spherical structure in the ovary that contains an oocyte surrounded by one or more layers of follicle cells.

Follicle-stimulating hormone (FSH) A hormone secreted by the anterior pituitary gland that in females stimulates the development of the follicles in the ovaries, resulting in the development of ova (eggs) and the production of estrogen, and in males stimulates sperm production.

Fontanels The "soft spots" in the skull of a newborn. The membranous areas in the skull of a newborn that hold the skull bones together before and shortly after birth. The fontanels are gradually replaced by bone.

Food chain The successive series of organisms through which energy (in the form of food) flows in an ecosystem. Each organism in the series eats or decomposes the preceding one. It begins with the photosynthesizers and flows to herbivores and then to carnivores.

Food defense Precautions designed to prevent the intentional contamination of food.

Food safety Precautions designed to prevent the unintentional contamination of food.

Food web The interconnection of all the feeding relationships (food chains) in an ecosystem.

Foodborne illness An illness that results from ingesting contaminated food or water. Those caused by pathogens (bacteria, viruses, parasites) are considered infections while those caused by chemicals or toxins are considered poisonings. Symptoms first appear in the gastrointestinal tract and include diarrhea and vomiting. Treatment usually involves giving fluids to prevent dehydration; antibiotics may be prescribed if the illness is caused by bacteria. Many go unreported.

Foramen magnum The opening at the base of the skull (in the occipital bone) through which the spinal cord passes.

Foramen ovale In the fetus, a small hole in the wall between the right atrium and left atrium of the heart that allows most blood to bypass the lungs.

Formed elements Cells or cell fragments found in the blood. They include platelets, white blood cells, and red blood cells.

Fossilization The process by which fossils form. Typically an organism dies in water and is buried under accumulating layers of sediments. Hard parts become impregnated with minerals from surrounding water and sediments. Eventually the fossil may be exposed when sediments are uplifted and wind erodes the rock formation.

Fossils The preserved remnants or impressions of past organisms. Most fossils occur in sedimentary rocks.

Founder effect Genetic drift associated with the colonization of a new place by a few individuals so that by chance alone the genetic makeup of the colonizing individuals is not representative of the population they left.

Fovea A small region on the retina that contains a high density of cones but no rods. Objects are focused on the fovea for sharp vision.

Frameshift mutation A mutation that occurs when the number of nucleotides inserted or deleted is not a multiple of three, causing a change in the codon sequence on the mRNA molecule as well as a change in the resulting protein.

Fraternal twins Individuals that develop when two oocytes are released from the ovaries and fertilized by different sperm. Such twins may or may not be the same gender and are as similar genetically as any siblings. They are also called dizygotic twins.

Free radicals Molecular fragments that contain an unpaired electron.

FSH See follicle-stimulating hormone.

Full-term infant Baby born at least 38 weeks after fertilization.


Gallbladder A muscular pear-shaped sac that stores, modifies, and then concentrates bile. Bile is released from the gallbladder into the small intestine.

Gamete A reproductive cell (sperm or egg) that contains only one copy of each chromosome. A sperm and egg fuse at fertilization, producing a zygote.

Gamete intrafallopian transfer (GIFT) A procedure in which eggs and sperm are collected from a couple and then inserted into the woman's oviduct, where fertilization may occur. If fertilization occurs, resulting embryos drift naturally into the uterus.

Ganglion (plural, ganglia) A collection of nerve cell bodies outside the central nervous system.

Gap junction A type of junction between cells that links the cytoplasm of adjacent cells through small holes, allowing physical and electrical continuity between cells.

GAS See general adaptation syndrome.

Gastric glands Any one of several glands in the stomach mucosa that contribute to the gastric juice (hydrochloric acid and pepsin).

Gastric juice The mixture of hydrochloric acid (HCl) and pepsin released into the stomach.

Gastrin A hormone released from the stomach lining in response to the presence of partially digested proteins. Gastrin triggers the production of gastric juice by the stomach.

Gastrointestinal (GI) tract A long tubular system specialized for the processing and absorption of food that begins at the mouth and continues to the esophagus, stomach, intestines, and anus. Several accessory glands empty their secretions into the GI tract to assist digestion.

Gastrula The embryo during gastrulation, when cells move to establish primary germ layers.

Gastrulation Cell movements that establish the primary germ layers of the embryo. The embryo during this period is called a gastrula.

Gated ion channel An ion channel that is opened to allow ions to pass through or closed to prevent passage by changes in the shape of a protein that functions as a gate.

Gene A segment of DNA on a chromosome that directs the synthesis of a specific polypeptide that will play a structural or functional role in the cell. Some genes have regulatory regions of DNA within their boundaries. Also, some genes code for RNA molecules that are needed for the production of the polypeptide but are not part of it.

Gene flow Movement of alleles between populations as a result of the movement of individuals. It is a cause of microevolution.

Gene pool All of the alleles of all of the genes of all individuals in a population.

Gene therapy Treating a genetic disease by inserting healthy functional genes into the body cells that are affected by the faulty gene.

General adaptation syndrome (GAS) A series of physiological adjustments made by our bodies in response to prolonged, extreme stress.

General senses The sensations that arise from receptors in the skin, muscles, joints, bones, and internal organs and include touch, pressure, vibration, temperature, a sense of body and limb position, and pain.

Genetic code The base triplets in DNA that specify the amino acids that go into proteins or that function as start or stop signals in protein synthesis. It is used to convert the linear sequence of bases in DNA to the sequence of amino acids in proteins.

Genetic drift The random change in allele frequencies within a population due to chance alone. It is a cause of microevolution that is usually negligible in large populations.

Genetic engineering The manipulation of genetic material for human practical purposes.

Genital herpes A sexually transmitted disease caused by the herpes simplex virus (HSV) that is usually characterized by painful blisters on the genitals. It is usually caused by HSV-2 but can be caused by HSV-1.

Genital warts Warts that form in the genital area caused by the human papillomaviruses (HPV). These viruses also cause cervical cancer and penile cancer.

Genome The complete set of DNA of an organism, including all of its genes.

Genotype The genetic makeup of an individual. It refers to precise alleles that are present.

Gestational diabetes Diabetes mellitus that develops during pregnancy. It is characterized by insulin resistance and normally resolves after delivery of the baby and placenta.

Gigantism A condition characterized by rapid growth and unusual height caused by abnormally high levels of growth hormone in childhood; also called giantism.

GIFT See gamete intrafallopian transfer.

Gingivitis An inflammation of the gums.

Gland Epithelial tissue that secretes a product.

Glaucoma A condition in which the pressure within the anterior chamber of the eye increases as a result of the buildup of aqueous humor. It can cause blindness.

Glial cells Nonexcitable cells in the nervous system that are specialized to support, protect, and insulate neurons. Also called neuroglial cells or neuroglia.

Global warming A long-term increase in atmospheric temperatures caused by a buildup of carbon dioxide (CO2) and other greenhouse gases in the atmosphere. Greenhouse gases trap heat in the atmosphere.

Glomerular (Bowman's) capsule A cuplike structure surrounding the glomerulus of a nephron.

Glomerular filtration The process by which water and dissolved substances move from the blood in the glomerulus to the inside of Bowman's capsule.

Glomerulus A tuft of capillaries within the renal corpuscle of a nephron.

Glottis The opening to the airways of the respiratory system from the pharynx into the larynx.

Glucagon The hormone secreted by the pancreas that elevates glucose levels in the blood.

Glucocorticoids Hormones secreted by the adrenal cortex that affect glucose homeostasis, thereby influencing metabolism and resistance to stress.

Gluconeogenesis The conversion of noncarbohydrate molecules to glucose.

Glycogen The storage polysaccharide of animals. This complex carbohydrate is stored in the liver and muscles where it serves as a short-term energy source that can be broken down to release energy-packed glucose molecules.

Glycolysis The splitting of glucose, a six-carbon sugar, into two three-carbon molecules called pyruvate. Glycolysis takes place in the cytoplasm of a cell and is the starting point for cellular respiration and fermentation.

GnRH See gonadotropin-releasing hormone.

Goiter, simple An enlarged thyroid gland caused by iodine deficiency.

Golgi complex An organelle consisting of flattened membranous disks that functions in protein processing and packaging.

Golgi tendon organs The highly branched nerve fibers located in the tendons that sense the degree of muscle tension.

Gonad An ovary in a female or a testis in a male. The gonads produce gametes (eggs or sperm) and sex hormones.

Gonadocorticoids The male and female sex hormones, androgens and estrogens, secreted by the adrenal cortex.

Gonadotropin-releasing hormone (GnRH) A hormone produced by the hypothalamus that causes the secretion of luteinizing hormone and follicle-stimulating hormone from the anterior pituitary gland.

Gonorrhea A sexually transmitted disease caused by the bacterium Neisseria gonorrhoeae that commonly causes urethritis and pelvic inflammatory disease. It may not cause symptoms, especially in women.

Graafian follicle A mature follicle in the ovary.

Graded potential A temporary local change in the membrane potential that varies directly with the strength of the stimulus.

Granulocytes White blood cells with granules in their cytoplasm. Examples are neutrophils, eosinophils, and basophils.

Graves' disease An autoimmune disorder caused by oversecretion of thyroid hormone. It is characterized by increased heart and metabolic rates, sweating, nervousness, and exophthalmos.

Gray matter Regions of the central nervous system that contain neuron cell bodies and unmyelinated axons. These regions are gray because they lack myelin. Gray matter is important in neural integration.

Greenhouse effect A process in which greenhouse gases trap heat in the atmosphere. Examples of greenhouse gases include carbon dioxide and methane. The greenhouse effect could lead to a rise in temperatures throughout the world.

Ground substance In connective tissue the ground substance forms the extracellular matrix that the cells are embedded in. It is composed of protein fibers and noncellular material.

Growth factor A type of signaling molecule that stimulates growth by stimulating cell division in target cells.

Growth hormone (GH) An anterior pituitary hormone with the primary function of stimulating growth through increases in protein synthesis, cell size, and rates of cell division. GH stimulates growth in general, especially bone growth.

Growth plate A plate of cartilage that separates each end of a long bone from its shaft that permits bone to grow. Also called an epiphyseal plate.

Gumma A large sore that forms during the third stage of syphilis.


Habitat The natural environment or place where an organism, population, or species lives.

Hair cells A type of mechanoreceptor that generates nerve impulses when bent or tilted. Hair cells in the inner ear are responsible for hearing and equilibrium.

Hair root plexus The nerve endings that surround the hair follicle and are sensitive to touch.

Haploid The condition of having one set of chromosomes, as in eggs and sperm.

HCG See human chorionic gonadotropin.

Heart A muscular pump that keeps blood flowing through an animal's body. The human heart has four chambers: two atria and two ventricles.

Heart attack The death of heart muscle cells caused by an insufficient blood supply; a myocardial infarction.

Heart murmur Heart sounds other than "lub dup" that are created by turbulent blood flow. Heart murmurs can indicate a heart problem, such as a malfunctioning valve.

Heartburn A burning sensation behind the breastbone that occurs when acidic gastric juice backs up into the esophagus.

Heimlich maneuver A procedure intended to force a large burst of air out of the lungs to dislodge material lodged in the trachea.

Helper T cell The kind of T lymphocyte that serves as the main switch for the entire immune response by presenting the antigen to B cells and by secreting chemicals that stimulate other cells of the immune system. It is also known as a T4 cell or a CD4 cell, after the receptors on its surface.

Hematocrit The percentage of red blood cells in blood by volume. It is a measure of the oxygen-carrying ability of the blood.

Hemodialysis The use of artificial devices, such as the artificial kidney machine, to cleanse the blood during kidney failure.

Hemoglobin The oxygen-binding pigment in red blood cells. It consists of four subunits, each made up of an iron-containing heme group and a protein chain.

Hemolytic disease of the newborn A condition in which the red blood cells of an Rh-positive fetus or newborn are destroyed by anti-Rh antibodies previously produced in the bloodstream of an Rh-negative mother.

Hemophilia An inherited blood disorder in which there is insufficient production of blood-clotting factors. Hemophilia results in excessive bleeding in joints, deep tissues, and elsewhere. Hemophilia usually occurs in males.

Hepatitis An inflammation of the liver. Hepatitis is usually caused by one of six viruses.

Herbivore An animal that eats primary producers (green plants or algae). A primary consumer.

Herpes simplex virus See HSV-1 and HSV-2.

Heterotroph An organism that cannot make its own food from inorganic substances and instead consumes other organisms or decaying material.

Heterozygote advantage The phenomenon in which the heterozygous condition is favored over either homozygous condition. It maintains genetic variation within a population in the face of natural selection.

Heterozygous The condition of having two different alleles for a particular gene.

Hinge joint A joint that permits motion in only one plane, such as the knee joint or the elbow.

Hippocampus The part of the limbic system of the brain that plays a role in converting short-term memory into long-term memory.

Histamine A substance released by basophils and mast cells during an inflammatory response that causes blood vessels to widen (dilate) and become more permeable.

Homeostasis The ability of living things to maintain a relatively constant internal environment in all levels of body organization.

Hominid A member of the family Hominidae, which now includes apes and humans. In the past, only humans and their immediate ancestors, such as species within the genus Australopithecus, were placed in Hominidae.

Hominin A member of the subfamily Homininae, which includes the human lineage and its immediate ancestors.

Homologous chromosomes A pair of chromosomes that bear genes for the same traits. One member of each pair came from each parent. Homologous chromosomes are the same size and shape and line up with one another during meiosis I.

Homologous structures Structures that have arisen from a common ancestry. Compare with analogous structure.

Homozygous The condition of having two identical alleles for a particular gene.

Hormonal implants A means of hormonal contraception consisting of silicon rods containing progesterone that are implanted under the skin in the upper arm and prevent pregnancy for up to 5 years.

Hormone A chemical messenger released by cells of the endocrine system that travels through the circulatory system to affect receptive target cells.

Host (vector) DNA The DNA that is recombined with pieces of DNA from another source (that might contain a desirable gene) in the formation of recombinant DNA.

