Organ Systems - Subject Review - SAT Biology E/M Subject Test

SAT Biology E/M Subject Test

Part II: Subject Review

Chapter 11 Organ Systems

The human body is made up of eleven different organ systems, each of which is specialized to carry out particular functions. Two systems, the nervous system and the endocrine system, control all the other organ systems, such as the circulatory system, the respiratory system, and the digestive system. This chapter will explore these systems as well as the other systems that make up the human body.


This chapter covers the basics of the eleven body systems, which are listed below.

• The nervous system detects and interprets information from the surrounding environment. It essentially controls most body functions.

• The endocrine system controls body functions through the use of chemical messengers called hormones.

• The circulatory system transports needed materials to the cells and carries away waste materials.

• The lymphatic system recaptures and filters fluid from the tissues and returns it to the blood stream.

• The respiratory system takes oxygen into the body and releases carbon dioxide.

• The digestive system takes food into the body, breaks it down, and absorbs the nutrients from the food.

• The urinary system removes metabolic wastes from the blood.

• The skeletal system supports the body, protects it, and allows movement (along with the muscular system).

• The muscular system makes it possible for the body to move.

• The skin protects the body and helps regulate body temperature.

• The reproductive system produces the cells necessary to produce offspring.


The nervous system consists of billions of nerve cells. Nerve cells are also called neurons. Neurons are highly specialized cells that carry impulses—electrical signals—between body parts. Here”s a typical neuron:

The cell body (also called the soma) has all the usual cellular material. It has a nucleus, ribosomes, mitochondria, and all the rest of the organelles. Neurons are different from other cells because the cell body has structures sticking off of it in all directions. In the body, anything that sticks off something else is called a process, so we can say that the neuron has processes extending from the cell body. The processes are called dendrites and axons. Most neurons have several dendrites but only one axon. To be ready for the test, you should be able to identify the picture on the previous page as a neuron and be able to label the cell body, nucleus, dendrites, and axon.

What Neurons Do

We already said that neurons are specialized to carry impulses from one place to another. The impulse always follows the same path. A neuron receives impulses at its dendrites. It transmits the impulse through the cell body and down the axon.

The direction in which an impulse travels through a neuron is dendritecell bodyaxon.

What Is This Impulse?

We said that the impulse is an electrical signal. To understand this more completely, we have to take a closer look at the neuron. When a neuron is resting (i.e., not carrying an impulse), we describe it as being polarized. That means it”s different on one side of its membrane than the other. The inside of the neuron is negatively charged when compared to the outside of the neuron.

The Resting Membrane Potential

All cells establish and maintain a resting membrane potential (RMP), wherein the inside of the cell is more negative than the outside. The RMP of most cells is around –70 mV; that is, the inside of the cells is about 70 mV more negative than the outside of the cell. The two membrane proteins that help set up and maintain the RMP are the Na+/K+ ATPase and the K+ leak channel.

How Does the Inside of the Neuron Get More Negative than the Outside?

All cells, including neurons, have protein channels in their membranes (review Chapter 4 if you”ve forgotten this). The protein channels can act in facilitated diffusion to allow molecules across the membrane down their concentration gradients. They can also act as pumps in active transport to move substances across the membrane against their concentration gradients.

There are two particular membrane proteins we”re interested in at the moment. The first is the sodium-potassium pump (Na+/K+ ATPase), which uses a molecule of ATP to move three sodium ions out of the cell and (simultaneously) two potassium ions into the cell. After these pumps run for a while, there is plenty of sodium outside the cell (and not much inside) and plenty of potassium inside the cell (and not much outside). Remember, because these molecules are charged (they”re ions), they cannot simply cross the membrane. So once the sodium is out, it”s out. And once the potassium is in, it”s in. Unless of course, there”s a sodium or potassium channel that can allow them to cross the membrane again, according to their gradients.

There are no sodium channels in the membrane, but there are potassium channels. This is the second protein we”re interested in. These particular potassium channels are referred to as leak channels, because they are always open and will always allow potassium to leak out of the cell, according to its gradient. (Remember, because of the ATPase, there is more potassium inside the cell than outside, so potassium will leak out of the cell.)

The bottom line is that many positively charged ions are being let out of the cell. Sodium ions are being pumped out, and potassium ions are leaking out. Many negatively charged things are left behind, inside the cell. Things like DNA and RNA and proteins. Because a lot of positive stuff is leaving the cell and a lot of negative stuff is staying behind, the cell is more negative on the inside, compared to the outside; 70 millivolts more negative, in fact, so that when we look at a cell we say that it rests at –70 mV.

Quick Quiz #1

Fill in the blanks and check the appropriate boxes:

1. The ______________________________ is a membrane protein that pumps three sodium ions out of the cell and two potassium ions into the cell.

2. Sodium ions [ can cannot ] cross back into the cell after being pumped out.

3. Dendrites [ receive transmit ] an electrical impulse.

4. The direction in which an impulse travels through a neuron is _________________________ to _________________________ to _________________________.

5. Potassium concentration is [ higher lower ] inside the cell than outside.

6. The resting membrane potential of the cell is ___________________.

7. Sodium concentration is [ higher lower ] inside the cell than outside.

8. The axon of a neuron carries the nerve impulse [ toward away from] the cell body.

Correct answers can be found in Chapter 15.

What”s Different About Neurons

Neurons, in addition to the two membrane proteins we discussed above (the Na+/K+ ATPase and the potassium leak channels), have voltage-gated channels in their membranes. Voltage-gated channels are channels that open when the cell membrane reaches a particular voltage. At the normal resting potential of the cell, –70 mV, the voltage-gated channels are closed. But if the cell membrane could reach –50 mV, these channels would open. This potential, the potential at which the voltage-gated channels open, is known as the threshold potential. (Don”t worry just yet about how the cell reaches threshold; we”ll talk about this a bit later.) There are two types of voltage-gated channels in neuron cell membranes: sodium voltage-gated channels and potassium voltage-gated channels.

So What?

Imagine the scene: A barrier separates the inside of the cell from the outside. Sodium ions, plentiful on the outside, long to get in. But they can”t cross the barrier. Potassium ions, plentiful on the inside, long to get out. Some of them do, through the leak channels. But many potassium ions stay behind. All of a sudden, the cell potential reaches –50 mV! Sodium voltage-gated channels slam open, and now sodium has a way to get across the barrier. Sodium ions flood into the cell from the outside! The inside of the cell gets very positive, until finally, at around +35 mV, the sodium channels close. Now the potassium voltage-gated channels open! Potassium has a way to get across the barrier, and potassium ions flood out of the cell, carrying a positive charge out of the cell and making the inside of the cell more negative again. At around –90 mV, the potassium voltage-gated channels close, and the only channels left running are the Na+/K+ ATPase and the potassium leak channels. The pump restores the balance of sodium and potassium, and the cell membrane potential again rests at –70 mV.

The sequence of events we”ve just described is known as an action potential. An action potential occurs at only a small portion of the neuron”s membrane.

Let”s take a quick look at some terms and definitions.

Polarized: the state of the membrane at rest, negative on the inside and positive on the outside.

Depolarization: the membrane potential moves in the positive direction.

Repolarization: the membrane potential returns to its resting value.

So let”s describe the scene above again, this time using the appropriate terminology. This might seem a little overwhelming, so let”s take it a sentence at a time. Picture in your mind what is going on in the neuron.

1. If a neuron is polarized and at the resting potential (–70 mV), and depolarizes slightly to the threshold potential (–50 mV), voltage-gated sodium and voltage-gated potassium channels will open.

2. The voltage-gated sodium channels open first, allowing sodium to enter the cell according to its concentration gradient (previously established by the Na+/K+ ATPase).

3. The entering sodium ions depolarize the cell further, allowing it to reach a maximum of +35 mV before the voltage-gated sodium channels close.

4. Then the voltage-gated potassium channels open, allowing potassium to exit the cell according to its concentration gradient (also previously established by the Na+/K+ ATPase).

5. The exiting potassium ions repolarize the cell, actually bypassing the resting membrane potential, to a minimum of –90 mV before the voltage-gated potassium channels close.

6. Finally, the Na+/K+ ATPase and the potassium leak channels return the membrane to its resting polarized state.

We said earlier that the function of a neuron is to transmit impulses—electrical signals. This “impulse” is nothing more than a traveling action potential. When one small portion of a neuron”s membrane fires an action potential, some of the sodium that rushes in from the opening of the voltage-gated channels travels down the inside of the membrane, bringing the next small portion up to threshold. As soon as threshold is reached, the opening of the voltage-gated channels occurs as we”ve described, and that portion of the membrane has an action potential. Some of the sodium travels down the inside of the membrane, bringing the next small portion up to threshold and causing it to fire an action potential. And so on, and so on, and so on, all the way down the axon.