HPV See human papillomavirus.

HSV-1 Herpes simplex virus 1. HSV-1 usually infects the upper half of the body and causes cold sores (fever blisters), but it can cause genital herpes if contact is made with the genital area.

HSV-2 Herpes simplex virus 2. HSV-2 causes genital herpes, but it can cause cold sores if contact is made with the mouth.

Human chorionic gonadotropin (HCG) A hormone produced by the cells of the early embryo (blastocyst) and the placenta that maintains the corpus luteum for approximately the first 3 months of pregnancy. HCG enters the bloodstream of the mother and is excreted in her urine. HCG forms the basis for many pregnancy tests.

Human papillomavirus (HPV) One of the group of viruses that commonly cause genital warts.

Hyaline cartilage A type of cartilage with a gel-like matrix that provides flexibility and support. It is found at the end of long bones as well as parts of the nose, ribs, larynx, and trachea.

Hydrocephalus A condition resulting from the excessive production or inadequate drainage of cerebrospinal fluid.

Hydrogen bond A weak chemical bond formed between a partially positively charged hydrogen atom in a molecule and a partially negatively charged atom in another molecule or in another region of the same molecule.

Hydrolysis The process by which polymers are broken apart by the addition of water.

Hydrophilic Water-loving. The heads of phospholipids (components of the plasma membrane) are hydrophilic.

Hydrophobic Water-fearing. The tails of phospholipids (components of the plasma membrane) are hydrophobic.

Hyperglycemia An elevated blood glucose level.

Hypertension High blood pressure. A high upper (systolic) value usually suggests that the person's arteries have become hardened and are no longer able to dampen the high pressure of each heartbeat. The lower (diastolic) value is generally considered more important because it indicates the pressure when the heart is relaxing.

Hyperthermia Abnormally elevated body temperature.

Hypertonic solution A solution with a higher concentration of solutes than plasma, for example.

Hypodermis The layer of loose connective tissue below the epidermis and dermis that anchors the skin to underlying tissues and organs.

Hypoglycemia Depressed levels of blood glucose often resulting from excess insulin.

Hypophysis See pituitary gland.

Hypothalamus A small brain region located below the thalamus that is essential to maintaining a stable environment within the body. The hypothalamus influences blood pressure, heart rate, digestive activity, breathing rate, and many other vital physiological processes. It acts as the body's "thermostat"; regulates food intake, hunger, and thirst; coordinates the activities of the nervous system and the endocrine system; and is part of the circuitry for emotions.

Hypothermia Abnormally low body temperature.

Hypothesis A testable explanation for a specific set of observations that serves as the basis for experimentation.

Hypotonic solution A solution with a lower concentration of solutes than plasma, for example.


ICSI See intracytoplasmic sperm injection.

Identical twins Individuals that develop from a single fertilized ovum that splits in two at an early stage of cleavage. Such twins have nearly identical genetic material and are always the same gender. They are also called monozygotic twins.

Immigration Movement of new individuals from other populations into an area.

Immune response The body's response to specific targets not recognized as belonging in the body.

Immune system The system of the body directly involved with body defenses against specific targets—pathogens, cells, or chemicals not recognized as belonging in the body.

Immunoglobulin (Ig) Any of the five classes of proteins that constitute the antibodies.

Implantation The process by which a blastocyst (pre-embryo) becomes embedded in the lining of the uterus. It normally occurs high up on the back wall of the uterus.

Impotence The inability to achieve or maintain an erection long enough for sexual intercourse.

In vitro fertilization (IVF) A procedure in which eggs and sperm are placed together in a dish in the laboratory. If fertilization occurs, early-stage embryos are then transferred to the woman's uterus, where it is hoped they will implant and complete development. It is a common treatment for infertility resulting from blocked oviducts.

Incomplete dominance In genetic inheritance, expression of the trait in a heterozygous individual is somewhere in between expression of the trait in a homozygous dominant individual and homozygous recessive individual.

Incomplete proteins Proteins that are deficient in one or more of the essential amino acids. Plant sources of protein are generally incomplete proteins.

Incontinence A condition characterized by the escape of small amounts of urine when sudden increases in abdominal pressure, perhaps caused by laughing, sneezing, or coughing, force urine past the external sphincter. This condition is common in women, particularly after childbirth, an event that may stretch or damage the external sphincter, making it less effective in controlling the flow of urine.

Incus The middle of three small bones in the middle ear that transmit information about sound from the eardrum to the inner ear. The incus is also known as the anvil.

Independent assortment Of chromosomes, the process by which homologous chromosomes and the alleles they carry segregate randomly during meiosis, creating mixes of maternal and paternal chromosomes in gametes. This is an important source of genetic variation in populations.

Independent assortment, law of In genetic inheritance, a principle that states that the alleles of unlinked genes (those that are located on different chromosomes) are randomly distributed to gametes.

Inductive reasoning A logical progression of thought proceeding from the specific to the general. It involves the accumulation of facts through observation until the sheer weight of the evidence forces some general statement about the phenomenon. A conclusion is reached on the basis of a number of observations.

Infancy The stage in postnatal development that roughly corresponds to the first year of life. It is a time of rapid growth when total body length usually increases by one-half and weight may triple.

Infertility The inability to conceive (become pregnant) or to cause conception (in the case of males).

Inflammatory response A nonspecific body response to injury or invasion by foreign organisms. It is characterized by redness, swelling, heat, and pain.

Influenza The flu. Influenza is a viral infection caused by any of the variants of influenza A or influenza B viruses.

Informed consent A document that a person must sign before participating in an experiment that lists all possible harmful consequences that might result from participation.

Inguinal canal A passage through the abdominal wall through which the testes pass in their descent to the scrotum.

Inhalation Breathing in. Inhalation (inspiration) is the movement of air into the respiratory system.

Inhibin A hormone produced in the testes that increases with sperm count and inhibits follicle-stimulating hormone secretion and, therefore, inhibits sperm production.

Inhibiting hormone A hormone that inhibits secretion of another hormone.

Inhibitory synapse A synapse in which the response of the receptors for that neurotransmitter on the postsynaptic membrane decreases the likelihood that an action potential will be generated in the postsynaptic neuron. The postsynaptic cell is inhibited because its resting potential becomes more negative than usual.

Innate defense responses Body defense responses that we are born with—barriers and chemical—to prevent entry of pathogens, and cells and chemicals that attack a pathogen if it breaks through outer barriers. These defense responses are nonspecific.

Inner cell mass A group of cells within the blastocyst that will become the embryo proper and some extraembryonic membranes.

Inner ear A series of passageways in the temporal bone that houses the organs for hearing (cochlea) and the sense of equilibrium (vestibular apparatus).

Insertion The end of the muscle that is attached to the bone that moves when the muscle contracts.

Insoluble fiber A fiber that does not easily dissolve in water. These fibers include cellulose, hemicellulose, and lignin.

Inspiration Inhalation. The movement of air into the respiratory system.

Inspiratory reserve volume The volume of air that can be forcefully brought into the lungs after normal inhalation.

Insulin The hormone secreted by the pancreas that reduces glucose levels in the blood.

Insulin resistance The condition in which the body's cells fail to adequately respond to insulin. It characterizes type 2 diabetes mellitus and gestational diabetes.

Insulin shock A condition that results from severely depressed glucose levels in which brain cells fail to function properly, causing convulsions and unconsciousness. Often the result of a diabetic injecting too much insulin.

Integral proteins Proteins embedded in the plasma membrane, either completely or incompletely spanning the bilayer.

Integumentary system The skin.

Intercalated disks Thickenings of the plasma membranes of cardiac muscle cells that strengthen cardiac tissue and promote rapid conduction of impulses throughout the heart.

Intercostal muscles The layers of muscles between the ribs that raise and lower the rib cage during breathing.

Interferon A type of defensive protein produced by T lymphocytes that slows the spread of viruses already in the body by interfering with viral replication. Interferons also attract macrophages and natural killer cells, which kill the viral-infected cell.

Interleukin 1 A chemical secreted by a macrophage that activates helper T cells in an immune response.

Interleukin 2 A chemical released by a helper T cell that activates both B cells and T cells.

Intermediate filament A component of the cytoskeleton made from fibrous proteins. It maintains cell shape and anchors organelles such as the nucleus.

Internal respiration Movement of oxygen from the blood to the tissues, and movement of carbon dioxide from the tissues to the blood.

Internal urethral sphincter A thickening of smooth muscle at the junction of the bladder and the urethra. The action of this sphincter is involuntary and keeps urine from flowing into the urethra while the bladder is filling.

Interneuron An association neuron. Neurons located within the central nervous system between sensory and motor neurons that serve to integrate information.

Interoceptor A sensory receptor located inside the body, where it monitors conditions. Interoceptors play a vital role in maintaining homeostasis. They are an important part of the feedback loops that regulate blood pressure, blood chemistry, and breathing rate. Interoceptors may also cause us to feel pain, hunger, or thirst, thereby prompting us to take appropriate action.

Interphase The period between cell divisions when the DNA, cytoplasm, and organelles are duplicated and the cell grows in size. Interphase is also the time in the cell's life cycle when the cell carries out its functions in the body.

Interstitial cells Cells located between the seminiferous tubules in the testes that produce the steroid sex hormones, collectively called androgens.

Intervertebral disks Pads of cartilage that help cushion the bones of the vertebral column.

Intracytoplasmic sperm injection (ICSI) A procedure in which a tiny needle is used to inject a single sperm cell into an egg. It is an option for treating infertility when the man has few sperm or sperm that lack the strength or enzymes necessary to penetrate the egg.

Intrauterine device (IUD) A means of contraception consisting of a small plastic device that either is wrapped with copper wire or contains progesterone. It is inserted into the uterus to prevent pregnancy.

Intrinsic factor A protein secreted by the stomach necessary for the absorption of vitamin B12from the small intestine.

Intron A portion of a newly formed mRNA that is cut out of the mature mRNA molecule and is not expressed (used to form a protein). An intervening sequence.

Ion An atom or group of atoms that carries an electric charge resulting from the loss or gain of electrons.

Ion channel A protein-lined pore or channel through a plasma membrane through which one type or a few types of ions can pass. Nerve cell ion channels are important in the generation and propagation of nerve impulses.

Ionic bond A chemical bond that results from the mutual attraction of oppositely charged ions.

Iris The colored portion of the eye. The iris regulates the amount of light that enters the eye.

Iron-deficiency anemia A reduction in the ability of the blood to carry oxygen due to an insufficient amount of iron in the diet, an inability to absorb iron from the digestive system, or blood loss.

Ischemia A temporary reduction in blood supply caused by obstructed blood flow. It causes reversible damage to heart muscle.

Islets of Langerhans See pancreatic islets.

Isotonic solution A solution with the same concentration of solutes as plasma, for example.

Isotopes Atoms that have the same number of protons but different numbers of neutrons.

IUD See intrauterine device.

IVF See in vitro fertilization.


Jaundice A condition in which the skin develops a yellow tone caused by the buildup of bilirubin in the blood and its deposition in certain tissues such as the skin. It is an indication that the liver is not handling bilirubin adequately.

Joint A point of contact between two bones; an articulation.

Junctional complexes Membrane specializations that attach adjacent cells to each other to form a contiguous sheet. There are three kinds of junctions between cells: tight junctions, desmosomes, and gap junctions.

Juxtaglomerular apparatus The region of the kidney nephron where the distal convoluted tubule contacts the afferent arteriole bringing blood into the glomerulus. Cells in this area secrete renin, an enzyme that triggers events eventually leading to increased reabsorption of sodium and water by the distal convoluted tubules and collecting ducts of nephrons.


Kaposi's sarcoma A cancer that forms tumors in connective tissue and manifests as pink or purple marks on the skin. It is common in people with a suppressed immune system, such as people living with HIV/AIDS, and is thought to be associated with a new virus in the herpes family, HHV-8.

Karyotype The arrangement of chromosomes based on physical characteristics such as length and location of the centromere. Karyotypes can be checked for defects in number or structure of chromosomes.

Keratinocytes Cells of the epidermis that undergo keratinization, the process in which keratin gradually replaces the contents of maturing cells.

Ketoacidosis A lowering of blood pH resulting from the accumulation of breakdown by-products of fats. This biochemical imbalance is characteristic of type 1 diabetes mellitus where it is called diabetic ketoacidosis (DKA).

Kidney stones Small, hard crystals formed in the urinary tract when substances such as calcium (the most common constituent), uric acid, or magnesium ammonium phosphate precipitate from urine as a result of higher than normal concentrations. They are also called renal calculi.

Kidneys Reddish, kidney bean-shaped organs responsible for filtering wastes and excess materials from the blood, assisting the respiratory system in the regulation of blood pH, and maintaining fluid balance by regulating the volume and composition of blood and urine.

Klinefelter syndrome A genetic condition resulting from nondisjunction of the sex chromosomes in which a person inherits an extra X chromosome that results in an XXY genotype. The person has a male appearance.

Krebs cycle See citric acid cycle.


Labia majora Two elongated skin folds lateral to the labia minora. They are part of the female external genitalia.

Labia minora Two small skin folds on either side of the vagina and interior to the labia majora. They are part of the external genitalia of a female.

Labor The process by which the fetus is expelled from the uterus and moved through the vagina and into the outside world. During true labor, uterine contractions occur at regular intervals, are often painful, and intensify with walking. Labor is usually divided into the dilation stage, expulsion stage, and placental stage. False labor is characterized by irregular contractions that fail to intensify or change with walking.

Lactation The production and ejection of milk from the mammary glands. The hormone prolactin from the anterior pituitary gland promotes milk production, and the hormone oxytocin released from the posterior pituitary gland makes milk available to the suckling infant by stimulating milk ejection, or let-down.

Lacteal A lymphatic vessel in an intestinal villus that aids in the absorption of lipids.

Lactic acid fermentation The process by which glucose is broken down by muscle cells when oxygen is low during strenuous exercise.