Very Fast Impulse Speeds: Myelin Sheath and Schwann Cells

In some neurons, the axon is wrapped with special cells called Schwann cells. This Schwann cell “wrapping” is called a myelin sheath. Many Schwann cells can sit on a single axon. The spaces between the Schwann cells are called nodes of Ranvier.

Myelin increases the speed at which an impulse can travel down the axon, because not all portions of the axon have to fire an action potential. The only portions that fire action potentials are the nodes of Ranvier. So the impulse seems to “jump” down the axon from node to node, and this increases the rate at which it reaches the end of the axon. This “jumping” type of conduction is called saltatory conduction, from the Latin word saltar meaning “to jump.” The largest myelinated neurons can conduct impulses at the speed of 100 meters per second (100 m/sec). That”s a little more than the length of a football field in one second—virtually instantaneous.

One last thing: For a very short while after firing an action potential, that portion of the membrane is not able to fire a second action potential (that is, until the sodium and potassium channels reset and the membrane is again at the resting potential). In any case, that short period of time is known as the refractory period. Having a short refractory period in the portion of the membrane that has just fired an action potential ensures that the action potential (the impulse) will only travel in one direction down the axon—away from the cell body.

Quick Quiz #2

Fill in the blanks and check the appropriate boxes:

1. Depolarization results from an [ influx efflux ] of [ sodium potassium ] ions.

2. Rapid, “jumping” conduction is called ________________________.

3. Threshold potential is [ –70 mV –50 mV ].

4. A return to the resting, polarized state is called _________________.

5. The small portion of a neuron”s membrane that is undergoing an action potential is relatively [ positive negative ] on the inside and [ positive negative ] on the outside.

6. The time during which a portion of the membrane is unable to fire an action potential (because of the fact that it has just fired one) is called the ______________________________________.

7. In a myelinated axon, action potentials occur only at the [ Schwann cells nodes of Ranvier ].

8. Repolarization results from an [ influx efflux ] of [ sodium potassium ] ions.

9. Ion channels that open at a particular membrane potential are said to be ______________________________________.

Correct answers can be found in Chapter 15.

What Happens When the Impulse (Action Potential) Reaches the End of the Axon?

When the nerve impulse reaches the end of an axon, it will either get transferred to another neuron”s dendrites, or it will get transferred to an organ (which will exhibit some effect because of being stimulated by the neuron). The point where the impulse gets transferred is called a synapse. A synapse is nothing more than a neuron-to-neuron junction, or a neuron-to-organ junction.

Most synapses in the body are chemical. In other words, they use a special chemical, called a neurotransmitter, to pass the impulse from one neuron to the next. There are many different neurotransmitters in the body. The most common is acetylcholine (ACh). Acetylcholine is the neurotransmitter you should remember for the exam.

So how does it work? How does acetylcholine pass a nerve impulse from one neuron to the next? Or from a neuron to an organ? First, let”s take a close-up look at the synapse itself.

The axon of the first neuron doesn”t actually contact the dendrites of the second neuron. There is a small gap between them called the synaptic cleft. In the terminal end of the axon are vesicles that contain the chemical neurotransmitter. On the dendrites are receptors that can bind to that neurotransmitter.

When an action potential reaches the terminal end of an axon, it causes the vesicles to fuse with the cell membrane. The neurotransmitter is released into the synaptic cleft by exocytosis. It diffuses instantly across the (very small) synaptic cleft where it binds to the receptors on the dendrites of the next neuron.

Usually the receptors on the second neuron are connected to ion channels, which open when the receptors bind the neurotransmitter. Suppose the receptors were connected to sodium channels. What might happen when the neurotransmitter binds?

When the neurotransmitter binds to the receptors, the sodium channels open and sodium rushes into the neuron (remember, sodium concentration is higher outside the cell than inside). This influx of positive ions causes the neuron to depolarize slightly. And if it depolarizes enough to reach the threshold for the voltage-gated channels—BOOM! An action potential fires in the second neuron. Let”s summarize:

1. An action potential travels down the axon of a neuron.

2. When the action potential reaches the terminal end of the axon, it causes vesicles containing a neurotransmitter to fuse with the cell membrane.

3. The neurotransmitter is released into the synaptic cleft by exocytosis.

4. The neurotransmitter diffuses across the cleft where it binds to receptors on the dendrites of the next neuron.

5. Binding of the neurotransmitter to the receptors opens ion channels in the next neuron.

6. If the ion channels allow sodium to enter the neuron, it will depolarize.

7. If the neuron depolarizes to threshold, voltage-gated channels will open, causing an action potential to fire.

Not All Neurotransmitters Are the Same

Not all neurotransmitters cause a cell to be stimulated (depolarize toward threshold). Some cause a cell to be inhibited, in other words, to move away from threshold. And don”t forget, a single neuron may receive impulses from many, many other neurons. Some impulses will cause the neuron to be stimulated, some will cause it to be inhibited. The neuron will take all the stimulatory input and all the inhibitory input and “add them up.” If there are more stimulatory inputs than inhibitory, the neuron will most likely fire an action potential. If there are more inhibitory inputs than stimulatory, the cell will NOT fire an action potential. This is called summation.

Quick Quiz #3

Fill in the blanks and check the appropriate boxes:

1. A neuron whose resting potential is moving away from threshold is said to be [ stimulated inhibited ].

2. The small space between the axon terminus of one neuron and the dendrites of the next neuron is called the ________________________.

3. A synapse can be found between a [ neuron and an organ neuron and a neuron both of these ].

4. The most common neurotransmitter in the body is ______________.

5. A neurotransmitter is released from [ vesicles receptors ] and binds to [ vesicles receptors ].

6. Receptors that open sodium channels would cause the neuron to ______________________.

7. A neuron will fire an action potential only if its membrane potential reaches ______________________________.

Correct answers can be found in Chapter 15.

The Nervous System”s Job As a Whole

So far we”ve only looked at the nervous system at the cellular level—the neuron level. But all of an organism”s neurons are put together into a complicated network. If an organism were a city”s entire electrical system, a neuron would be a single wire. The nervous system would be all of the wires in the whole city—on every street, on every power line and pole, in every wall of every floor of every building and home. In the human nervous system, billions of neurons run every which way, with synapses all over the place, carrying impulses here, there, everywhere.

The brain and the spinal cord are made completely out of neurons. The brain and the spinal cord are referred to as the central nervous system (CNS). Any neurons outside of the brain and spinal cord, like those in our organs and skin, are part of the peripheral nervous system (PNS).

The CNS is like a command station at a military base. Decisions are made here, and information is processed here. The PNS is like a network of phone lines that connect the command station to all other centers on the base. Information from the command station is sent along these phone lines to all other centers so that orders are carried out. New information can be sent to the command station from other centers along these same phone lines. That new information will be processed, decisions will be made, and new orders will be sent along the phone lines.

This, then, is the true function of the nervous system as a whole. It receives information from the body”s sense organs (eyes, ears, etc.). This sensory information is carried by the PNS to the CNS, where it is processed and integrated with other information. The CNS makes some decisions and sends commands out to the body through the PNS. There are three types of neurons involved here:

Here is something to remember about all of the neurons in the nervous system: They all fire the same type of action potential. In other words, all action potentials are exactly alike. There is no such thing as a “big” action potential or a “small” action potential. There is no such thing as a “short” action potential or a “long” action potential. As soon as threshold is reached and the voltage-gated channels open, the action potential occurs automatically, in exactly the same way it did the last time the neuron reached threshold, and in exactly the same way it will the next time the neuron reaches threshold.

However, the sensations picked up by the sensory neurons and sent to the CNS certainly DO differ in strength. We might sense something as a little bit warm or as too hot to touch. The prick of a pin is hardly irritating, but a broken ankle is excruciatingly painful. If all action potentials are the same, how does the CNS “know” when a sensation is strong or weak?

The answer lies in how frequently threshold is reached and, thus, how frequently action potentials are fired. Weak stimuli might cause the neuron to fire two action potentials in a one-second period, while strong stimuli might cause the neuron to fire 20 action potentials in the same one-second period. Very frequent action potentials are interpreted by the CNS as a strong sensation, whereas less frequent action potentials are interpreted as weak sensations.

The Subdivisions of the Nervous System

So far we”ve looked at two of the subdivisions of the nervous system, the CNS and the PNS. Let”s take a closer look at the central nervous system:

Spinal cord: The spinal cord is primarily involved in primitive, reflex actions.

Cerebrum: The cerebral cortex is our conscious mind. This is where voluntary actions occur, such as movement, speech, and problem solving. This is where we have conscious awareness of sensations, such as smells, sights, hot, and cold.

Cerebellum: The cerebellum coordinates muscle movement and balance, so that movement is smooth and coordinated.

Medulla: Involuntary acts originate here, such as breathing and blood pressure regulation. This is a relatively primitive region.

Hypothalamus: The hypothalamus maintains body homeostasis—a constant internal environment regardless of changing external conditions. It monitors things like hormone levels, electrolyte balance, and temperature. It also controls the pituitary gland.

Now let”s take a closer look at the peripheral nervous system (PNS):

The PNS has two subdivisions: the somatic nervous system and the autonomic nervous system. The somatic nervous system is a voluntary system, meaning that we have conscious control over the organs that this subdivision controls. The only organs controlled and monitored by the somatic system are the skeletal muscles. The somatic nervous system uses acetylcholine (ACH) as a neurotransmitter. In other words, to stimulate a skeletal muscle, a somatic motor neuron releases a little ACh onto the muscle. The ACh binds to receptors on the muscle, and this causes the muscle to depolarize and contract.

The autonomic nervous system (ANS), as its name implies, is an autonomous, or involuntary system. We do NOT have conscious control over the organs controlled by this subdivision. Some examples of the organs controlled by the autonomic nervous system are the heart, the digestive organs, the blood vessels, and the pancreas. The autonomic nervous system can be further subdivided into the sympathetic division (which tends to increase body activity) and the parasympathetic division (which tends to decrease body activity).

The sympathetic division is sometimes known as the “fight or flight” system. This division of the ANS helps prepare your body for stress situations by increasing the rate and force of your heartbeat, increasing blood pressure, increasing breath rate, and diverting blood flow away from your digestive organs and toward skeletal muscles. The primary neurotransmitter used by the sympathetic division is norepinephrine.

A Neuro–Endocrine Connection

One of the first things triggered by the sympathetic “fight or flight” system is the release of the hormone epinephrine (adrenaline) from the adrenal medulla. Epinephrine is very much like the neurotransmitter norepinephrine, and causes the same effects in the body. However, because it is a hormone, it is released into the bloodstream, where it is present for a much longer time (minutes) than norepinephrine is at a synapse (milliseconds). Thus, it prolongs and enhances the effects of the sympathetic response, making sure that your body is able to deal with a stressful situation for as long as it takes to resolve it.

The parasympathetic division is sometimes known as the “resting and digesting” system. This division of the ANS is most active when you are at rest. It decreases the rate and force of your heartbeat, decreases blood pressure, decreases breath rate, and diverts blood flow to the digestive organs and away from skeletal muscles. It also stimulates activity in the digestive system, such as movement of food through the stomach and intestines and secretion of digestive enzymes. The primary neurotransmitter used by the parasympathetic division is acetylcholine, just like in the somatic (voluntary skeletal muscle) division.

Not Just for Humans

Much of this discussion about the nervous system relates to humans. The nervous system in other organisms of the vertebrate group, such as fish, amphibians, and birds, is very similar. In all these organisms, the central nervous system is made up of the brain and spinal cord. Nerves transmit impulses to and from the brain and spinal cord and make up the peripheral nervous system.

The nervous systems of arthropods (such as many of the insects we see) and annelids (segmented worms) are made up of a ventral nerve cord and a brain. There are a series of ganglia (clusters of nerve cell bodies) along the nerve cord and neurons branch from the ganglia.

Quick Quiz #4

Fill in the blanks and check the appropriate boxes:

1. The CNS consists of the _______________________ and the ______________________________.

2. Motor neurons are part of the [ CNS PNS ].

3. Interneurons are part of the [ CNS PNS ].

4. The _________________________ maintains body homeostasis.

5. Conscious awareness of ourselves and our surroundings is controlled by the [ cerebrum cerebellum ] of the brain.

6. The [ somatic sympathetic ] division of the PNS controls the skeletal muscles.

7. The primary neurotransmitter used by the parasympathetic division of the ANS is

(A) sympathetic

(B) parasympathetic

(C) somatic

(D) norepinephrine

(E) acetylcholine

8. The [ sympathetic somatic parasympathetic ] division of the PNS is in control of a person watching TV.

9. Which part of the brain smoothes and coordinates body movement?

(A) Medulla

(B) Brain stem

(C) Cerebellum

(D) Cerebrum

(E) Spinal cord

10. Neurons of the PNS are [ entirely separated from connected to ] neurons of the CNS.

11. Conscious thought processes are carried out by the _________________________.

Correct answers can be found in Chapter 15.


We”ve seen how the nervous system helps to control body functions. The nervous system is extremely fast. Action potentials last only about two to three milliseconds, so actions controlled by the nervous system are virtually instantaneous. Consider a pain withdrawal reflex (controlled by the nervous system). If you touch something painful, like a hot stove, your hand immediately pulls back, even before you consciously realize you”ve touched something hot.

The endocrine system is also a control system of the body, but it operates on a much slower time scale than the nervous system.

Endocrine System

Consider some of the things controlled by the endocrine system:

Ejection of breast milk: 1–2 minutes

Regulation of blood glucose: about 15 minutes

Regulation of extracellular sodium: about 1–2 hours

Female reproductive cycle: average of 28 days

Puberty: average of 5 years

How does the endocrine system control the body? Through the use of hormones. Hormones are chemicals made by special glands (called endocrine glands), then secreted (released) into the bloodstream. Once a hormone is in the blood, it goes everywhere in the body; however, it has effects on only some of the organs in the body. What makes some organs respond to a hormone and other organs ignore the same hormone? For a hormone to have an effect on an organ, that organ must have receptors for the hormone. No receptors, no effect. The organs that are affected by a particular hormone are called target organs for that hormone.

Peptides and Steroids

Hormones come in two classes: peptide hormones (which are amino acid-based) and steroid hormones (which are cholesterol-based). These general classes of hormones act in slightly different ways. Let”s take a look.

Peptide hormones are made from amino acids. They are essentially protein molecules, but some are very small and are referred to as peptides. Because they cannot cross cell membranes, peptide hormones must bind to receptors outside the cell (on the extracellular surface). Peptide hormones generally cause their effects rapidly. They do this by turning existing enzymes in the cell on or off. Some examples of peptide hormones are insulin, prolactin, and glucagon.

Steroid hormones are made from cholesterol. They are lipids and can easily cross the cell membrane, so they bind to receptors inside the cell (intracellular). Steroid hormones generally cause their effects more slowly than peptide hormones. They cause their effects by binding to DNA and changing which genes get transcribed. Some examples of steroids are aldosterone, estrogen, and testosterone.

Quick Quiz #5

Fill in the blanks and check the appropriate boxes:

1. Peptide hormones have receptors [ outside inside ] the cell, and steroids have receptors [ outside inside ] the cell.

2. The organs that are affected by a particular hormone are referred to as that hormone”s _____________________________.

3. The endocrine system is [ faster slower ] than the nervous system.

4. Peptide hormones cause their effects [ more less ] rapidly than steroid hormones.

5. Steroid hormones cause their effects by _______________________.

6. Steroid hormones are derived from [ cholesterol amino acids ].

Correct answers can be found in Chapter 15.

The Endocrine Glands

The endocrine glands (glands that produce hormones) are scattered throughout the body. Hormones are then circulated through the body by blood.

Male and Female Organs

The Pituitary Gland—the “Master” Endocrine Organ

The pituitary gland is sometimes referred to as the “master” endocrine organ because its hormones control many of the other endocrine glands in the body. But the pituitary gland itself is controlled by the hypothalamus of the brain, so it really isn”t the “master” after all. The pituitary gland has two lobes: the anterior lobe and the posterior lobe.

The anterior pituitary gland makes and secretes six different hormones:

1. Growth hormone (GH): This hormone targets all tissues and organs in the body and causes them to grow. It is especially important in childhood and adolescence, but in adults it can stimulate the rate at which older cells are replaced with newer cells (called cell-turnover rate).

2. Thyroid stimulating hormone (TSH): This hormone does exactly what its name implies. It stimulates the thyroid gland to secrete thyroid hormones.

3. Adrenocorticotropic hormone (ACTH): This hormone stimulates the adrenal cortex (the outer layer of the adrenal gland) to secrete its hormones.

4. Follicle stimulating hormone (FSH): This hormone”s target organs are the gonads (the male and female reproductive organs). In the female it stimulates the ovaries, causing maturation of ova and the release of estrogen. In the male it stimulates the testes to make sperm.

5. Luteinizing hormone (LH): This hormone also targets the gonads. In the female it stimulates the ovaries, causing development of a corpus luteum (we”ll talk more about this later). In the male it stimulates the testes to make testosterone.

6. Prolactin: This hormone is released only after childbirth. It stimulates the mammary glands to make breast milk.

The release of these hormones is controlled by special releasing hormones from the hypothalamus. For example, to cause the anterior pituitary to secrete prolactin, the hypothalamus releases prolactin-releasing hormone (prolactin-RH). To cause the anterior pituitary to release thyroid-stimulating hormone (TSH), the hypothalamus releases TSH-releasing hormone (TSH-RH). For every hormone released by the anterior pituitary there is a corresponding releasing hormone from the hypothalamus.

The posterior pituitary gland stores and secretes two hormones:

1. Oxytocin: This hormone causes the uterus to contract during childbirth and also causes the mammary glands to release milk during breastfeeding.