Lacuna (plural, lacunae) A tiny cavity. It contains osteocytes (bone cells) in the matrix of bone and cartilage cells in the matrix of cartilage.

Lanugo Soft, fine hair that covers the fetus beginning about the third or fourth month after conception.

Large intestine The final segment of the gastrointestinal tract, consisting of the cecum, colon, rectum, and anal canal. The large intestine helps absorb water, forms feces, and plays a role in defecation.

Laryngitis An inflammation of the larynx in which the vocal cords become swollen and can no longer vibrate and produce sound.

Larynx The voice box or Adam's apple. A boxlike cartilaginous structure between the pharynx and the trachea held together by muscles and elastic tissue.

Latent period Pertaining to muscle contraction, the interval between the reception of the stimulus and the beginning of muscle contraction.

Latin binomial The two-part scientific name that consists of the genus name followed by the specific epithet.

Lens A transparent, semispherical body of tissue behind the iris and pupil that focuses light on the retina.

Leukemia A cancer of the blood-forming organs that causes white blood cell numbers to increase. The white blood cells are abnormal and do not effectively defend the body against infectious agents.

Leukocytes White blood cells. They are cells of the blood including neutrophils, eosinophils, basophils, monocytes, B lymphocytes, and T lymphocytes. Leukocytes are involved in body defense mechanisms and the removal of wastes, toxins, or damaged, abnormal cells.

LH See luteinizing hormone.

Life expectancy The average number of years a newborn is expected to live.

Ligament A strong band of connective tissue that holds the bones together, supports the joint, and directs the movement of the bones.

Limbic system A collective term for several structures in the brain involved in emotions and memory.

Linkage The tendency for a group of genes located on the same chromosome to be inherited together.

Lipid A compound, such as a triglyceride, phospholipid, or steroid, that does not dissolve in water.

Lipid-soluble hormone A hormone that moves easily through the plasma membrane of cells and combines with receptors inside target cells to activate certain genes and stimulate protein synthesis. Steroid hormones are lipid-soluble hormones. Steroids are derived from cholesterol and secreted by the ovaries, testes, and adrenal glands.

Liver A large organ that functions mainly in the production of plasma proteins, the excretion of bile, the storage of energy reserves, the detoxification of poisons, and the interconversion of nutrients.

Local signaling molecules Chemical messengers, such as neurotransmitters, prostaglandins, growth factors, and nitric oxide, that act locally rather than traveling to distant sites within the body.

Locus The point on a chromosome where a particular gene is found.

Longitudinal fissure A deep groove that separates the cerebrum into two hemispheres.

Long-term memory Memory that stores a large amount of information for hours, days, or years.

Loop of the nephron (Loop of Henle) A section of the renal tubule that resembles a hairpin turn. Reabsorption occurs here.

Loose connective tissue Connective tissue, such as areolar and adipose tissue, that contains many cells in which the fibers of the matrix are fewer in number and more loosely woven than those found in dense connective tissue.

Lower esophageal sphincter A ring of muscle at the juncture of the esophagus and the stomach that controls the flow of materials between the esophagus and the stomach. It relaxes to allow food into the stomach and contracts to prevent too much food from moving back into the esophagus.

Lumen The hollow cavity or channel of a tubule through which transported material flows.

Luteinizing hormone (LH) A hormone secreted by the anterior pituitary gland that in females stimulates ovulation and the formation of the corpus luteum (which produces estrogen and progesterone) and prepares the mammary glands for milk production. In males, it stimulates testosterone production by the interstitial cells within the testes.

Lymph The fluid within the vessels of the lymphatic system. It is derived from the fluid that bathes the cells of the body (interstitial fluid).

Lymph nodes Small nodular organs found along lymph vessels that filter lymph. The lymph nodes contain macrophages and lymphocytes, cells that play an essential role in the body's defense system.

Lymphatic system A body system consisting of lymph, lymphatic vessels, and lymphatic tissue and organs. The lymphatic system helps return interstitial fluid to the blood, transports the products of fat digestion from the digestive system to the blood, and assists in body defenses.

Lymphatic vessels The vessels through which lymph flows. A network of vessels that drains interstitial fluid and returns it to the blood supply and transports the products of fat digestion from the digestive system to the blood supply.

Lymphocyte A type of white blood cell important in nonspecific and specific (immune) body defenses. The lymphocytes include B lymphocytes (B cells) that transform into plasma cells and produce antibodies and T lymphocytes (T cells) that are important in defense against foreign or infected cells.

Lymphoid organs Various organs that belong to the lymphatic system, including the tonsils, spleen, thymus, and Peyer's patches.

Lymphoid tissue The type of tissue that predominates in the lymphoid organs except the thymus. The organs in which lymphoid tissue is found are the lymph nodes, tonsils, and spleen. Lymphoid tissue is an important component of the immune system.

Lymphoma Cancer of the lymphoid tissues. In people with AIDS, it commonly affects the B cells.

Lysosomal storage diseases Disorders such as Tay-Sachs disease caused by the absence of lysosomal enzymes. In these disorders, molecules that would normally be degraded by the missing enzymes accumulate in the lysosomes and interfere with cell functioning.

Lysosome An organelle that serves as the principal site of digestion within the cell.

Lysozyme An enzyme present in tears, saliva, and certain other body fluids that kills bacteria by disrupting their cell walls.


Macroevolution Large-scale evolutionary changes such as those that might result from long-term changes in the climate or position of the continents. Examples include mass extinctions and the evolution of mammals.

Macromolecule A giant molecule of life such as a nucleic acid, protein, or polysaccharide. A macromolecule is formed by the joining together of smaller molecules.

Macrophage A large phagocytic cell derived from a monocyte that lives in loose connective tissue and engulfs anything detected as foreign.

Magnetic resonance imaging (MRI) A means of visualizing a region of the body, including the brain, in which the picture results from differences in the way the hydrogen nuclei in the water molecules within the tissues vibrate in response to a magnetic field created around the area to be pictured.

Malignant tumor A cancerous tumor. An abnormal mass of tissue with cells that can invade surrounding tissue and spread to multiple locations throughout the body.

Malleus The first of three small bones in the middle ear that transmit information about sound from the eardrum to the inner ear. The malleus is also known as the hammer.

Malnourishment A form of hunger that occurs when the diet is not balanced and the right foods are not eaten.

Mammary glands The milk-producing glands in the breasts.

Mass The measure of how much matter is in an object.

Mast cells Small, mobile connective tissue cells often found near blood vessels. In response to injury, mast cells release histamine, which dilates blood vessels and increases blood flow to an area, and heparin, which prevents blood clotting.

Matrix In connective tissue the matrix is the material that the cells are embedded in. The matrix consists of ground substance, which is made up of protein fibers and noncellular material.

Matter Anything that takes up space and has mass. Matter is made up of atoms.

Mechanical digestion A part of the digestive process that involves physically breaking food into smaller pieces.

Mechanoreceptor A sensory receptor that is specialized to respond to distortions in the receptor itself or in nearby cells. Mechanoreceptors are responsible for the sensations we describe as touch, pressure, hearing, and equilibrium.

Medulla oblongata The part of the brain stem containing reflex centers for some of life's most vital physiological functions: the pace of the basic breathing rhythm, the force and rate of heart contraction, and blood pressure. It connects the spinal cord to the rest of the brain.

Medullary cavity The cavity in the shaft of long bones that is filled with yellow marrow.

Medullary rhythmicity center The region of the brain stem controlling the basic rhythm of breathing.

Meiosis A type of cell division that occurs in the gonads and gives rise to gametes. As a result of two divisions (meiosis I and meiosis II), haploid gametes are produced from diploid germ cells.

Meissner's corpuscles Encapsulated nerve cell endings that are common on the hairless, sensitive areas of the skin, such as the lips, nipples, and fingertips, and tell us exactly where we have been touched.

Melanin A pigment produced by the melanocytes of the skin. There are two forms of melanin: black-to-brown and yellow- to-red.

Melanocytes Spider-shaped cells located at the base of the epidermis that manufacture and store melanin, a pigment involved in skin color and absorption of ultraviolet radiation.

Melanoma The least common and most dangerous form of skin cancer. It arises in the melanocytes, the pigment-producing cells of the skin.

Melatonin A hormone secreted by the pineal gland that reduces jet lag and promotes sleep.

Membranous epithelium A type of epithelial tissue that forms linings and coverings. Depending on its location, it may be specialized to either protect, secrete, or absorb.

Memory cell A lymphocyte (B cell or T cell) of the immune system that forms in response to an antigen and that circulates for a long period of time; such cells are able to mount a quick immune response to a subsequent exposure to the same antigen.

Meninges Three protective connective tissue membranes that surround the central nervous system: the dura mater, the pia mater, and the arachnoid.

Meningitis An inflammation of the meninges (protective coverings of the brain and spinal cord).

Menopause The end of a female's reproductive potential when ovulation and menstruation cease.

Menstrual (uterine) cycle The sequence of events that occurs on an approximately 28-day cycle in the uterine lining (endometrium) that involves the thickening of and increased blood supply to the endometrium and the loss of the endometrium as menstrual flow.

Merkel cells Cells of the epidermis found in association with sensory neurons where the epidermis meets the dermis.

Merkel disk The Merkel cell-neuron combination that functions as a sensory receptor for light touch, providing information about objects contacting the skin. It is found on both the hairy and the hairless parts of the skin.

Mesoderm The primary germ layer that gives rise to muscle, bone, connective tissue, and organs such as the heart, kidneys, ovaries, and testes.

Messenger RNA (mRNA) A type of RNA synthesized from and complementary to a region of DNA that attaches to ribosomes in the cytoplasm and specifies the amino acid order in the protein. It carries the DNA instructions for synthesizing a particular protein.

Metabolism The sum of all chemical reactions within living cells.

Metaphase In mitosis, the phase when the chromosomes, guided by the fibers of the mitotic spindle, form a line at the center of the cell. As a result of this alignment, when the chromosomes split at the centromere, each daughter cell receives one chromatid from each chromosome and thus a complete set of the parent cell's chromosomes.

Metastasize To spread from one part of the body to another part not directly connected to the first part. Cancerous tumors metastasize and form new tumors in distant parts of the body.

MHC markers Molecules on the surface of body cells that label the cell as "self."

Microevolution Changes in populations at the genetic level. The causes include genetic drift, gene flow, natural selection, and mutation.

Microfilament A component of the cytoskeleton made from the globular protein actin. Microfilaments form contractile units in muscle cells and aid in pinching dividing cells in two.

Microtubule A component of the cytoskeleton made from the globular protein tubulin. Microtubules are responsible for the movement of cilia and flagella and serve as tracks for the movement of organelles and vesicles.

Microvilli Microscopic cytoplasm-filled extensions of the cell membrane that serve to increase the absorptive surface area of the cell.

Midbrain A region of the brain stem that coordinates reflex responses to auditory and visual stimuli.

Middle ear An air-filled space in the temporal bone that includes the tympanic membrane (eardrum) and three small bones (the malleus, incus, and stapes; sometimes called the hammer, anvil, and stirrup). It serves as an amplifier of sound pressure waves.

Mineralocorticoids Hormones secreted by the adrenal cortex that affect mineral homeostasis and water balance.

Minipill A birth control pill that contains only progesterone.

Mitochondrion (plural, mitochondria) An organelle within which most of cellular respiration occurs in a eukaryotic cell. Cellular respiration is the process by which oxygen and an organic fuel such as glucose are consumed and energy is released and used to form ATP.

Mitosis A type of nuclear division occurring in somatic cells in which two identical cells, called daughter cells, are generated from a single cell. The original cell first replicates its genetic material and then distributes a complete set of genetic information to each of its daughter cells. Mitosis is usually divided into prophase, metaphase, anaphase, and telophase. Mitosis is essential to cell division.

Mitral valve A heart valve located between the left atrium and left ventricle that prevents the backflow of blood from the ventricle to the atrium. It is also called the bicuspid valve or the left atrioventricular (AV) valve.

Molecular clock The idea that there is a constant rate of divergence of macromolecules (such as proteins) from one another. This idea is based on the notion that single nucleotide changes in DNA (point mutations) and the amino acid changes in proteins that can be produced by some point mutations occur with steady, clocklike regularity. It permits comparison of molecular sequences to estimate the time of separation between species (the more differences in sequence, the more time that has elapsed since the common ancestor).

Molecule A chemical structure composed of atoms held together by covalent bonds.

Monoclonal antibodies Defensive proteins specific for a particular antigen secreted by a clone of genetically identical cells descended from a single cell.

Monocyte The largest white blood cell. Monocytes are active in fighting chronic infections, viruses, and intracellular bacterial infections. A monocyte can transform into a phagocytic macrophage.

Monohybrid cross A genetic cross that considers the inheritance of a single trait from individuals differing in the expression of that trait.

Monomer A small molecule that joins with identical molecules to form a polymer.

Mononucleosis A viral disease caused by the Epstein-Barr virus that results in an elevated level of monocytes in the blood. It causes fatigue and swollen glands, and there is no available treatment.

Monosaccharide The smallest molecular unit of a carbohydrate. Monosaccharides are known as simple sugars.

Monosomy A condition in which there is only one representative of a chromosome instead of two representatives.

Mons veneris A round fleshy prominence over the pubic bone in a female. Part of the female external genitalia.

Morning sickness The nausea and vomiting experienced by some women early in pregnancy. It is not restricted to the morning and may be caused, in part, by high levels of the hormone human chorionic gonadotropin (HCG).

Morphogenesis The development of body form that begins during the third week after fertilization.

Morula A solid ball of 12 or more cells produced by successive divisions of the zygote. The name reflects its resemblance to the fruit of the mulberry tree.

Mosaic evolution The phenomenon whereby various traits evolve at their own rates.

Motor (efferent) neuron A neuron specialized to carry information away from the central nervous system to an effector, either a muscle or a gland.

Motor unit A motor neuron and all the muscle fibers (cells) it stimulates.

MRI See magnetic resonance imaging.

mRNA See messenger RNA.

Mucosa The innermost layer of the gastrointestinal tract. It secretes mucus that helps lubricate the tube, allowing food to slide through easily.

Mucous membranes Sheets of epithelial tissue that line many passageways in the body that open to the exterior. Mucous membranes are specialized to secrete and absorb.

Mucus A sticky secretion that serves to lubricate body parts and trap particles of dirt and other secretions. It also helps protect the stomach from the action of gastric juice.

Multiple alleles Three or more alleles of a particular gene existing in a population. The alleles governing the ABO blood types provide an example.

Multiple sclerosis An autoimmune disease in which the body's own defense mechanisms attack myelin sheaths in the nervous system. As a patch of myelin is repaired, a hardened region called a sclerosis forms.

Multipolar neuron A neuron that has at least three processes, including an axon and a minimum of two dendrites. Most motor neurons and association neurons are multipolar.

Multiregional hypothesis The idea that modern humans evolved independently in several different areas from distinctive local populations of Homo erectus. Compare with Out of Africa hypothesis.

Muscle fiber A muscle cell.

Muscle spindles Specialized muscle fibers with sensory nerve cell endings wrapped around them that report to the brain whenever a muscle is stretched.

Muscle tissue Tissue composed of muscle cells that contract when stimulated and passively lengthen to the resting state. There are three types of muscle tissue: skeletal, smooth, and cardiac.

Muscle twitch Contraction of a muscle in response to a single stimulus.

Muscularis The muscular layers of the gastrointestinal tract. These layers help move food through the gastrointestinal system and mix food with digestive secretions.

Mutation A change in the base sequence of the DNA of a gene. A mutation may occur spontaneously or be caused by outside sources, such as radiation or chemical agents.

Myelin sheath An insulating layer around axons that carry nerve impulses over relatively long distances that is composed of multiple wrappings of the plasma membrane of certain glial cells. Outside the brain and spinal cord, Schwann cells form the myelin sheath. The myelin sheath greatly increases the speed at which impulses travel and assists in the repair of damaged axons. The Schwann cells that form the myelin sheath are separated from one another by short regions of exposed axon called nodes of Ranvier.

Myocardial infarction A heart attack. A condition in which a part of the heart muscle dies because of an insufficient blood supply.

Myocardium Cardiac muscle tissue that makes up the bulk of the heart. The contractility of the myocardium is responsible for the heart's pumping action.

Myofibril A rodlike bundle of contractile proteins (myofilaments) found in skeletal and cardiac muscle cells essential to muscle contraction.

Myofilament A contractile protein within muscle cells. There are two types: myosin (thick) filaments and actin (thin) filaments.

Myoglobin An oxygen-binding pigment in muscle fibers.

Myometrium The smooth muscle layer in the wall of the uterus.

Myosin filaments The thick filaments in muscle cells composed of the protein myosin and essential to muscle contraction. A myosin molecule is shaped like a golf club with two heads.

Myosin heads Club-shaped ends of a myosin molecule that bind to actin filaments and can swivel, causing actin filaments to slide past the myosin filaments, which causes muscle contraction. They are also called cross-bridges.

Myxedema A condition characterized by swelling of the facial tissues because of the accumulation of interstitial fluids caused by undersecretion of thyroid hormone.


Nasal cavities Two chambers in the nose, separated by the septum.

Nasal conchae The three convoluted bones within each nasal cavity that increase surface area and direct airflow.

Nasal septum A thin partition of cartilage and bone that divides the inside of the nose into two nasal cavities.

Natural killer (NK) cells A type of cell in the immune system. These cells, probably lymphocytes, roam the body in search of abnormal cells and quickly kill them.

Natural selection The process by which some individuals live longer and produce more offspring than other individuals because their particular inherited characteristics make them better suited to their local environment.

Nearsightedness Myopia. Nearsightedness is a visual condition in which nearby objects can be seen more clearly than distant objects. Nearsightedness occurs because either the eyeball is elongated or the lens is too thick, causing the image to be focused in front of the retina.

Negative feedback mechanism The homeostatic mechanism in which the outcome of a process feeds back on the system, shutting down the process.

Nephrons Functional units of the kidneys responsible for the formation of urine. These microscopic tubules number 1 million to 2 million per kidney and perform filtration (only certain substances are allowed to pass out of the blood and into the nephron), reabsorption (some useful substances are returned from the nephron to the blood), and secretion (the nephron directly removes wastes and excess materials in the blood and adds them to the filtered fluid that becomes urine).

Nerve A bundle of parallel axons, dendrites, or both from many neurons. A nerve is usually covered with tough connective tissue.

Nervous tissue Tissue consisting of two types of cells, neurons and neuroglia, that make up the brain, spinal cord, and nerves.

Neural tube The embryonic structure that gives rise to the brain and spinal cord.

Neuroglia (neuroglial cells) Cells of the nervous system that support, insulate, and protect nerve cells; also called glial cells.

Neuromuscular junction The area of contact between the terminal end of a motor neuron and the sarcolemma of a skeletal muscle fiber. When an action potential reaches the terminal end of the motor neuron, acetylcholine is released, triggering events that can lead to muscle contraction.

Neurons Nerve cells involved in intercellular communication. A neuron consists of a cell body, dendrites, and an axon. Neurons are excitable cells in the nervous system specialized to generate and transmit electrochemical signals called action potentials or nerve impulses.

Neurotransmitter A chemical released from the axon tip of a neuron that affects the activity of another cell (usually a nerve, muscle, or gland cell) by altering the electrical potential difference across the membrane of the receiving cell.

Neurula The embryo during neurulation (formation of the brain and spinal cord from ectoderm).

Neurulation A series of events during embryonic development when the central nervous system (brain and spinal cord) forms from the ectoderm. During this period, the embryo is called a neurula.

Neutrophil A phagocytic white blood cell important in defense against bacteria and removal of cellular debris. Most abundant of white blood cells.

Niche The role of a species in an ecosystem. The niche includes the habitat, food, nest sites, and so on. It describes how a member of a particular species uses materials in the environment and how it interacts with other organisms.

Nitric oxide A local signaling molecule that dilates blood vessels; also functions as a neurotransmitter.

Nitrification The conversion of ammonia to nitrate (NO3-) by nitrifying bacteria living in the soil.

Nitrogen fixation The process of converting nitrogen gas to ammonium (a nitrogen-containing molecule that can be used by living organisms). The process of nitrogen fixation is carried out by nitrogen-fixing bacteria living in nodules on the roots of leguminous plants such as peas and alfalfa.

Node of Ranvier A region of exposed axon between Schwann cells forming a myelin sheath. In myelinated nerves, the impulse jumps from one node of Ranvier to the next, greatly increasing the speed of conduction. This type of transmission is called saltatory conduction.

Nondisjunction Failure of the members of a pair of homologous chromosomes or the sister chromatids to separate during mitosis or meiosis. Nondisjunction results in cells with an abnormal number of chromosomes.

Norepinephrine Noradrenaline. A hormone secreted by the adrenal medulla, along with epinephrine, in response to stress. They initiate the physiological "fight-or-flight" reaction. Norepinephrine is also a neurotransmitter found in both the central and peripheral nervous systems. In the central nervous system, it is important in the regulation of mood, in the pleasure system of the brain, arousal, and dreaming sleep. Norepinephrine is thought to produce an energizing "good" feeling. It is also thought to be essential in hunger, thirst, and sex drive.

Notochord The flexible rod of tissue that develops during gastrulation and signals where the vertebral column will form. The notochord defines the axis of the embryo, gives the embryo some rigidity, and prompts overlying ectoderm to begin formation of the central nervous system. During development, vertebrae form around the notochord. The notochord eventually degenerates, existing only as the pulpy, elastic material in the center of intervertebral disks.

Nuclear envelope The double membrane that surrounds the nucleus.

Nuclear pore An opening in the nuclear envelope that permits communication between the nucleus and the cytoplasm.

Nucleolus A specialized region within the nucleus that forms and disassembles during the course of the cell cycle. It plays a role in the generation of ribosomes, organelles involved in protein synthesis.

Nucleoplasm The chromatin and the aqueous environment within the nucleus.

Nucleotide A subunit of DNA composed of one five-carbon sugar (either ribose or deoxyribose), one phosphate group, and one of five nitrogen-containing bases. Nucleotides are the building blocks of nucleic acids (DNA and RNA).

Nucleus The command center of the cell containing almost all the genetic information.


Oil glands Glands associated with the hair follicles that produce sebum. They are also called sebaceous glands.

Olfactory receptor Sensory receptors that respond to odorous molecules; sensory receptors for the sense of smell.

Oligosaccharide A chain of a few monosaccharides (simple sugars) joined together by dehydration synthesis. Disaccharides, formed by the joining of two monosaccharides, are an example.

Omnivore An organism that feeds on a variety of food types, such as plants and animals.

Oocyte A cell whose meiotic divisions will produce an ovum and up to three polar bodies.

Oogenesis The production of ova (eggs), including meiosis and maturation.

Oogonium (plural, oogonia) A germ cell in an ovary that divides, giving rise to oocytes.

Opsin One of several proteins that can be bound to retinal to form the visual pigments in rods and cones.

Optic nerve One of two nerves, one from each eye, responsible for bringing processed electrochemical messages from the retina to the brain for interpretation.

Organ A structure with a specific function composed of two or more different tissues.

Organ of Corti The spiral organ. The portion of the cochlea in the inner ear that contains receptor cells that sense vibrations caused by sound. It is most directly responsible for the sense of hearing.

Organ system A group of organs with a common function.

Organelle A component within a cell that carries out specific functions. Some organelles, such as the nucleus and mitochondria, have membranes while others, such as ribosomes and microtubules, do not.

Origin In reference to a muscle, the end of the muscle that is attached to the bone that remains relatively stationary during a movement.

Osmosis A special case of diffusion in which water moves across the plasma membrane or any other selectively permeable membrane from a region of lower concentration of solute to a region of higher concentration of solute.

Osteoarthritis An inflammation in a joint that is caused by degeneration of the surfaces of the joint by wear and tear.

Osteoblast A bone-forming cell.

Osteoclast A large cell responsible for the breakdown and absorption of bone.

Osteocytes Mature bone cells found in lacunae that are arranged in concentric rings around the central canal. Cytoplasmic projections from osteocytes extend tiny channels that connect with other osteocytes.

Osteon The structural unit of compact bone that appears as a series of concentric circles of lacunae. The lacunae contain bone cells around a central canal containing blood vessels and nerves.

Osteoporosis A decrease in bone density that occurs when the destruction of bone outpaces the formation of new bone, causing bone to become thin, brittle, and susceptible to fracture.

OT See oxytocin.

Otoliths Granules of calcium carbonate embedded in gelatinous material in the utricle and saccule of the inner ear. Otoliths cause the gelatin to slide over and bend sensory hair cells when the head is moved. The bending generates nerve impulses that are sent to the brain for interpretation as the position of the head.

Out of Africa hypothesis The idea that modern humans evolved from Homo erectus in Africa and later migrated to Europe, Asia, and Australia. It suggests a single origin for Homo sapiens. Compare with multiregional hypothesis.

Outer ear The external appendage on the outside of the head (pinna) and the canal (the external auditory canal) that extends to the eardrum. It functions as the receiver for sound vibrations.

Oval window A membrane-covered opening in the inner ear (cochlea) through which vibrations from the stirrup (stapes) are transmitted to fluid within the cochlea.

Ovarian cycle The sequence of events in the ovary that leads to ovulation. The cycle is approximately 28 days long and is closely coordinated with the menstrual cycle.

Ovary One of the female gonads. The female gonads produce the ova (eggs) and the hormones estrogen and progesterone.

Oviduct One of two tubes that conduct the ova from the ovary to the uterus in the female reproductive system. It is also called a uterine tube or a fallopian tube.

Ovulation The release of the secondary oocyte from the ovary.

Ovum (plural, ova) A mature egg; a large haploid cell that is the female gamete.

Oxygen debt The amount of oxygen required after exercise to oxidize the lactic acid formed during exercise.

Oxyhemoglobin Hemoglobin bound to oxygen.

Oxytocin (OT) The hormone released at the posterior pituitary that stimulates uterine contractions and milk ejection.


Pacemaker See sinoatrial (SA) node.

Pacinian corpuscle A large encapsulated nerve cell ending that is located deep within the skin and near body organs that responds when pressure is first applied. It is important in sensing vibration.

Pain receptor A sensory receptor that is specialized to detect the physical or chemical damage to tissues that we sense as pain. Pain receptors are sometimes classified as chemoreceptors, because they often respond to chemicals liberated by damaged tissue, and occasionally as mechanoreceptors, because they are stimulated by physical changes, such as swelling, in the damaged tissue.

Palate The roof of the mouth. The front region of the palate, the hard palate, is reinforced with bone. The tongue pushes against the hard palate while mixing food with saliva. The soft palate is farther to the back of the mouth and consists of only muscle. The soft palate prevents food from entering the nose during swallowing.

Pancreas An accessory organ behind the stomach that secretes digestive enzymes, bicarbonate ions to neutralize the acid in chyme, and hormones that regulate blood sugar.

Pancreatic islets Small clusters of endocrine cells in the pancreas; also called islets of Langerhans.

Parasympathetic nervous system The branch of the autonomic nervous system that is active during restful conditions. Its effects generally oppose those of the sympathetic nervous system. The parasympathetic nervous system adjusts bodily functions so that energy is conserved during nonstressful times.

Parathormone See parathyroid hormone.

Parathyroid glands Four small, round masses at the back of the thyroid gland that secrete parathyroid hormone (parathormone).

Parathyroid hormone (PTH) A hormone released from the parathyroid glands that increases blood calcium levels by stimulating osteoclasts to break down bone. PTH, also called

parathormone, is secreted when blood calcium levels are too low.

Parental generation The parents—the individuals in the earliest generation under consideration in a genetic cross.