2. Antidiuretic hormone (ADH): This hormone causes the kidneys to retain water. It is also known as vasopressin.

An Annoying Fact
About the Posterior
Pituitary Hormones

ADH and oxytocin aren”t
made in the posterior
pituitary. They”re
actually made by the
hypothalamus, and
then transported to and
stored in the posterior
pituitary. Release of the
hormones is by an action
potential from the

The Thyroid Gland

The thyroid gland is located in the anterior part of the neck. It secretes two hormones: thyroid hormone, also known as thyroxine, and calcitonin. Thyroxine affects most of the body”s cells. It makes them increase their rate of metabolism, meaning that they work harder and use more energy. If you think of the body as a car, thyroxine steps on the gas.

Thyroxine contains iodine. If you don”t eat enough iodine, you can”t make enough thyroxine. If that happens, you develop hypothyroidism (hypo means “lower than,” so hypothyroidism means lower than normal levels of thyroid hormone). A person with hypothyroidism has a low metabolic rate; they can gain weight and become sluggish. The opposite of hypothyroidism is hyperthyroidism—an overproduction of thyroxine. This produces a higher than normal metabolic rate, accompanied by symptoms such as weight loss and a fast heart rate.

Calcitonin activates special cells in bone that remove calcium from the blood and use it to build new bone. The overall effect is to reduce blood calcium levels (calcitonin “tones down” blood calcium).

The Parathyroid Glands

There are four parathyroid glands. They are very small and are found on the back of the thyroid gland. The parathyroid glands secrete parathyroid hormone (also called parathormone). Parathyroid hormone functions as the opposite of calcitonin; it activates special cells in bone that dissolve the bone to release calcium into the blood, so the overall effect of parathyroid hormone is to increase the amount of calcium in the blood. Calcium is used in many different situations, ranging from nerve impulse conduction to heart contraction to blood clotting, so it is very important to maintain its concentration to be relatively constant.

The Adrenal Glands

The adrenal glands sit on top of the kidneys. Even though they are located on top of the kidney, they are not part of the kidney. The adrenal glands have two parts: the adrenal medulla (the inner part) and the adrenal cortex (the outer part).

The adrenal medulla secretes two hormones: epinephrine (also known as adrenaline) and norepinephrine (also known as noradrenaline). Most of its output (about 80%) is epinephrine. These two hormones have very similar chemical structures, which means that they can bind to the same receptors and have the same effects on organs.

Remember when we talked about the sympathetic division of the autonomic nervous system? We referred to it as the “fight or flight” system, because it increases body activity, especially the heart rate, in preparation for stressful activity. Remember also that the sympathetic neurons use norepinephrine as a neurotransmitter? In other words, to affect the organs it innervates, the sympathetic system releases a very small amount of norepinephrine onto the organ at a synapse. The norepinephrine binds to receptors on the organ, and this causes the effects of the sympathetic division.

The first thing that gets stimulated by the sympathetic neurons in a stress situation is the adrenal medulla. The effect is to release a lot of epinephrine (and some norepinephrine) into the blood. These hormones can bind to the same receptors on organs that norepinephrine from the sympathetic neurons does. The only difference is that norepinephrine from a sympathetic neuron is present only for a few milliseconds at a synapse, whereas norepinephrine and epinephrine from the adrenal medulla are present in the blood for a LOT longer: at the very least a few minutes and up to a few hours depending on the situation and the stress level. Otherwise, the effect on the organs is the same.

So … bottom line:

The effect of epinephrine and norepinephrine from the adrenal medulla is to increase and prolong the effects of the sympathetic nervous system.

Imagine you are walking along the side of a road when all of a sudden a car swerves out and almost hits you. You jump quickly out of the way and are safe, but a few minutes after the event your heart rate is still elevated and you still feel “shaky.” The stimulation from the sympathetic system came and went in about three milliseconds, but the effect of the epinephrine from the adrenal medulla is prolonged. This is what keeps your heart rate up and keeps your body “on alert” even though the immediate danger has passed. It takes a while for the epinephrine to get cleared out of the blood.

Glands: A Quick Review

Let”s quickly review the major glands in the body and their functions:



anterior pituitary gland

makes and secretes growth hormone, thyroid stimulating hormone, adrenocorticotropic hormone, follicle stimulating hormone, luteinizing hormone, prolactin

posterior pituitary gland

stores and secretes oxytocin, antidiuretic hormone

thyroid gland

secretes thyroxine and calcitonin

parathyroid glands

secrete parathyroid hormone (parathormone)

adrenal medulla

secretes epinephrine and norepinephrine

adrenal cortex

secretes steroids


secretes hormones and digestive enzymes

gonads (primary sex organs)

testes—produce male steroids called androgens ovaries—produce female steroids called estrogens and progesterone

The adrenal cortex secretes many different hormones. All are steroids, and they come in three main classes: glucocorticoids, mineralocorticoids, and sex steroids.

One of the targets for the glucocorticoids is the liver. The glucocorticoids cause the liver to produce glucose from fats and proteins and to release that “new” glucose into the blood. This is called gluconeogenesis (gluco = glucose, neo = new, genesis = formation). Glucocorticoids also target other body cells to use fats for fuel instead of glucose, causing an increase in blood glucose levels and increased body metabolism. These hormones are also strong anti-inflammatory agents. The primary glucocorticoid is cortisol. (You”ve seen this substance in over-the-counter creams available at drug stores to reduce inflammation and itching associated with some skin conditions.)

The targets for the mineralocorticoids are the kidneys. The primary mineralocorticoid is aldosterone, and aldosterone causes the kidney to retain sodium—in other words, to remove sodium from the urine and return it to the body. When the sodium is returned, water comes with it, so the effect on the body is to retain both sodium AND water. This increases blood volume and blood pressure.

The sex steroids from the adrenal cortex (testosterone, etc.) are of little consequence because the primary source of these hormones are the gonads (ovaries and testes) and not the adrenal cortex.

The Pancreas

The pancreas has multiple functions; not only does it secrete hormones, it also secretes many digestive enzymes. Secreting enzymes is an exocrine role, which we”ll discuss later on. Let”s talk now about the pancreas” endocrine role—secreting insulin and glucagon.

Insulin and glucagon are produced by special cells in the pancreas called Islets of Langerhans, or simply islet cells. Insulin is secreted whenever blood glucose levels are high, such as after a meal. Insulin affects pretty much all the cells in the body; it allows them to take glucose out of the blood so they can use it in cellular respiration (glycolysis, PDC, Krebs cycle, and electron transport) to produce energy. Insulin also stimulates the liver to store glucose as glycogen. Consequently, blood glucose levels go down.

Glucagon has the opposite effect on blood glucose. Glucagon is released whenever blood glucose is low, such as between meals, when you haven”t eaten in a while. The target organ for glucagon is the liver; it causes the liver to break down glycogen (stored glucose) and to release free glucose into the blood. This is called glycogenolysis (lysis = to break). The liver is the only organ that stores glycogen for this purpose. Consequently, blood glucose levels go up. Note that this is different from the effect that cortisol has on the liver. Both cortisol and glucagon cause the liver to release glucose, but cortisol stimulates the production of “new” glucose from fats and proteins (gluconeogenesis), whereas glucagon stimulates the breakdown of glycogen into free glucose (glycogenolysis).

The Gonads

The gonads are the male and female primary sex organs. The primary sex organs are those that produce the gametes (the sex cells—sperm and ova). The male primary sex organ—the male gonad—is the testis or testicle (plural = testes). The female primary sex organ—the female gonad—is the ovary (plural = ovaries).

The gonads, in addition to producing gametes, secrete hormones. All the hormones produced and secreted by the gonads are steroids. Because these hormones are involved in maintaining sexual characteristics, they are collectively referred to as the sex steroids.

The testes, as we”ve already seen in Chapter 7, produce sperm. They also produce male sex steroids, called androgens. The primary androgen—the primary male sex steroid—is testosterone. Testosterone has many targets in the body. It is responsible for developing the male secondary sex characteristics during puberty and for maintaining them during adulthood. Male secondary sex characteristics include broader shoulders and narrower hips, deeper voice, facial hair, chest hair, axillary hair, pubic hair, and enlarged external genitalia. Testosterone is also necessary for normal and adequate sperm production.

The ovaries, again as we”ve already seen in Chapter 7, produce ova. They also produce two types of female sex steroids, the estrogens and progesterone. There are several forms of estrogens; the most common is estradiol. Estradiol frequently is simply referred to as “estrogen.” Estrogen and progesterone have multiple targets in the female body, just as testosterone does in the male body. They are responsible for developing female secondary sex characteristics during puberty and for maintaining them during adulthood. Female secondary sex characteristics include a narrower waist and wider hips, breasts and mammary glands, axillary and pubic hair, softer skin, and a higher-pitched voice.

Estrogen and progesterone also regulate the menstrual cycle. Estrogen stimulates the growth of the uterine lining in the first half of the cycle, and progesterone enhances and maintains the lining in the second half of the cycle. We”ll take a closer look at the menstrual cycle later on.


That”s a lot of information. For the SAT Biology E/M Subject Test, you need to know which hormones are secreted from which glands, the target organs of the hormones, the specific effects of the hormones on the target organs, and the general effects of the hormones on the body. Go back over all the information we”ve given you about the different glands and their hormones, and then take these practice quizzes.

Quick Quiz #6

On each blank line place the letter that designates the appropriate hormone.

A. estrogen

K. growth hormone



C. aldosterone

M. testosterone

D. epinephrine

N. progesterone

E. prolactin

O. cortisol

F. glucagon

P. norepinephrine

G. parathormone




I. insulin

S. thyroxine

J. oxytocin

T. calcitonin

1. The pancreatic islet cells secrete _______ and _______.

2. The ovaries secrete _______ and ______.

3. The anterior pituitary secretes _______, _______, _______, _______, _______, and _______.

4. The thyroid gland secretes ______ and ______.

5. The adrenal cortex secretes _______ and _______.

6. The posterior pituitary secretes _______ and _______.

7. The adrenal medulla secretes _______ and _______.

8. The testes secrete _______.

9. The parathyroid gland secretes _______.

Correct answers can be found in Chapter 15.

Quick Quiz #7

On each blank line place the letter that designates the appropriate target organ. Letters can be used more than once.

A. all cells in the body

G. adrenal cortex

B. bones

H. kidneys

C. male body

I. female body

D. thyroid gland

J. testes

E. uterus

K. liver

F. mammary glands

L. ovaries and testes

1. estrogen / progesterone _______

2. ACTH _______

3. aldosterone _______

4. epinephrine / norepinephrine _______

5. prolactin _______

6. glucagon _______

7. parathormone _______

8. LH / FSH _______

9. insulin _______

10. oxytocin _______

11. growth hormone _______

12. testosterone _______

13. cortisol _______

14. ADH _______

15. TSH _______

16. thyroxine _______

17. calcitonin _______

Correct answers can be found in Chapter 15.

Quick Quiz #8

On each blank line place the letter that designates the appropriate effect in the body.

A. produces breast milk

K. maintains female sex characteristics, builds uterine lining

B. contracts uterus, releases breast milk

L. releases testosterone in male, forms corpus luteum in female

C. causes gluconeogenesis, increases blood glucose levels

M. maintains and enhances uterine lining

D. maintains male sex characteristics

N. prolongs and enhances “fight or flight” response

E. causes release of hormones from adrenal cortex

O. causes thyroid gland to release thyroxine

F. increases body metabolism

P. causes spermatogenesis in male, oogenesis in female

G. growth of the body

Q. causes kidneys to retain sodium

H. breaks down glycogen, increases blood glucose levels

R. allows cells to take up glucose, decreases blood glucose levels

I. builds bone, decreases blood calcium

S. breaks down bone, increases blood calcium

J. causes kidneys to retain water

1. estrogen _______

2. ACTH _______

3. aldosterone _______

4. epinephrine / norepinephrine_______

5. prolactin _______

6. glucagon _______

7. parathormone _______

8. LH _______

9. insulin _______

10. oxytocin _______

11. growth hormone _______

12. testosterone _______

13. cortisol _______

14. ADH _______

15. TSH _______

16. thyroxine _______

17. FSH _______

18. progesterone _______

19. calcitonin _______

Correct answers can be found in Chapter 15.


The circulatory system is designed to move stuff around the body. It transports oxygen, carbon dioxide, glucose, hormones, waste products, lipids, etc. Essentially, the circulatory system consists of a pump (the heart), a network of tubing (the blood vessels), and a fluid (the blood). Let”s start by talking about the blood.

The Blood

Any organism with a closed circulatory system has blood. A closed circulatory system just means that the blood is carried in vessels. But some organisms do not have a closed system; their blood (called hemolymph in these organisms) is not carried in vessels; it simply bathes the organs in their body cavities. Examples of organisms with open circulatory systems are the arthropods (insects, etc.).

Blood consists of two main things: (1) fluid and (2) cells that float around in the fluid. The fluid is called plasma, and the cells that float around are red blood cells, white blood cells, and platelets.

Plasma is mostly water. It has a lot of stuff dissolved in it, such as glucose, hormones, ions, and gases. The glucose makes it sticky. It also has a lot of protein in it, like albumin (the most abundant protein in blood), fibrinogen, and lipoproteins. All the blood proteins are made by the liver, and plasma makes up about 50% of the blood volume.

Most of the cells in the plasma are red blood cells. In fact, red blood cells make up about 45% of the total blood volume. Red blood cells are shaped like biconcave disks, which look sort of like disks that have been squashed in the center, on both sides. Red blood cells are filled with a protein called hemoglobin. There is so much hemoglobin in red blood cells that there is no room for organelles or a nucleus. Red blood cells are the only cells in the body that do not have a nucleus.

Hemoglobin can bind oxygen. Because red blood cells contain hemoglobin, and hemoglobin carries oxygen, we say that red blood cells carry oxygen and deliver it to cells all over the body. Really, though, it”s the hemoglobin that carries the oxygen.

Hemoglobin is made partly of iron. That means that if you don”t get enough iron in your diet, you can”t produce enough hemoglobin. Your red blood cells can”t bind enough oxygen, and your body”s cells get shortchanged. They don”t receive all the oxygen they need to carry out cellular respiration to make energy (ATP). When that happens, you have anemia. One of the most obvious symptoms of anemia is fatigue. It makes sense—if you can”t use cellular respiration to make ATP, you don”t have enough energy and you”re easily fatigued.

The remaining 5% of the blood volume is made up of white blood cells and platelets. White blood cells are very important in fighting off disease. Most of the white blood cells are phagocytes, which means they are very good at phagocytosis (eating stuff). What do they eat? Well, viruses, bacteria, parasites, dead cells, and sick cells, to name a few—anything that”s potentially harmful to your body. Some of the white blood cells are lymphocytes, which participate in very specific disease defense called immunity. Lymphocytes come in two forms: B-cells and T-cells.

B-cells make antibodies. Antibodies are just markers that can bind to foreign things in the body and mark them for destruction (like by phagocytosis). For example, if you get infected with the chickenpox virus, some of the B-cells in your blood will make antibodies that can bind to the chickenpox virus and mark it for destruction. Because there are millions of different viruses, bacteria, parasites, and other potentially harmful things, you have millions of different B-cells that can make antibodies that are specific for each of the million different potentially harmful things.

Blood Cells: A Quick Review

• All blood cells are made in the bone marrow.

• Blood cells include red cells, white cells, and platelets.

• Red blood cells contain hemoglobin (which contains iron), and hemoglobin binds oxygen and carries it around the body.

• Some white blood cells help to fight infection by phagocytizing harmful things.

• B-cells are a type of white blood cell that makes antibodies against very specific foreign things.

• Helper T-cells help B-cells and other T-cells reproduce, and killer T-cells kill cells that have been infected by a virus.

• AIDS is caused by a virus (HIV) that infects and kills helper T-cells. Without helper T-cells, the body cannot fight infection.

• Platelets are necessary for blood clotting.

T-cells have two jobs. First, they help the B-cells and other T-cells divide and proliferate. T-cells that do this are called helper T-cells. Second, they kill any cells that have been infected by viruses. Because viruses are not complete cells they cannot reproduce without some help. So they act as parasites, living inside our cells and essentially turning them into virus factories. The easiest way to kill the virus, then, is to kill the cell it has infected. That”s what this type of T-cell does, and they are called killer T-cells.

Here”s another thing to know: AIDS is a disease caused by a virus called HIV (Human Immunodeficiency Virus). HIV infects and lives in helper T-cells, killing the helper T-cells in the process. Because the helper T-cells are so important to B-cells and killer T-cells, without the helper T-cells the other two cell types can”t reproduce and fight infection. As a consequence, many, many infections spring up. Patients with AIDS often die of these infections.

Platelets are very, very small structures that are important in blood clotting. If a person is deficient in platelets, the blood does not clot. Platelets secrete a substance that activates a chain of events that ultimately converts a soluble blood protein—fibrinogen—into insoluble threads called fibrin. The fibrin threads form “nets” that trap blood cells and more platelets to form a clot. The process requires calcium, vitamin K, and many other chemicals.

Finally, all blood cells—red, white, and platelets—are made in the bone marrow inside bones.

Quick Quiz #9

Fill in the blanks and check the appropriate boxes:

1. [ Red blood cells White blood cells ] function in the immune system.

2. Hemoglobin contains ______________________ and can bind ______________________.

3. Approximately what percentage of the blood is made of red blood cells?

(A) 5%

(B) 20%

(C) 45%

(D) 50%

(E) 80%

4. B-cells make ______________________________.

5. Insufficient iron in the diet leads to insufficient __________________and the disease _________________________.

6. [ Killer Helper ] T-cells are T-cells that help B-cells and other T-cells reproduce.