Parkinson's disease A progressive disorder that results from the death of dopamine-producing neurons that lie in the heart of the brain's movement control center, the substantia nigra. Parkinson's disease is characterized by slowed movements, tremors, and muscle rigidity.

Parturition Birth, which usually occurs about 38 weeks after fertilization.

Passive immunity Temporary immune resistance that develops when a person receives antibodies that were produced by another person or animal.

Pathogen A disease-causing organism.

PCR See polymerase chain reaction.

PDD-NOS See pervasive developmental disorder not otherwise specified.

Pectoral girdle The bones that connect the arms to the rib cage. The pectoral girdle is composed of the shoulder blades (scapulae) and the collarbones (clavicles).

Pedigree A diagram showing the genetic connections among individuals in an extended family that is often used to trace the expression of a particular trait in that family.

Pelvic girdle The bones that connect the legs to the vertebral column. The pelvic girdle is composed of the paired hipbones.

Pelvic inflammatory disease (PID) A general term for any bacterial infection of a woman's pelvic organs. PID is usually caused by sexually transmitted bacteria, especially those that cause chlamydia and gonorrhea.

Penis The cylindrical external reproductive organ of a male through which most of the urethra extends and that serves to deliver sperm into the female tract during sexual intercourse.

Pepsin A protein-splitting enzyme initially secreted in the stomach in the inactive form of pepsinogen that is activated into pepsin by hydrochloric acid.

Peptic ulcer A local defect in the surface of the stomach or small intestine characterized by dead tissue and inflammation.

Peptide A chain containing only a few amino acids.

Perforin A type of protein released by a natural killer cell that creates numerous pores (holes) in the target cell, making it leaky. Fluid is then drawn into the leaky cell because of the high salt concentration within, and the cell bursts.

Pericardium The fibrous sac enclosing the heart that holds the heart in the center of the chest without hampering its movements.

Perichondrium The layer of dense connective tissue surrounding cartilage that contains blood vessels, which supply the cartilage with nutrients.

Periodontitis An inflammation of the gums and the tissues around the teeth.

Periosteum The membranous covering that nourishes bone.

Peripheral nervous system The part of the nervous system outside the brain and spinal cord. It keeps the central nervous system in continuous contact with almost every part of the body. It is composed of nerves and ganglia. The two branches are the somatic and the autonomic nervous systems.

Peripheral proteins Proteins attached to the inner or outer surface of the plasma membrane.

Peristalsis Rhythmic waves of muscular contraction and relaxation in the walls of hollow tubular organs, such as the digestive organs, that push contents through the tubes.

Pervasive developmental disorder not otherwise specified (PDD-NOS) A subthreshold condition used to describe individuals with impaired communication and social interaction or who exhibit stereotyped behavior, but who do not meet the full criteria for autism, Asperger syndrome, or certain psychiatric disorders.

PET scan See positron emission tomography.

pH A measure of hydrogen ion concentration of a solution; values range from 0 to 14 on the pH scale.

pH scale A scale for measuring the concentration of hydrogen ions. The scale ranges from 0 to 14, with a pH of 7 being neutral, a pH of less than 7 being acidic, and a pH of greater than 7, basic.

Phagocytes Scavenger cells specialized to engulf and destroy particulate matter, such as pathogens, damaged tissue, or dead cells.

Phagocytosis The process by which cells such as white blood cells ingest foreign cells or substances by surrounding the foreign material with cell membrane. It is a type of endocytosis.

Pharynx The space shared by the respiratory and digestive systems that is commonly called the throat. The pharynx is a passageway for air, food, and liquid.

Phenotype The observable physical and physiological traits of an individual. Phenotype results from the inherited alleles and their interactions with the environment.

Phospholipid An important component of cell membranes. It has a nonpolar "water-fearing" tail (made up of fatty acids) and a polar "water-loving" head (containing an R group, glycerol, and phosphate).

Photoreceptor A sensory receptor specialized to detect changes in light intensity. Photoreceptors are responsible for the sensation we describe as vision.

Phylogenetic trees Generalized descriptions of the history of life. They depict hypotheses about evolutionary relationships among species or higher taxa.

Physical dependence A condition in which continued use of a drug is needed to maintain normal cell functioning.

Pia mater The innermost layer of the meninges (the connective tissue layers that protect the central nervous system).

PID See pelvic inflammatory disease.

Piloerection Contraction of the arrector pili muscles causing hairs to stand on end and form a layer of insulation.

Pineal gland The gland that produces the hormone melatonin and is located at the center of the brain.

Pinna The visible part of the ear on each side of the head that gathers sound and channels it to the external auditory canal.

Pinocytosis A type of endocytosis in which droplets of fluid and the dissolved substances therein are engulfed by cells.

Pituitary dwarfism A condition caused by insufficient growth hormone during childhood.

Pituitary gland The endocrine organ connected to the hypothalamus by a short stalk. It consists of the anterior and posterior lobes and is also called the hypophysis.

Pituitary portal system The system in which a capillary bed in the hypothalamus connects to veins that lead into a capillary bed in the anterior lobe of the pituitary gland. It allows hormones of the hypothalamus to control the secretion of hormones from the anterior pituitary gland.

Placebo In a controlled experiment to test the effectiveness of a drug, it is a substance that appears to be identical to a drug but has no known effect on the condition for which it is taken.

Placenta The organ that delivers oxygen and nutrients to the embryo and later fetus and carries carbon dioxide and wastes away from each. The placenta is also called the afterbirth.

Placenta previa The condition in which the placenta forms in the lower half of the uterus, entirely or partially covering the cervix. It may cause premature birth or maternal hemorrhage and usually makes vaginal delivery impossible.

Placental stage The third (and final) stage of true labor. It begins with delivery of the baby and ends when the placenta detaches from the wall of the uterus and is expelled from the mother's body.

Plaque A bumpy layer consisting of smooth muscle cells filled with lipid material, especially cholesterol, that bulges into the channel of an artery and reduces blood flow. Another type of plaque is a buildup of food material and bacteria on teeth that leads to tooth decay.

Plasma A straw-colored liquid that makes up about 55% of blood. It serves as the medium for transporting materials within the blood. Plasma consists of water (91%-93%) with substances dissolved in it (7%-9%).

Plasma cell The effector cell, produced from a B lymphocyte, that secretes antibodies.

Plasma membrane The thin outer boundary of a cell that controls the movement of substances into and out of the cell.

Plasma proteins Proteins dissolved in plasma, including albumins, which are important in water balance between cells and the blood; globulins, which are important in transporting various substances in the blood; and antibodies, which are important in the immune response.

Plasmid A small, circular piece of self-replicating DNA that is separate from the chromosome and found in bacteria.

Plasmids are often used as vectors in recombinant DNA research.

Plasmin An enzyme that breaks down fibrin and dissolves blood clots. Plasmin is formed from plasminogen.

Plasminogen A plasma protein. It is the inactive precursor of plasmin.

Platelet A cell fragment of a megakaryocyte that releases substances necessary for blood clotting. It is formed in the red bone marrow and is sometimes called a thrombocyte.

Platelet plug A mass of platelets clinging to the protein fiber collagen at a damaged blood vessel to prevent blood loss.

Pleiotropy One gene having many effects.

PMS See premenstrual syndrome.

Point mutation A mutation that involves changes in one or a few nucleotides in DNA.

Polar body Any of three small nonfunctional cells produced during the meiotic divisions of an oocyte. The divisions also produce a mature ovum (egg).

Polygenic inheritance Inheritance in which several independently assorting or loosely linked genes determine the expression of a trait.

Polymer A large molecule formed by the joining together of many smaller molecules of the same general type (monomers).

Polymerase chain reaction (PCR) A technique used to amplify (increase) the quantity of DNA in vitro using primers, DNA polymerase, and nucleotides.

Polypeptide A chain containing 10 or more amino acids.

Polysaccharide A complex carbohydrate formed when large numbers of monosaccharides (most commonly glucose) join together to form a long chain through dehydration synthesis. Most polysaccharides store energy or provide structure.

Polysome A cluster of ribosomes simultaneously translating the same messenger RNA (mRNA) strand.

Pons A part of the brain that connects upper and lower levels of the brain.

Population A group of potentially interacting individuals of the same species living in a distinct geographic area.

Population dynamics Changes in population size over time.

Portal system A system whereby a capillary bed drains to veins that drain to another capillary bed.

Positive feedback mechanism The mechanism by which the outcome of a process feeds back on the system, further stimulating the process.

Positron emission tomography (PET) A method that can be used to measure the activity of various brain regions. The person being scanned is injected with a radioactively labeled nutrient, usually glucose, that is tracked as it flows through the brain. The radioisotope emits positively charged particles, called positrons. When the positrons collide with electrons in the body, gamma rays are released. The gamma rays can be detected and recorded by PET receptors. Computers then use the information to construct a PET scan that shows where the radioisotope is being used in the brain.

Postnatal period The period of development after birth. It includes the stages of infancy, childhood, adolescence, and adulthood.

Postsynaptic neuron The neuron located after the synapse. The receiving neuron in a synapse. The membrane of the postsynaptic neuron has receptors specific for certain neurotransmitters.

Precapillary sphincter A ringlike muscle that acts as a valve that opens and closes a capillary bed. Contraction of the precapillary sphincter squeezes the capillary shut and directs blood through a thoroughfare channel to the venule. Relaxation of the precapillary sphincter allows blood to flow through the capillary bed.

Pre-embryo The developing human from fertilization through the second week of gestation (the pre-embryonic period).

Pre-embryonic period The period during prenatal development that extends from fertilization through the second week. Cleavage and implantation follow fertilization.

Prefrontal cortex An association area of the cerebral cortex that is important in decision making.

Premature infant A baby born before 37 weeks of gestation.

Premenstrual syndrome (PMS) A collection of uncomfortable symptoms, including irritability, stress, and bloating, that appears 7 to 10 days before a woman's menstrual period and is associated with hormonal cycling.

Prenatal period The period of development before birth. It is further subdivided into the pre-embryonic period (from fertilization through the second week), the embryonic period (from the third through the eighth weeks), and the fetal period (from the ninth week until birth).

Presynaptic neuron The neuron located before the synapse. The sending neuron in a synapse. The neuron in a synapse that releases neurotransmitter from its synaptic knobs into the synaptic cleft.

Primary consumer An herbivore. An animal that eats primary producers (green plants or algae).

Primary germ layers The layers produced by gastrulation from which all tissues and organs form. They are ectoderm, mesoderm, and endoderm.

Primary motor area A band of the frontal lobe of the cerebral cortex that initiates messages that direct voluntary movements.

Primary productivity In ecosystems, the gross primary productivity is the amount of light energy that is converted to chemical energy in the bonds of organic molecules during a given period. The net primary productivity is the amount of productivity left after the photosynthesizers have used some of the energy stored in organic molecules for their own metabolic activities.

Primary response The immune response that occurs during the body's first encounter with a particular antigen.

Primary somatosensory area A band of the parietal lobe of the cerebral cortex to which information is sent from receptors in the skin regarding touch, temperature, and pain and from receptors in the joints and skeletal muscles.

Primary spermatocyte The original large cell that develops from a spermatogonium during sperm development in the seminiferous tubules. It undergoes meiosis and gives rise to secondary spermatocytes.

Primary structure The precise sequence of amino acids of a protein. This sequence, determined by the genes, dictates a protein's structure and function.

Primary succession The sequence of changes in the species making up a community over time that begins in an area where no community previously existed and ends with a climax community.

Primordial germ cells Cells that migrate from the yolk sac of the developing human to the ovaries or testes, where they differentiate into immature cells that will eventually become oocytes or sperm.

PRL See prolactin.

Prodrome The symptoms that precede recurring outbreaks of a disease such as genital herpes.

Producers In ecosystems, the producers are the organisms that convert energy from the physical environment into chemical energy in the bonds of organic molecules through photosynthesis or chemosynthesis. The producers form the first trophic level.

Product A material at the end of a chemical reaction.

Progesterone A sex hormone produced by the corpus luteum in the ovary. Progesterone helps prepare the endometrium (lining) of the uterus for pregnancy and maintains the endometrium.

Programmed cell death A genetically programmed series of events that causes a cell to self-destruct. It is also called apoptosis.

Prokaryotic cell A cell that lacks a nucleus and other membrane-enclosed organelles. The prokaryotes include bacteria and archaea.

Prolactin (PRL) An anterior pituitary hormone that stimulates the mammary glands to produce milk.

Promoter A specific region on DNA next to the "start" sequence that controls the expression of the gene.

Prophase In mitosis, the phase when the chromosomes begin to thicken and shorten, the nucleolus disappears, the nuclear envelope begins to break down, and the mitotic spindle forms in the cytoplasm.

Prostaglandins The lipid molecules found in and released by the plasma membranes of most cells. They are often called local hormones (or local signaling molecules) because they exert their effects on the secreting cells themselves or on nearby cells.

Prostate gland An accessory reproductive gland in males that surrounds the urethra as it passes from the bladder. Its secretions contribute to semen and serve to activate the sperm and to counteract the acidity of the female reproductive tract.

Proteins The macromolecules composed of amino acids linked by peptide bonds. The functions of proteins include structural support, transport, movement, and regulation of chemical reactions.

Prothrombin A plasma protein synthesized by the liver that is important in blood clotting. It is converted to an active form (thrombin) by thromboplastin that is released from platelets.

Prothrombin activator A blood protein that converts prothrombin to thrombin as part of the blood-clotting process.

Protozoans A group of single-celled organisms with a well- defined eukaryotic nucleus. Protozoans can cause disease by producing toxins or by releasing enzymes that prevent host cells from functioning normally.

Proximal convoluted tubule The section of the renal tubule where reabsorption and secretion occur.

PTH See parathyroid hormone.

Puberty The time when secondary sexual characteristics such as pubic and underarm hair develop. This period usually occurs slightly earlier in girls (from 12 to 15 years of age) than in boys (from 13 to 16 years of age).

Pulmonary arteries Blood vessels that carry blood low in oxygen from the right ventricle to the lungs, where it is oxygenated.