7. T-cells are [ white red ] blood cells.

8. Blood cells involved in blood clotting are called _________________.

9. Plasma is mainly composed of

(A) glucose

(B) water

(C) oxygen

(D) red blood cells

(E) white blood cells

Correct answers can be found in Chapter 15.

Blood Typing

Blood type is determined by the membrane proteins that sit on the surface of red blood cells. You might remember from Chapter 6 that proteins are made by reading codons on mRNA and connecting amino acids in the order specified by the codons, and that the order of codons on mRNA is determined by the sequence of nucleotides in DNA. Do you remember that a portion of a DNA chromosome that gives rise (ultimately) to a protein is called a gene? Well, if you do, it should not surprise you that there are genes that determine which proteins are produced and inserted into the membranes of red blood cells. The gene for the most common form of blood typing (the ABO blood typing system) is called the I gene.

The I gene comes in three versions (three alleles): IA, IB, and i. IA codes for type-A protein, IB codes for type-B protein, and i codes for the absence of protein (no protein). In Chapter 8 you learned that everyone has two copies of each of their genes (two alleles), one on each of a pair of homologous chromosomes. We also said that if both copies of the alleles are the same, the people are said to be homozygous and their phenotype (their trait) is easy to determine. So, if people have two copies of the IA allele (their genotype is IAIA), then all they can make is type-A protein and that”s all they”ll have on their red blood cells. They are said to have blood type A.

The same is true for people who are homozygous for the IB allele. Their genotype is IBIB, all they can make is type-B protein, and that”s all they”ll have on their red blood cells. They are said to have blood type B.

People who are homozygous for i (genotype ii) don”t make any protein at all and have no proteins on the surface of their red cells. They have blood type O.

What about heterozygotes? To determine the phenotype of a heterozygote we have to know which alleles are dominant and which are recessive. For this gene, the i allele is recessive to both the IA allele and the IB allele, and the IA allele and the IB allele are said to be codominant. Codominant means that if these two alleles are found together in a heterozygote, neither gene is silent (repressed). They are both expressed independently. So how does this work for blood typing? Let”s take a look.

A heterozygote who has the genotype IAi has one allele that codes for type-A protein and one allele that codes for no protein. Because IA is dominant to i, the cells make type-A protein and the person has blood type A. The same thing is true for a heterozygote with the genotype IBi. One allele codes for type-B protein, and the other codes for no protein. Because IB is dominant to i, the cells make type-B protein and the person has blood type B.

A heterozygote who has the genotype IAIB has a unique phenotype. Because IA and IB are codominant, both will be expressed in this heterozygote. The cells will make both type-A protein AND type-B protein. The person”s blood type will be AB.

Here”s a summary table of blood types and the genotypes that produce them:

Blood type










What to Expect

There are two types of questions about blood typing that you might see on the SAT Biology E/M Subject Test. The first type of question will ask about blood transfusions—which blood types can donate to other blood types, and which blood types can receive which blood types. The second type is more of a genetics question.

Blood Transfusion Questions

Here”s what you have to remember about donating and receiving blood:

If a person”s body does not recognize the proteins on the newly received red blood cells, the newly received red blood cells will clump up and be destroyed.

The clumping of the red blood cells is also called agglutination. This clumping is part of a transfusion reaction and it can be fatal.

For example, a person with blood type AB recognizes both types of protein—type-A protein and type-B protein. So people with blood type AB can receive blood from all other blood types. They can receive type A blood, because their bodies recognize type-A protein. They can receive type B blood, because their bodies recognize type-B protein. And they can receive type O blood, because type O blood has no proteins to be recognized. Blood type AB is sometimes referred to as the universal recipient, because they can receive all other blood types.

People with type A blood can recognize only type-A protein. Type-B protein looks foreign to their bodies. Therefore, people with type A blood can receive type A blood, and they can receive type O blood (no proteins), but they cannot receive type B blood. Type B blood given to a type A person will cause a transfusion reaction. They also cannot receive type AB blood, because the red blood cells of type AB blood have both type-A protein AND type-B protein. Type AB blood given to a type A person will also cause a transfusion reaction.

The same is true for people with type B blood. They can recognize only type-B protein, and type-A protein looks foreign to them. A person with type B blood can receive either type B blood or type O blood, but not types A or AB.

Last, people with type O blood can receive only type O blood. Their bodies recognize neither type-A protein nor type-B protein. Therefore, type A blood, type B blood, and type AB blood will all cause transfusion reactions in a type O person. But notice that type O blood can be given to any other blood type. That”s because its red blood cells have no proteins on them that could cause reactions in the receiving person. For this reason, type O blood is sometimes referred to as the universal donor.

Here”s another summary table:

Genetics Questions

The other type of question you might be asked is more like a typical genetics question. You will be asked to determine the probability of blood types in the children of two people whose blood types you are given. Here are a few practice questions. Don”t forget to draw a Punnett square if you think it will help you.

Practice Question 1: A man with blood type AB marries a woman who is homozygous for blood type A (genotype IAIA). What is the probability that they will produce a child with blood type B?

Practice Question 2: Which blood type(s) are NOT possible from a cross between a person with blood type AB and a person heterozygous for blood type B (IBi)?

Practice Question 3: Could a woman with blood type B and a man with blood type A produce a child with blood type O?

(Answers can be found in Chapter 15.)

Quick Quiz #10

Fill in the blanks and check the appropriate boxes:

1. The genotype(s) for blood type O is (are) ______________________.

2. Blood type AB is sometimes called the [ universal donor universal recipient ].

3. Blood type A can receive blood from blood type(s) ______________________________.

4. Blood type B can donate blood to blood type(s) ______________________________.

5. The probability of a man homozygous for blood type B and a woman homozygous for blood type A producing a child with blood type A is [ 0% 25% 50% 75% 100% ].

6. The genotype(s) for blood type A is (are) ______________________.

7. Blood type AB can donate blood to blood type [ AB A B O all of them ].

8. Alleles IA and IB are said to be ______________________________.

Correct answers can be found in Chapter 15.

The Blood Vessels

At the center of the circulatory system is the heart, which acts as a pump. The blood vessels lead away from the heart and enter the tissues, then return to the heart. The heart pumps blood through the blood vessels.

This diagram represents a very basic circuit. Any vessel that carries blood away from the heart is called an artery. Arteries carry blood Away from the heart. The blood pressure inside arteries is relatively high, and they have thick, muscular walls that regulate their diameters to regulate blood flow. Blood moves through arteries mostly through momentum, because of the big “push” it gets from the heart. As they travel away from the heart, arteries get smaller and smaller and branch out (not shown above) into arterioles and, ultimately, capillaries.

Capillaries are the smallest blood vessels in the body and are the site of exchange between blood and tissues. Oxygen and nutrients (such as glucose) leave the blood and enter the tissues, and carbon dioxide and other waste products leave the tissues and are picked up by the blood. Blood flow is slow enough here so that there is enough time for this exchange. And, again, even though it”s not shown above, arterioles branch into thousands and thousands of capillaries. For this reason, blood pressure drops substantially in the capillaries.

Capillaries then merge to form larger vessels, called venules, which merge to form even larger vessels called veins, which carry blood back to the heart. Any vessel that returns blood to the heart is called a vein. Blood loses most of its forward momentum and pressure as it crosses through the capillaries, so the pressure in the veins is relatively low. As your body moves around and skeletal muscles contract, veins get squeezed, and this pushes the blood along toward the heart. It”s just like when you pick up a garden hose and squeeze it and water runs out. But because we want blood to run in only one direction in the veins (water in a hose would run out in both directions) the veins have valves, which ensure that the blood keeps moving in the direction we want it to—toward the heart. Veins do not have muscular walls. They do not regulate blood flow. They are passive receivers, taking whatever they get from the capillaries and moving it along to the heart.

The Blood Vessels: A Quick Review

• Any vessel that carries blood away from the heart is an artery.

• Any vessel that returns blood to the heart is a vein.

• Arteries branch into smaller and smaller vessels, ultimately becoming capillaries.

• Capillaries are the sites of exchange between blood and tissues.

• Capillaries merge into larger and larger vessels, called veins.

• Arteries have higher pressure and muscular walls, can regulate blood flow, and do NOT have valves.

• Veins have lower pressure and no muscle, are passive receivers, and DO have valves.

Oxygen-Rich or Oxygen-Poor?

An important point to note is that the type of blood being carried—oxygen-rich or oxygen-poor—has absolutely nothing to do with the type of vessel carrying it. In other words, arteries and veins are designated as arteries or veins based only on the direction they are carrying the blood—either away from or toward the heart.

It is true that most arteries carry blood that is rich in oxygen. And most veins carry blood that is relatively oxygen-poor; but it doesn”t have to be that way. We”ll see an example of this in a little while, when we discuss blood flow through the heart.