Pulmonary circuit (or circulation) The pathway that transports blood from the right ventricle of the heart to the lungs and back to the left atrium of the heart.

Pulmonary veins Blood vessels that carry oxygenated blood from the lungs to the left atrium of the heart.

Pulp The center of a tooth that contains the tooth's life- support systems.

Pulse The rhythmic expansion of an artery created by the surge of blood pushed along the artery by each contraction of the ventricles of the heart. With each beat of the heart, the wave of expansion begins, moving along the artery at the rate of 6 to 9 meters per second.

Punnett square A diagrammatic method used to determine the probable outcome of a genetic cross. The possible allele combinations in the gametes of one parent are used to label the columns and the possible allele combinations of the other parent are used to label the rows. The alleles of each column and each row are then paired to determine the possible genotypes of the offspring.

Pupil The small hole through the center of the iris through which light passes to enter the eye. The size of the pupil is altered to regulate the amount of light entering the eye.

Purkinje fibers The specialized cardiac muscle cells that deliver an electrical signal from the atrioventricular bundle to the individual heart muscle cells in the ventricles.

Pyloric sphincter A ring of muscle between the stomach and small intestine that regulates the emptying of the stomach into the small intestine.

Pyramid of biomass A diagram in which blocks represent the amount of biomass (dry body mass of organisms) available at each trophic (feeding) level.

Pyramid of energy A diagram in which blocks represent the decreasing amount of energy available at each trophic (feeding) level.

Pyramid of numbers A diagram of the number of individuals at each trophic level.

Pyrogen A fever-producing substance.

Pyruvate The three-carbon compound produced by glycolysis, which is the first phase of cellular respiration.


Quaternary structure The shape of an aggregate protein. It is determined by the mutually attractive forces between the protein's subunits.


Radioisotopes Isotopes that are unstable and spontaneously decay, emitting radiation in the form of gamma rays and alpha and beta particles.

RAS See reticular activating system.

RDS See respiratory distress syndrome.

Receptor A protein molecule located in the cytoplasm and on the plasma membrane of cells that is sensitive to chemical messengers.

Receptor potential An electrochemical message (a change in the degree of polarization of the membrane) generated in a sensory receptor in response to a stimulus. Receptor potentials vary in magnitude with the strength of the stimulus.

Recessive allele The allele whose effects are masked in the heterozygous condition. The recessive allele often produces a nonfunctional protein.

Recombinant DNA Segments of DNA from two sources that have been combined in vitro and transferred to cells in which their information can be expressed.

Recruitment A process of increasing the strength of muscle contraction by increasing the number of motor units being stimulated.

Rectum The final section of the gastrointestinal tract. The rectum receives and temporarily stores the feces.

Red blood cell See erythrocyte.

Red marrow Blood cell-forming connective tissue found in the marrow cavity of certain bones.

Reduction division The first meiotic division (meiosis I) that produces two cells, each of which contains one member of each homologous pair (23 chromosomes with replicates attached in humans).

Referred pain Pain felt at a site other than the area of origin.

Reflex A simple, stereotyped reaction to a stimulus.

Reflex arc A neural pathway consisting of a sensory receptor, a sensory neuron, usually at least one interneuron, a motor neuron, and an effector.

Refractory period The interval following an action potential during which a neuron cannot be stimulated to generate another action potential.

Relaxin The hormone released from the placenta and the ovaries. It initiates labor and facilitates delivery by dilating the cervix and relaxing the ligaments and cartilage of the pubic bones.

Releasing hormone A hormone that stimulates hormone secretion by another gland.

Renal corpuscle The portion of the nephron where water and small solutes are filtered from the blood. It consists of the glomerulus and Bowman's (glomerular) capsule.

Renal cortex The outer region of the kidney, containing renal columns.

Renal failure A decrease or complete cessation of glomerular filtration.

Renal medulla The inner region of the kidney. It contains cone-shaped structures called renal pyramids.

Renal pelvis The innermost region of the kidney; the chamber within the kidney.

Renal tubule The site of reabsorption and secretion by the nephron. It consists of the proximal convoluted tubule, the loop of the nephron (also called the loop of Henle), and the distal convoluted tubule.

Renin An enzyme released by cells of the juxtaglomerular apparatus of nephrons. Renin converts angiotensinogen, a protein produced by the liver and found in the plasma, into another protein, angiotensin I. These actions of renin initiate a series of hormonal events that leads to increased reabsorption of sodium and water by the distal convoluted tubules and collecting ducts of nephrons.

Rennin The gastric enzyme that breaks down milk proteins.

Replication Copying from a template, as occurs during the synthesis of new DNA from preexisting DNA.

Repolarization The return of the membrane potential to approximately its resting value. Repolarization of the nerve cell membrane during an action potential occurs because of the outflow of potassium ions.

RER See rough endoplasmic reticulum.

Residual volume The amount of air that remains in the lungs after a maximal exhalation.

Resistance phase The phase of the general adaptation syndrome that follows the alarm reaction. It is characterized by anterior pituitary release of adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), and growth hormone (GH), and the sustainment of metabolic demands by the body's fat reserves.

Respiratory distress syndrome (RDS) A condition in newborns caused by an insufficient amount of surfactant, causing the alveoli of the lungs to collapse and thereby making breathing difficult.

Respiratory system The organ system that carries out gas exchange. The respiratory system includes the nose, pharynx, larynx, trachea, bronchi, and lungs.

Resting potential The separation of charge across the plasma membrane of a neuron when the neuron is not transmitting an action potential. It is primarily caused by the unequal distributions of sodium ions, potassium ions, and large negatively charged proteins on either side of the plasma membrane. The resting potential of a neuron is about -70 mV.

Restriction enzyme An enzyme that recognizes a specific sequence of bases in DNA and cuts the DNA at that sequence. Restriction enzymes are used to prepare DNA containing "sticky ends" during the creation of recombinant DNA. Their natural function in bacteria is to control the replication of viruses that infect the bacteria.

Reticular activating system (RAS) An extensive network of neurons that runs through the medulla and projects to the cerebral cortex. It filters out unimportant sensory information before it reaches the brain and controls changing levels of consciousness.

Reticular fibers Interconnected strands of collagen in certain connective tissues that branch extensively. Networks of reticular fibers support soft tissues, including the liver and spleen.

Retina The light-sensitive innermost layer of the eye containing numerous photoreceptors (rods and cones).

Retinal The light-absorbing portion of pigment molecules in the photoreceptors. Retinal combines with one of four opsins (proteins) to form the light-absorbing pigments in rods and cones.

Retrovirus Any one of the viruses that contain only RNA and carry out transcription from RNA to DNA (reverse transcription).

Rh factor A group of antigens found on the surface of the red blood cells of most people. A person who has these antigens is said to be Rh+. A person who lacks these antigens is Rh-.

Rheumatoid arthritis An inflammation of a joint caused by an autoimmune response. It is marked by inflammation of the synovial membrane and excess synovial fluid accumulation in the joints, causing swelling, pain, and stiffness.

Rhodopsin The light-absorbing pigment in the photoreceptors called rods. Rhodopsin is responsible for black and white vision.

Rhythm method of birth control A method of reducing the risk of pregnancy by avoiding intercourse on all days that might result in sperm and egg meeting.

Ribonucleic acid (RNA) A single-stranded nucleic acid that contains ribose (a five-carbon sugar), phosphate, adenine, uracil, cytosine, or guanine. RNA plays a variety of roles in protein synthesis.

Ribosomal RNA (rRNA) A type of RNA that combines with proteins to form the ribosomes, structures on which protein synthesis occurs. The most abundant form of RNA.

Ribosome The site where protein synthesis occurs in a cell. It consists of two subunits, each containing ribosomal RNA and proteins.

RNA (ribonucleic acid) A single-stranded nucleic acid that contains ribose (a five-carbon sugar), phosphate, adenine, uracil, cytosine, or guanine. RNA plays a variety of roles in protein synthesis.

RNA polymerase One of the group of enzymes necessary for the synthesis of RNA from a DNA template. It binds with the promoter on DNA that aligns the appropriate RNA nucleotides and links them together.

Rods Photoreceptors containing rhodopsin and responsible for black and white vision. Rods are extremely sensitive to light.

Root The part of a tooth that is below the gum line. It is covered with a calcified, yet living and sensitive, connective tissue called cementum.

Root canal A channel through the root of a tooth that contains the blood vessels and nerves.

Rough endoplasmic reticulum (RER) Endoplasmic reticulum that is studded with ribosomes. It produces membrane.

Round window A membrane-covered opening in the cochlea that serves to relieve the pressure caused by the movements of the oval window.

rRNA See ribosomal RNA.

Rugae Folds in the mucosa of the lining of the empty stomach's walls that can unfold, allowing the stomach to expand as it fills.


SAD See seasonal affective disorder.

Salinization An accumulation of salts in soil caused by irrigation over a long period of time that makes the land unusable.

Saliva The secretion from the salivary glands that helps moisten and dissolve food particles in the mouth, facilitating taste and digestion. An enzyme in saliva (salivary amylase) begins the chemical digestion of starch.

Salivary amylase An enzyme in saliva that begins the chemical digestion of starches, breaking them into shorter chains of sugars.

Salivary glands Exocrine glands in the facial region that secrete saliva into the mouth to begin the digestion process.

Saltatory conduction The type of nerve transmission along a myelinated axon in which the nerve impulse jumps from one node of Ranvier to the next. Saltatory conduction greatly increases the speed of nerve conduction.

Sarcomere The smallest contractile unit of a striated or cardiac muscle cell.

Sarcoplasmic reticulum An elaborate form of smooth endoplasmic reticulum found in muscle fibers. The sarcoplasmic reticulum takes up, stores, and releases calcium ions as needed in muscle contraction.

Schizophrenia A mental illness characterized by hallucinations and disordered thoughts and emotions that is caused by excessive activity at dopamine synapses in the midbrain. As a result, dopamine is no longer in the proper balance with glutamate, a neurotransmitter released by neurons in the cerebral cortex.

Schwann cell A type of glial cell in the peripheral nervous system that forms the myelin sheath by wrapping around the axon many times. The myelin sheath insulates axons, increases the speed at which impulses are conducted, and assists in the repair of damaged neurons.

Science A systematic approach to acquiring knowledge through carefully documented investigation and experimentation.

Scientific method A procedure underlying most scientific investigations that involves observation, formulating a hypothesis, making predictions, experimenting to test the predictions, and drawing conclusions. Experimentation usually involves a control group and an experimental group that differ in one or very few factors (variables). New hypotheses may be generated from the results of experimentation.

Sclera The white part of the eye that protects and shapes the eyeball and serves as a site of attachment for muscles that move the eye.

SCN See suprachiasmatic nucleus.

Scrotum A loose fleshy sac containing the testes.

Seasonal affective disorder (SAD) A form of depression associated with winter months when overproduction of melatonin is triggered by short day length.

Sebaceous glands See oil glands.

Sebum An oily substance made of fats, cholesterol, proteins, and salts secreted by the oil glands.

Second messenger A molecule in the cytoplasm of a cell that is activated when a water-soluble hormone binds to a receptor on the surface of the cell. Second messengers influence the activity of enzymes and ultimately the activity of the cell to produce the effect of the hormone.

Secondary consumer A carnivore. An animal that obtains energy by eating other animals.

Secondary oocyte A haploid cell formed by meiotic division of a primary oocyte. It is released from an ovary at ovulation.

Secondary response The immune response during the body's second or subsequent exposures to a particular antigen. The secondary immune response is much quicker than is the primary response because memory cells specific for the antigen are present.

Secondary spermatocyte A haploid cell formed by meiotic division of a primary spermatocyte during sperm development in the seminiferous tubules.

Secondary structure The bending and folding of the chain of amino acids of a protein to produce shapes such as coils, spirals, and pleated sheets. These shapes form as a result of hydrogen bonding between different parts of the polypeptide chain.

Secondary succession The sequence of changes in the species making up a community over time that takes place after some disturbance destroys the existing life. Soil is already present.

Secretin A hormone released by the small intestine that inhibits the secretion of gastric juice and stimulates the release of bicarbonate ions from the pancreas and the production of bile in the liver.

Segregation, law of A genetic principle that states that the alleles for each gene separate (segregate) during meiosis and gamete formation, so half of the gametes bear one allele and the other half bear the other allele.

Selectively permeable A characteristic of the plasma membrane because it permits some substances to move across and denies access to others.

Semen The fluid expelled from the penis during male orgasm. Semen consists of sperm and the secretions of the accessory glands.

Semicircular canals Three canals in each ear that are oriented at right angles to one another and contain sensory receptors that precisely monitor any sudden movement of the head. They detect body position and movement.

Semiconservative replication Replication of DNA; the two strands of a DNA molecule become separated and each serves as a template to create a new double-stranded DNA. Each new double-stranded molecule consists of one new strand and one old strand.

Semilunar valves Heart valves located between each ventricle and its connecting artery that prevent the backflow of blood from the artery to the ventricle. Whereas the cusps of the atrioventricular (AV) valves are flaps of connective tissue, those of the semilunar valves are small pockets of tissue attached to the inner wall of their respective arteries.

Seminal vesicles A pair of male accessory reproductive glands located posterior to the urinary bladder. Their secretions contribute to semen and serve to nourish the sperm cells, reduce the acidity in the vagina, and coagulate sperm.

Seminiferous tubules Coiled tubules within the testes where sperm are produced.

Sensory adaptation A gradual decline in the responsiveness of a sensory receptor that results in a decrease in awareness of the stimulus.

Sensory (afferent) neuron A nerve cell specialized to conduct nerve impulses from the sensory receptors toward the central nervous system.

Sensory receptors The structures specialized to respond to changes in their environment (stimuli) by generating electrochemical messages that are eventually converted to nerve impulses if the stimulus is strong enough. The nerve impulses are then conducted to the brain, where they are interpreted to build our perception of the world.

Septum (of heart) A wall that separates the right and left sides of the heart.