More about the Capillaries

We”ve mentioned already that capillaries are the sites of exchange between blood and tissues. We”ve also mentioned that the rate of blood flow and blood pressure both decrease in the capillaries. Here”s why: Imagine a river, and imagine that at one place the river gets very narrow. The same amount of water now has to squeeze through a narrower place. What happens? The water speeds up. The water pressure goes up. These areas are sometimes referred to as rapids, or whitewater, because the water moves very fast and white foam forms. Now imagine that a little farther downriver there is a place where the banks widen. What happens here? The water slows down. The pressure is lower. There”s more room for the water.

The arteries are like the narrow part of the river. Blood flow is rapid and pressure is high. But as the arteries branch out into smaller and smaller vessels, the combined diameter of all these tiny vessels is actually greater than the diameter of the artery. This is like the wide part of the river. Blood flow slows down and the pressure drops.

On the artery side of a capillary, the blood pressure is higher than on the vein side of the capillary. On the artery side, the pressure forces fluid (essentially blood plasma) out of the capillaries and into the tissues. On the vein side, where pressure is lower, some of the fluid returns to the capillaries, but not all of it; some of it remains in the tissues. So there is a net loss of fluid to the tissues. This fluid is called intercellular fluid.

The Lymphatic System

Somehow we have to recapture that fluid and return it to the blood vessels. If we don”t, blood volume will go down as fluid is lost to the tissues, and the tissues will swell up. Luckily, there is a specialized system of vessels that do just this. The lymphatic system is a network of vessels that begins at the tissues and ends at the veins, just before the heart. It recaptures the extra fluid from the tissues, and it filters that fluid to remove anything potentially harmful from it before it gets returned to the blood. The fluid is filtered through structures called lymph nodes or glands.

Lymph nodes are just concentrated areas of white blood cells. By passing through the lymph nodes on its way back to the veins, the fluid is exposed to many, many white blood cells. These white blood cells destroy anything harmful before it can get dumped into the general circulation.


Edema is the swelling of parts of the body due to trapped tissue fluid. Because the flow of lymph through lymphatic vessels depends on nearby muscle contraction, remaining in one position can restrict this flow. The problem can get worse if a person is standing upright because the combination of poor flow and gravity can trap fluid in the lowest parts of the body. This is why people who stand in one place for a long time (e.g. assembly line workers) can have problems with swollen feet and ankles.

The lymphatic vessels are very similar to veins. They have low pressure, no muscle in their walls, and valves that keep the fluid moving in the right direction. Fluid moves through the vessels the same way that blood moves through the veins—by nearby skeletal muscles that squeeze the vessels when they contract. The fluid inside lymphatic vessels is called lymph.

Quick Quiz #11

Fill in the blanks and check the appropriate boxes:

1. [ Veins Capillaries Arteries ] are blood vessels that return blood to the heart.

2. _________________________ return excess tissue fluid to the blood vessels.

3. The type of blood being carried [ does does not ] matter when determining whether a blood vessel is an artery or a vein.

4. Blood pressure in the veins is [ high low ].

5. Blood pressure in the arteries is [ high low ].

6. Blood flow in the capillaries is [ fast slow ].

7. [ Lymph nodes Lymph vessels ] filter potentially harmful things from the lymph before it is returned to the veins.

8. The diameter of an artery is [ larger smaller ] than the combined diameters of the capillaries.

9. Lymphatic vessels [ do do not ] have valves.

10. Blood moves through the veins and lymph moves through the lymphatic vessels when nearby _________________________________ contract and squeeze the vessels.

Correct answers can be found in Chapter 15.

The Heart

The heart, of course, is the pump that moves the blood through the vessels. It consists of four chambers: two smaller, weaker chambers on the top called atria (singular = atrium) and two larger, stronger chambers on the bottom called ventricles.

Whenever you think about the heart, you think about it as though it were in a person, and the person is facing you. That person”s right side would be on your left side, and the left side would be on your right. That”s why in the picture above, the right side of the heart is on your left, and the left side of the heart is on your right. We”re thinking about it as though it were in a person.

Blood enters the heart from veins that empty into the atria and leaves the heart through arteries that exit from the ventricles. The heart actually pumps the blood in two separate circuits that run simultaneously. The right side of the heart pumps blood through the pulmonary circuit, and the left side of the heart pumps blood through the systemic circuit.

Blood Pathway Through the Heart: The Specifics

The pulmonary circuit takes blood that”s returning from the tissues of the body and pumps it to the lungs. Blood coming from the tissues is fairly low in oxygen. It has delivered its oxygen to the tissues and picked up carbon dioxide in exchange. It needs to go to the lungs to get rid of the carbon dioxide and pick up a fresh supply of oxygen.

This oxygen-poor blood enters the heart at the right atrium, through the two largest veins in the body, the anterior vena cava and the posterior vena cava. It passes from the right atrium into the right ventricle, and then from the right ventricle into the pulmonary artery on its way to the lungs. (Remember earlier when we said that all arteries carry blood away from the heart and that most arteries carry blood that is oxygen-rich? Here”s an example of an artery that carries oxygen-poor blood. It still follows the rule about direction of flow, however; it carries blood AWAY from the heart.)

At the lungs, the blood picks up oxygen and delivers its carbon dioxide. It returns to the heart through the pulmonary veins, which empty into the left atrium. (Pulmonary veins follow the direction rule also; veins always return blood to the heart. But these are an example of veins that carry oxygen-rich blood, whereas most veins carry blood that is oxygen-poor.) From the left atrium, the blood passes into the left ventricle, then exits the heart through the largest artery in the body, the aorta. The aorta carries this oxygen-rich blood to all the tissues of the body. The left side of the heart pumps the blood through the systemic circuit. The systemic circuit takes oxygen-rich blood that”s returning from the lungs and pumps it to the rest of the body.

“Lub-Dup, Lub-Dup”

To keep the blood moving in the right direction, the heart has valves. There”s a set of valves between the atria and the ventricles, called the atrioventricular valves (the AV valves), and there is a set of valves between the ventricles and the arteries, called the semilunar valves. The closing of the valves produces the characteristic sound of the heartbeat—“lub-dup, lub-dup, lub-dup.” The AV valves close first, at the beginning of heart contraction (producing the “lub” sound) and the semilunar valves close second, at the end of the heart contraction (producing the “dup” sound).

This contraction and relaxation in your heart happens automatically, about 72 times per minute. A special conduction system makes sure that your heart beats rhythmically. The beat begins in tissues in the right atrium called the sinoatrial (SA) node (also known as the pacemaker). The part of the cycle in which contraction occurs is called systole, and the part in which relaxation occurs is called diastole. Systolic pressure in the heart is created by the contraction of the ventricles and diastolic pressure is created by the relaxation of the ventricles. The average human blood pressure is 120/80. High blood pressure (also known as hypertension) is a serious health issue.

One last thing worth mentioning before we look at the lungs and the pulmonary circuit in a little more detail: The pictures of the heart above are not absolutely anatomically correct. The chambers, the veins, and the valves are all okay, but arteries exit the ventricles at the top of the heart, not out the bottom. We”ll see a more correct picture when we look at the heart in conjunction with the lungs.

Not Just for Humans

The circulatory and respiratory systems of other organisms are similar to the human systems. Fish have a two-chambered heart where blood moves in a single circuit. Blood is pumped from the heart to the gills where it receives oxygen.

Amphibians, turtles, and snakes and lizards all have three-chambered hearts. Like humans, these organisms also have two circuits of blood flow. All adult forms of these animals have lungs, for the exchange of gases. It is important to note that in most amphibians, the skin is also a site for the exchange of gas.

Crocodiles, alligators, and birds all have four-chambered hearts with two circuits of blood flow. These animals all have lungs that function like those of mammals.

All arthropods have an open circulatory system. Blood is pumped from the heart to the organs. Arthropods that live in the water have gills. Terrestrial arthropods have tracheae, which lead from the inside of the body to the outside to exchange gases.


The circulatory system”s job is to move the blood and the gases, oxygen, and carbon dioxide (as well as anything else that”s carried in the blood, like glucose, waste products, and hormones) around the body. But without the respiratory system, there would be no gases to transport. The respiratory system”s job is to move air into and out of the lungs. This process is called ventilation. The respiratory system also exchanges oxygen and carbon dioxide with the blood. This is gas exchange. The respiratory system also plays a role in regulating the pH of the body.

The Conduction Zone—No Gas Exchange

Some parts of the respiratory system are designed for ventilation only and not for the exchange of oxygen and carbon dioxide. Because these parts are designed to conduct air in and out only, they are referred to as the conduction zone.