SER See smooth endoplasmic reticulum.

Serosa A thin layer of connective tissue that forms the outer layer of the gastrointestinal tract. It secretes a fluid that reduces friction between contacting surfaces.

Serotonin A neurotransmitter in the central nervous system thought to promote a generalized feeling of well-being.

Serous membranes Sheets of epithelial tissue that line the thoracic and abdominal cavities and the organs within them. Serous membranes secrete a fluid that lubricates the organs within these cavities.

Sex chromosomes The X and Y chromosomes. The pair of chromosomes involved in determining gender.

Sex-influenced inheritance An autosomal genetic trait that is expressed differently in males and females, usually because of the presence of sex hormones.

Sex-linked gene A gene located on the X chromosome.

Sexual dimorphism A difference in appearance between males and females within a species.

Short-term memory Memory of new information that lasts for a few seconds or minutes.

Sickle-cell anemia A type of anemia caused by a mutation that results in a change in one amino acid in a globin chain of hemoglobin (the iron-containing protein in red blood cells that transports oxygen). Such a change causes the red blood cell to assume a crescent (sickle) shape when oxygen levels are low. The sickle-shaped cells clog small blood vessels, leading to pain and tissue damage from insufficient oxygen.

Simple diffusion The spontaneous movement of a substance from a region of higher concentration to a region of lower concentration.

Simple epithelial tissue Epithelial tissue with only a single layer of cells.

Simple goiter An enlarged thyroid gland caused by iodine deficiency.

Sinoatrial (SA) node A region of specialized cardiac muscle cells located in the right atrium near the junction of the superior vena cava that sets the pace of the heart rate at about 70 to 80 beats a minute. It is also known as the pacemaker. The SA node sends out an electrical signal that spreads through the muscle cells of the atria, causing them to contract.

Sinuses Large, air-filled spaces in the bones of the face.

Sinusitis Inflammation of the mucous membranes of the sinuses making it difficult for the sinuses to drain their mucous fluid.

Skeletal muscle A contractile tissue. One of three types of muscle in the body. Skeletal muscle cells are cylindrical, have many nuclei, and have stripes (striations). Skeletal muscle provides for conscious, voluntary control over contraction. It attaches to bones and forms the muscles of the body. It is also called striated muscle.

Skeleton A framework of bones and cartilage that functions to support and protect internal organs and to permit body movement.

Sliding filament model A model of the mechanism of muscle contraction in which the myofilaments actin and myosin slide across one another, causing a sarcomere to shorten. When enough sarcomeres shorten, the muscle contracts.

Slow-twitch muscle cells Muscle fibers that are specialized to contract slowly but with incredible endurance when stimulated. They contain an abundant supply of myoglobin and mitochondria and are richly supplied with capillaries. They depend on aerobic pathways to generate adenosine triphosphate (ATP) during muscle contraction.

Small intestine The organ located between the stomach and large intestine responsible for the final digestion and absorption of nutrients.

Smooth endoplasmic reticulum (SER) Endoplasmic reticulum without ribosomes. It produces membrane and detoxifies drugs.

Smooth muscle A contractile tissue characterized by the lack of visible striations and by unconscious control over its contraction. It is found in the walls of blood vessels and airways and in organs such as the stomach, intestines, and bladder.

Sodium-potassium pump A molecular mechanism in a plasma membrane that uses cellular energy in the form of adenosine triphosphate (ATP) to pump ions against their concentration gradients. Typically, each pump ejects three sodium ions from the cell while bringing in two potassium ions.

Soluble fiber A type of dietary fiber that either dissolves or swells in water. This type of fiber includes the pectins, gums, mucilages, and some hemicelluloses. Soluble fiber has a gummy consistency.

Somatic cells All body cells except for gametes (egg and sperm). Somatic cells contain the diploid number of chromosomes, which in humans is 46.

Somatic nervous system The part of the peripheral nervous system that carries information to and from the central nervous system, resulting in voluntary movement and sensations.

Somites Blocks formed from mesoderm cells of the developing embryo that eventually form skeletal muscles of the neck and trunk, connective tissues, and vertebrae.

Source (donor) DNA DNA containing the "gene of interest" that will be combined with host DNA to form recombinant DNA.

Special senses The sensations of smell, taste, vision, hearing, and the sense of balance or equilibrium.

Speciation The formation of a new species.

Species A population or group of populations whose members are capable of successful interbreeding under natural conditions. Such interbreeding must produce fertile offspring.

Sperm A mature male gamete. A spermatozoon.

Spermatid A haploid cell that is formed by mitotic division of a haploid secondary spermatocyte and that develops into a spermatozoon.

Spermatocyte A cell developed from a spermatogonium during sperm development in the seminiferous tubules.

Spermatogenesis The series of events within the seminiferous tubules that gives rise to physically mature sperm from germ cells. It involves meiosis and maturation.

Spermatogonium (plural, spermatogonia) The undifferentiated male germ cells in the seminiferous tubules that give rise to spermatocytes.

Spermicides A means of contraception that consists of spermkilling chemicals in some form of a carrier, such as foam, cream, jelly, film, or tablet.

Sphincter A ring of muscle between regions of a system of tubes that controls the flow of materials from one region to another past the sphincter.

Sphygmomanometer A device for measuring blood pressure. A sphygmomanometer consists of an inflatable cuff that wraps around the upper arm attached to a device that can measure the pressure within the cuff.

Spina bifida A birth defect in which the neural tube fails to develop and close properly. The mother's taking vitamins and folic acid before conception appears to reduce the chance of having a baby with spina bifida. Some cases can be improved with surgery.

Spinal cord A tube of neural tissue that is continuous with the medulla at the base of the brain and extends about 45 cm (17 in.) to just below the last rib. It conducts messages between the brain and the rest of the body and serves as a reflex center.

Spinal nerves Thirty-one pairs of nerves that arise from the spinal cord. Each spinal nerve services a specific region of the body. Spinal nerves carry both sensory and motor information.

Spiral organ (of Corti) The portion of the cochlea in the inner ear that contains receptor cells that sense vibrations caused by sound. It is most directly responsible for the sense of hearing.

Spleen The largest lymphoid organ; it contains a reservoir of lymphocytes and removes old or damaged red blood cells from the blood.

Spongy bone The bone formed from a latticework of thin struts of bone with marrow-filled areas between the struts. It is found in the ends of long bones and within the breastbone, pelvis, and bones of the skull. Spongy bone is less dense than compact bone and is made of an irregular network of collagen fibers surrounded by a calcium matrix.

Sprain Damage to a ligament (a strap of connective tissue that holds bones together).

Squamous cell carcinoma The second most common form of skin cancer that arises in the newly formed skin cells as they flatten and move toward the skin surface.

Squamous epithelium A type of epithelial tissue composed of flattened cells. It forms linings and coverings.

Stapes The last of three small bones in the middle ear that transmit information about sound from the eardrum to the inner ear. The stapes is also known as the stirrup.

Starch The storage polysaccharide in plants.

Statistical significance In a scientific experiment, statistical significance is a measure of the probability that the results are due to chance.

Stem cell A type of cell that divides continuously and can give rise to other types of cells.

Steroid A lipid, such as cholesterol, consisting of four carbon rings with functional groups attached.

Steroid hormones A group of closely related hormones chemically derived from cholesterol and secreted primarily by the ovaries, testes, and adrenal glands.

Stimulus Changes in the internal or external environment that a sensory receptor can detect and respond to by generating electrochemical messages.

Stomach A muscular sac that is well designed for the storage of food, the liquefaction of food, and the initial chemical digestion of proteins.

Stratified epithelial tissue Epithelial tissue with several layers of cells.

Strep throat A sore throat that is caused by Streptococcus bacteria.

Stress incontinence A mild form of incontinence characterized by the escape of small amounts of urine when sudden increases in abdominal pressure force urine past the external urethral sphincter.

Striated muscle See skeletal muscle.

Stroke A cerebrovascular accident. A condition in which nerve cells die because the blood supply to a region of the brain is shut off, usually because of hemorrhage or atherosclerosis. The extent and location of the mental or physical impairment caused by a stroke depend on the region of the brain involved.

Submucosa The layer of the digestive tract between the mucosa and the muscularis; the submucosa contains blood vessels, lymph vessels, and nerves.

Substrate The material on which an enzyme works.

Succession, ecological The sequence of changes in the species making up a community over time.

Summation (of muscle contraction) A phenomenon that results when a muscle is stimulated to contract before it has time to completely relax from a previous contraction. The response to each stimulation builds on the previous one.

Superovulation The ovulation of several oocytes. It is usually prompted by administration of hormones.

Suppressor T cell A type of T lymphocyte that turns off the immune response when the level of antigen falls by releasing chemicals that dampen the activity of both B cells and T cells.

Surface-to-volume ratio The physical relationship that dictates that increases in the volume of a cell occur faster than increases in its surface area. This relationship explains why most cells are small.

Surfactant Phospholipid molecules coating the alveolar surfaces that prevent the alveoli from collapsing.

Suture An immovable joint between the bones of the skull.

Sweat glands Exocrine glands found in the skin. One type of sweat gland is functional throughout life and releases sweat onto the surface of the skin. Another type releases its secretions into hair follicles and becomes functional at puberty.

Sympathetic nervous system The branch of the autonomic nervous system responsible for the "fight-or-flight" responses that occur during stressful or emergency situations. Its effects are generally opposite to those of the parasympathetic nervous system.

Synapse The site of communication between a neuron and another cell, such as another neuron or a muscle cell.

Synapsis The physical association of homologous pairs of chromosomes that occurs during prophase I of meiosis. The term literally means "bringing together."

Synaptic cleft The gap between two cells forming a synapse, for example, two communicating nerve cells.

Synaptic knob A small bulblike swelling of an axon terminal that releases neurotransmitter. An axon terminal.

Synaptic vesicle A tiny membranous sac containing molecules of a neurotransmitter. Synaptic vesicles are located in the synaptic knobs of axon endings, and they release their contents when an action potential reaches the synaptic knob.

Synergistic muscles Two or more muscles that work together to cause movement in the same direction.

Synovial cavity A fluid-filled space surrounding a synovial joint formed by a double-layered capsule. The fluid within the cavity is called synovial fluid.

Synovial fluid A viscous, clear fluid within a synovial cavity that acts as both a shock absorber and a lubricant between the bones.

Synovial joint A freely movable joint. A synovial joint is surrounded by a fluid-filled cavity. This is the most abundant type of joint in the body.

Synovial membranes Membranes that line the cavities of freely movable joints and secrete a fluid that lubricates the joint.

Syphilis A sexually transmitted disease caused by the bacterium Treponema pallidum. If untreated, it can progress through three stages and cause death. The first stage is characterized by a painless craterlike bump called a chancre that forms at the site where the bacterium entered the body. The second stage involves a rash covering the body, and the third stage is characterized by gummas.

Systematic biology The discipline that deals with the naming, classification, and evolutionary relationships of organisms. It is also called systematics.

Systemic circuit (of circulation) The pathway of blood from the left ventricle of the heart to the cells of the body and back to the right atrium.

Systole Contraction of the heart. Atrial systole is contraction of the atria. Ventricular systole is contraction of the ventricles.

Systolic pressure The highest pressure in an artery during each heartbeat. The higher of the two numbers in a blood pressure reading. In a typical, healthy adult, the systolic pressure is about 120 mm Hg.


T cell See T lymphocyte.

T lymphocyte T cell. A type of white blood cell. Some T lymphocytes attack and destroy cells that are not recognized as belonging in the body, such as an infected cell or a cancerous cell.

T4 cell A helper T cell. The kind of T lymphocyte that serves as the main switch for the entire immune response by presenting the antigen to B cells and by secreting chemicals that stimulate other cells of the immune system.

Tanning The buildup of melanin in the skin in response to ultraviolet (UV) exposure.

Target cell A cell with receptors that recognize and bind a specific hormone.

Taste bud A structure consisting of receptors responsible for the sense of taste surrounded by supporting cells. Taste buds are located primarily on the surface epithelium and certain papillae of the tongue.

Taste hairs Microvilli that project into a pore at the tip of the taste bud that bear the receptors for certain chemicals found in food.

Taxon (plural, taxa) A group under consideration, such as a genus, family, or order.

TB See tuberculosis.

Tectorial membrane A membrane that forms the roof of the organ of Corti (the actual organ of hearing). It projects over and is in contact with the sensory hair cells. Pressure waves caused by sound cause the sensory cells to push against the tectorial membrane and bend, resulting in nerve impulses that are carried to the brain by the auditory nerve.

Telomerase The enzyme that synthesizes telomeres.

Telomere Pieces of DNA at the tips of chromosomes that protect the ends of the chromosomes.

Telophase In mitosis, the phase when a nuclear envelope forms around the group of chromosomes at each pole, the mitotic spindle is disassembled, and nucleoli reappear. The chromosomes also become less condensed and more threadlike in appearance.

Temporomandibular joint (TMJ) syndrome A group of symptoms including headaches, toothaches, and earaches caused by physical stress on the mandibular joint.

Tendinitis Inflammation of a tendon caused by excessive stress on the tendon.

Tendon A band of connective tissue that connects muscle to bone.

Tertiary structure The three-dimensional shape of proteins formed by hydrogen, ionic, and covalent bonds between different side chains.

Testes The male gonads. The male reproductive organs that produce sperm and the hormone testosterone.

Testosterone A sex hormone needed for sperm production and the maintenance of male reproductive structures. Testosterone is produced primarily by the interstitial cells of the testes.

Tetanus A smooth, sustained contraction of muscle caused when stimuli are delivered in such rapid succession that there is no time for muscle relaxation.

TH See thyroid hormone.

Thalamus A brain structure located below the cerebral hemispheres that is important in sensory experience, motor activity, stimulation of the cerebral cortex, and memory.

Theory A broad-ranging explanation for some aspect of the universe that is consistent with many observations and experiments.

Thermoreceptor A sensory receptor specialized to detect changes in temperature.

Threshold The degree to which the voltage difference across the plasma membrane of a neuron or other excitable cell must change to trigger an action potential.

Thrombin A plasma protein that is important in blood clotting that is formed from prothrombin by thromboplastin. It converts fibrinogen to fibrin, which forms a web that traps blood cells and forms the clot.

Thrombocyte See platelet.

Thrombus A stationary blood clot that forms in the blood vessels. A thrombus can block blood flow.

Thymopoietin A hormone produced by the thymus gland that promotes the maturation of T lymphocytes.

Thymosin A hormone produced by the thymus gland that promotes the maturation of T lymphocytes.

Thymus gland A gland located on the top of the heart that secretes the hormones thymopoietin and thymosin. It decreases in size as we age.

Thyroid gland The shield-shaped structure at the front of the neck that synthesizes and secretes thyroid hormone and calcitonin.

Thyroid hormone (TH) A hormone released by the thyroid gland that regulates blood pressure and the body's metabolic rate and production of heat. It also promotes normal development of several organ systems.

Thyroid-stimulating hormone (TSH) The anterior pituitary hormone that acts on the thyroid gland to stimulate the synthesis and release of thyroid hormones.

Tidal volume The amount of air inhaled or exhaled during a normal breath.

Tight junction A type of junction between cells in which the membranes of neighboring cells are attached, forming a seal to prevent fluid from flowing across the epithelium through the minute spaces between adjacent cells.

Tissue A group of cells that work together to perform a common function.

Tolerance A progressive decrease in the effectiveness of a drug with continued use.

Tongue The large skeletal muscle studded with taste buds that aids in speech and eating.

Total lung capacity The total volume of air contained in the lungs after the deepest possible breath. It is calculated by adding the residual volume to the vital capacity.

Trabecula (plural, trabeculae) A supporting bar or strand of spongy bone that forms an internal strut that braces the bone from within.

Trachea The tube that conducts air into the thoracic cavity toward the lungs. The trachea is reinforced with C-shaped rings of cartilage to prevent it from collapsing during inhalation and exhalation.

Trait A phenotypically expressed characteristic.

Transcription The process by which a complementary singlestranded messenger RNA (mRNA) molecule is formed from a single-stranded DNA template. As a result, the information in DNA is transferred to RNA.

Transfer RNA (tRNA) A type of RNA that binds to a specific amino acid and transports it to the appropriate region of messenger RNA (mRNA). Transfer RNA acts as an interpreter between the nucleic acid language of mRNA and the amino acid language of proteins.

Transgenic organism An organism that contains certain genes from another species that code for a desired trait. It can be created, for example, by injecting foreign DNA into an egg cell or an early embryo.

Transition reaction The phase of cellular respiration that follows glycolysis and involves pyruvate reacting with coenzyme A (CoA) in the matrix of the mitochondrion to form acetyl CoA. The acetyl CoA then enters the citric acid cycle.

Translation Protein synthesis. The process of converting the nucleotide language of messenger RNA (mRNA) into the amino acid language of a protein.

Transverse tubules T tubules. The tiny, cylindrical inpocketings of the muscle fiber's plasma membrane that carry nerve impulses to almost every sarcomere.

Tricuspid valve A heart valve located between the right atrium and right ventricle that prevents the backflow of blood from the ventricle to the atrium. It is also called the right atrioventricular (AV) valve.

Triglycerides The lipids composed of one molecule of glycerol and three fatty acids. They are known as fats when solid and oils when liquid.

Trisomy A condition in which there are three representatives of a chromosome instead of only two representatives.

tRNA See transfer RNA.

Trophic level The feeding level of one or more populations in a food web. The producers form the first trophic level. Herbivores, which eat the producers, form the second trophic level. Carnivores that eat herbivores form the third trophic level. Carnivores that eat other carnivores form the fourth trophic level.

Trophoblast A group of cells within the blastocyst that gives rise to the chorion, the extraembryonic membrane that will become part of the placenta.

Tropic hormone A hormone that influences another endocrine gland.

Tropomyosin A protein on the thin (actin) filaments in muscle cells that works with troponin to prevent actin and myosin from binding in the absence of calcium ions.

Troponin A protein on the thin (actin) filaments in muscle cells that works with tropomyosin to prevent actin and myosin from binding in the absence of calcium ions.

TSH See thyroid-stimulating hormone.

Tubal ligation A sterilization procedure in females in which each oviduct is cut and sealed to prevent sperm from reaching the eggs.

Tubal pregnancy An ectopic pregnancy in which the embryo implants in an oviduct. This is the most common type of ectopic pregnancy.

Tuberculosis (TB) A highly contagious disease caused by a rod-shaped bacterium, Mycobacterium tuberculosis. TB is spread by coughing.

Tubular reabsorption The process by which useful materials are removed from the filtrate within the renal tubule and returned to the blood.

Tubular secretion The process by which wastes and excess ions that escaped glomerular filtration are removed from the blood and added to the filtrate within the renal tubule.

Tumor A neoplasm. An abnormal growth of cells. A tumor forms from the new growth of tissue in which cell division is uncontrolled and progressive.

Turner syndrome A genetic condition resulting from nondisjunction of the sex chromosomes in which a person has 22 pairs of autosomes and a single, unmatched X chromosome (XO). The person has a female appearance.

Tympanic membrane The eardrum. A membrane that forms the outer boundary of the middle ear and that vibrates in response to sound waves. Vibrations of the tympanic membrane are transferred through the middle ear by three small bones (malleus, incus, and stapes; also known as the hammer, anvil, and stirrup) to the inner ear, where hearing occurs when neural messages are generated in response to the pressure waves caused by the vibrations.

Type 1 diabetes mellitus An autoimmune disorder characterized by abnormally high glucose in the blood due to insufficient production of insulin by cells in the pancreas. It cannot be prevented and there is no cure at this time.

Type 2 diabetes mellitus A condition characterized by increased resistance to insulin by body cells. It can be prevented or delayed through changes in lifestyle.


Umbilical cord The ropelike connection between the embryo (and later the fetus) and the placenta. It consists of blood vessels (two umbilical arteries and one umbilical vein) and supporting connective tissue.

Undernourishment Starvation. A form of hunger that occurs when inadequate amounts of food are eaten.

Ureters Tubular organs that carry urine from the kidneys to the urinary bladder.

Urethra The muscular tube that transports urine from the floor of the urinary bladder to the outside of the body. In males, it also conducts sperm from the vas deferens out of the body through the penis.

Urethritis Inflammation of the urethra caused by bacteria.

Urinalysis An analysis of the volume, microorganism content, and physical and chemical properties of urine.

Urinary bladder The muscular organ that temporarily stores urine until it is released from the body.

Urinary incontinence Lack of voluntary control over urination. Incontinence is the norm for infants and children younger than 2 or 3 years old, because nervous connections to the external urethral sphincter are incompletely developed. In adults, incontinence may result from damage to the external sphincter (often caused, in men, by surgery on the prostate gland), disease of the urinary bladder, and spinal cord injuries that disrupt the pathways along which travel impulses related to conscious control of urination. In any age group, urinary tract infection can result in incontinence.

Urinary retention The failure to completely or normally expel urine from the bladder. This condition may result from lack of the sensation to urinate, as might occur temporarily after general anesthesia, or from contraction or obstruction of the urethra, a condition caused, in men, by enlargement of the prostate gland. Immediate treatment for retention usually involves use of a urinary catheter to drain urine from the bladder.

Urinary system The system that consists of two kidneys, two ureters, one urinary bladder, and one urethra. Its main function is to regulate the volume, pressure, and composition of the blood.

Urinary tract infection (UTI) An infection caused by bacteria in the urinary system. Most bacteria enter the urinary system by moving up the urethra from outside the body.

Urination The process, involving both involuntary and voluntary actions, by which the urinary bladder is emptied. It is also called voiding or micturition.

Urine The yellowish fluid produced by the kidneys. It contains wastes and excess materials removed from the blood. Urine produced by the kidneys travels down the ureters to the urinary bladder, where it is stored until being released from the body through the urethra.

Uterine (menstrual) cycle The sequence of events that occurs on an approximately 28-day cycle in the uterine lining (endometrium) that involves the thickening of and increased blood supply to the endometrium and the loss of the endometrium as menstrual flow.

Uterus A hollow muscular organ in the female reproductive system in which the embryo implants and develops during pregnancy.

UTI See urinary tract infection.


Vaccination A procedure that introduces a harmless form of the disease-causing organism into the body to stimulate immune responses against that antigen.

Vagina A muscular tube in the female reproductive system that extends from the uterus to the vulva and serves to receive the penis during sexual intercourse and as the birth canal.

Vaginitis An inflammation of the vagina.

Variable In a controlled experiment, the factor that differs between the control group and experimental groups.

Varicose veins Veins that have become stretched and distended because blood is prevented from flowing freely and so accumulates, or "pools," in the vein. A common cause of varicose veins is weak valves within the veins.

Vas deferens A tubule that conducts sperm from the epididymis to the urethra.

Vasectomy A sterilization procedure in men in which the vas deferens on each side is cut and sealed to prevent sperm from leaving the man's body.

Vasoactive intestinal peptide (VIP) A hormone released by the small intestine into the bloodstream that triggers the small intestine to release intestinal juices.

Vasoconstriction A decrease in the diameter of blood vessels, commonly of the arterioles. Blood flow through the vessel is reduced, and blood pressure rises as a result of vasoconstriction.

Vasodilation An increase in the diameter of blood vessels, commonly of the arterioles. Blood flow through the vessels increases, and blood pressure decreases as a result of vasodilation.

Vasopressin See antidiuretic hormone.

Vector (disease) An organism that transports a pathogen between hosts.

Vector (DNA) A biological carrier, usually a plasmid or a virus, that ferries the recombinant DNA to the host cell.

Vein A blood vessel formed by the merger of venules that transports blood back toward the heart. Veins have walls that are easily stretched, so they serve as blood reservoirs, holding up to 65% of the body's total blood supply.

Vena cava One of two large veins that empty oxygen-depleted blood from the body to the right atrium of the heart. The superior vena cava delivers blood from regions above the heart. The inferior vena cava delivers blood from regions below the heart.

Ventilation In respiration, breathing.

Ventral nerve root The portion of a spinal nerve that arises from the front (anterior) side of the spinal cord and contains axons of motor neurons. It joins with the dorsal nerve root to form a single spinal nerve, which passes through the opening between the vertebrae.

Ventricle One of the two lower chambers of the heart that receive blood from the atria. The ventricles function as the main pumps of the heart. The right ventricle pumps blood to the lungs. The left ventricle pumps blood to body tissues.

Ventricular fibrillation Rapid, ineffective contractions of the ventricles of the heart, which render the ventricles useless as pumps and stop circulation.

Venule A small blood vessel that receives blood from the capillaries. Venules merge into larger vessels called veins. The exchange of materials between the blood and tissues across the walls of a venule is minimal.

Vertebra One of a series of joined bones that forms the vertebral column.

Vertebral column The "backbone." It is composed of 26 vertebrae (7 cervical, 12 thoracic, 5 lumbar, 1 sacrum, and 1 coccyx) and associated tissues. The spinal cord passes through a central canal within the vertebrae.

Vesicle A membrane-bound sac formed during endocytosis.

Vestibular apparatus A closed fluid-filled maze of chambers and canals within the inner ear that monitors the movement and position of the head and functions in the sense of balance.

Vestibule A space or cavity at the entrance to a canal. In the inner ear, the vestibule is a structure consisting of the utricle and saccule.

Villi (singular, villus) Small fingerlike projections on the small intestine wall that increase surface area for absorption.

VIP See vasoactive intestinal peptide.

Virus A minute infectious agent that consists of a nucleic acid encased in protein. A virus cannot replicate outside a living host cell.

Vital capacity The maximal amount of air that can be moved into and out of the lungs during forceful breathing.

Vitiligo A condition in which melanocytes disappear from areas of the skin, leaving white patches in their wake.

Vitreous humor The jellylike fluid filling the posterior cavity of the eye between the lens and the retina that helps to keep the eyeball from collapsing and holds the thin retina against the wall of the eye.

Vocal cords Folds of tissue in the larynx that vibrate when air passes through them, producing sound.

Vulva External genitalia of a female that surround the opening of the vagina and urethra.


Water footprint A measure of the amount of water used by a person or population for personal use and for the production of and waste removal of products consumed.

Water-soluble hormone A hormone that cannot pass through the plasma membrane on its own, so it influences target cells indirectly, through second messenger systems. Second messenger systems initiate enzyme cascades within the cell that ultimately activate certain enzymes. Water-soluble hormones include protein and peptide hormones, such as those secreted by the pancreas and pituitary gland.

Water-soluble vitamin A vitamin that dissolves in water. Water- soluble vitamins include vitamin C and the various B vitamins.

White blood cells See leukocytes.

White matter Regions of the central nervous system that are white owing to the presence of myelinated nerve fibers. White matter is important in neural communication over distances.


X-linked genes Genes located on the X chromosome. Most X-linked genes have no corresponding allele on the Y chromosome and will be expressed in a male, and in a female if she is homozygous.


Yellow marrow A connective tissue found in the shaft of long bones that stores fat. It forms from red marrow, and, if the need arises, it can convert back to red marrow and form blood cells.

Yolk sac The extraembryonic membrane that is the primary source of nourishment for embryos in many species of vertebrates. In humans, however, the yolk sac does not provide nourishment (human embryos and fetuses receive nutrients from the placenta). In humans, the yolk sac is a site of blood cell formation and contains cells, called primordial germ cells, that migrate to the gonads, where they differentiate into immature cells that will eventually become sperm or oocytes.


Zygote The diploid cell resulting from the joining of an egg nucleus and a sperm nucleus. The first cell of a new individual.

Zygote intrafallopian transfer (ZIFT) A procedure in which zygotes created by the union of egg and sperm in a dish in the laboratory are inserted into the woman's oviducts. Zygotes travel on their own from the oviducts to the uterus.