The conduction zone begins at the nose, where air is warmed, filtered, and humidified. The air travels down the throat (called the pharynx), past the voicebox (called the larynx), and into the windpipe (called the trachea). The trachea then branches into two tubes, one that leads to the right lung and one that leads to the left lung. These two tubes are called the right and left primary bronchi. The primary bronchi continue to branch into smaller and smaller tubes, called bronchioles. The walls of the trachea and all of the bronchial tubes are lined with tall cells that secrete mucus. This mucus helps to trap dirt and dust, but it is difficult for gases to diffuse through it. The cells also have cilia, which sweep the “dirty” mucus upward and out of the system.

The Respiratory Zone—Gas Exchange

The smallest bronchioles contain “bubbles” of tissue that have very, very thin walls, and there is very little mucus in these areas. These bubbles are called alveoli (singular = alveolus), and this is where gas exchange takes place. The bronchioles actually end in clumps of alveoli that resemble clusters of grapes. Many capillaries surround these clusters of alveoli. Because the walls of the alveoli are so thin, it is very easy for carbon dioxide in the blood to pass through the capillary walls and alveolar walls and into the alveoli. Oxygen travels in the opposite direction—from the alveoli, through the alveolar walls, through the capillary walls, and into the blood. This is passive diffusion—the gases are hydrophobic (lipid soluble), and they simply move down their concentration gradients. Oxygen concentration is higher in the alveoli than in the blood, so oxygen moves from the alveoli to the blood. Carbon dioxide concentration is higher in the blood than in the alveoli, so carbon dioxide moves from the blood to the alveoli.

Full of Hot Air

A fair amount of water is
also exhaled along with
carbon dioxide. That”s
why you can “steam up”
a window or a mirror by
breathing on it.

Where Do the Carbon Dioxide and Water Come From?

Remember the Krebs cycle and electron transport? Well, if you do, then you remember that carbon dioxide is a waste product of the Krebs cycle and that oxygen is the final electron acceptor in the electron transport chain; when it accepts electrons it becomes reduced to water. So carbon dioxide and water are just the natural waste products of your cells as they run the Krebs cycle and electron transport and produce ATP.

This is also the reason we need oxygen in the first place. Remember that these processes are aerobic, which means that they can”t occur without oxygen.

The Circulatory and Respiratory Systems Combined: A Quick Review

1. Oxygen-rich blood leaves the left ventricle through the aorta.

2. This blood travels through the aorta, the arteries, the arterioles, and the capillaries.

3. Exchange occurs between the blood in the capillaries and the cells. Oxygen and glucose are delivered to the cells.

4. The cells use the oxygen and glucose in cellular respiration to make ATP. They produce carbon dioxide, water, and other waste products in the process.

5. Exchange occurs between the cells and the blood in the capillaries. Carbon dioxide, water, and other waste products are delivered to the blood.

6. This blood (which is oxygen-poor) passes from capillaries to venules, from venules to veins, and from veins to the largest veins in the body, the superior and inferior vena cavae.

7. The anterior (superior) and posterior (inferior) vena cavae deliver the blood to the right atrium of the heart, which passes it on to the right ventricle.

8. The right ventricle of the heart sends this oxygen-poor blood through the right and left pulmonary arteries, into the right and left lungs.

9. The pulmonary arteries divide many times to form hundreds of thousands of pulmonary capillaries, which surround the alveoli of the lungs.

10. Exchange occurs between the blood in the pulmonary capillaries and the alveoli. Carbon dioxide and water are delivered to the alveoli (the other waste products are filtered out by the kidney—more on that later). Oxygen is delivered to the blood.

11. The pulmonary capillaries merge to form pulmonary venules, which merge to form pulmonary veins, which merge to form the large left and right pulmonary veins.

12. The pulmonary veins carry this oxygen-rich blood to the left atrium of the heart, which passes it on to the left ventricle.

13. The left ventricle pumps the blood out to the body”s cells through the aorta, and the whole cycle starts over again.

The Circulatory and Respiratory Systems

Quick Quiz #12

Fill in the blanks and check the appropriate boxes:

1. Blood leaves the heart from [ ventricles atria ] and enters the heart at [ ventricles atria ].

2. The aorta carries [ oxygen-rich oxygen-poor ] blood [ away from toward ] the heart, and the two vena cavae carry [ oxygen-rich oxygen-poor ] blood [ away from toward ] the heart.

3. From the right atrium, blood passes immediately to the

(A) left ventricle

(B) right ventricle

(C) left atrium

(D) pulmonary artery

(E) posterior vena cava

4. The ______________________________ are the site of exchange between blood and tissue.

5. The pulmonary arteries carry [ oxygen-rich oxygen-poor ] blood [ away from toward ] the heart, and the pulmonary veins carry [ oxygen-rich oxygen-poor ] blood [ away from toward ] the heart.

6. Blood that enters the right atrium after touring the entire body is [ oxygen-rich oxygen-poor ].

7. The [ semilunar atrioventricular ] valves separate the ventricles from the arteries.

8. Where does gas exchange in the lungs occur?

(A) The aorta

(B) The capillaries

(C) The trachea

(D) Pulmonary arteries

(E) Alveolus

9. Moving air into and out of the lungs is called __________________.

10. The [ pulmonary systemic ] circuit sends blood to the lungs.

11. The _________________________ zone of the lungs is where gas exchange takes place.

12. The first heart sound is the closing of the [ semilunar atrioventricular ] valves at the [ end beginning ] of heart contraction.

13. From the right ventricle [ oxygen-rich oxygen-poor ] blood is passed to the [ pulmonary arteries pulmonary veins ] and then to the lungs.

14. The larynx is part of the [ conduction respiratory ] zone.

15. Oxygen-rich blood enters the heart at the _____________________.

16. Blood gases (oxygen and carbon dioxide) are [ hydrophilic hydrophobic ].

Correct answers can be found in Chapter 15.

pH Regulation by the Respiratory System

It”s very important that the body maintain a constant pH, because our cells” enzymes stop working if the pH becomes too acidic or too alkaline. Remember that acidity is measured on the pH scale, which ranges from 1 (really acidic) to 14 (really basic, or alkaline). A neutral pH is 7; our blood pH is approximately 7.4, which is slightly alkaline. Blood pH is maintained in a very narrow range: 7.35 to 7.45. The two systems that regulate blood pH are the respiratory system and the renal (kidney) system. But the respiratory system is by far the faster regulator of the two.

Carbon dioxide, being fairly hydrophobic, does not dissolve well in the plasma, which is mostly water. So carbon dioxide is converted to carbonic acid and then the bicarbonate ion through the following chemical reaction:

Not all of the carbonic acid converts to bicarbonate, so both carbonic acid and bicarbonate are found in the plasma. Because these substances are hydrophilic, they dissolve very easily in the plasma.

What Does This Have to Do With the Respiratory System?

You can think of the carbon dioxide that your body produces as an acid. So if your body gets too acidic (the pH goes down too far), it needs to reverse that by getting rid of some acid. It”s very easy and fast to get rid of acid by getting rid of some of the extra carbon dioxide. How do we do that? By breathing faster.

What if your body gets too alkaline (the pH goes up)? That”s also very easy to fix. Simply breathe more slowly. Less carbon dioxide will exit the body, and the extra acid will help bring the pH back down to normal.

Of course, you don”t consciously have to do this. Your medulla oblongata (in your brain) does it for you. It monitors your pH, and if your pH is out of balance (too acidic or too alkaline), your medulla oblongata will adjust your respiratory rate accordingly.

This is actually the reverse of what most people think. Most people think we breathe faster or slower based on our body”s need for oxygen. But really we breathe faster or slower based on our body”s need to get rid of, or retain, carbon dioxide (to adjust pH).

How Do We Breathe?

The lungs themselves contain very little muscle, so they cannot expand on their own. All of the muscles that expand and contract the lungs (by expanding and contracting the chest cavity) are found in the chest wall and along the bottom of the lungs (the diaphragm). The lungs are stuck to the inside wall of the chest cavity. So any change in the size of the chest cavity produces a corresponding change in size of the lungs.

The diaphragm is the primary muscle of breathing. Other muscles in the chest wall help, too, but the diaphragm does most of the work. When it is relaxed, it curves up under the lungs:

When it contracts, it flattens out, increasing the size of the chest cavity:

When the chest cavity increases in “volume” there is a decrease in air pressure in the chest cavity, causing air to rush into the lungs. This is called inspiration.

Relaxed exhaling, or passive expiration, is accomplished by simply relaxing the diaphragm. The diaphragm returns to its normal, curved state, thereby reducing the volume of the chest cavity (and the lungs). This pressure on the lungs forces the air out of the system.

Force it Out

This same principle is
used to help people who
are choking. The Heimlich
maneuver is a forcible
increase in abdominal
pressure that helps
dislodge an object stuck in
the trachea

A forced expiration moves air out of the lungs rapidly, such as when you blow up a balloon or blow out a candle. This is accomplished by contracting the abdominal muscles; imagine that someone is going to punch you in the stomach and you tighten up your stomach muscles. This increases pressure in the abdominal cavity, rapidly pushes the diaphragm upwards, and forcibly expels the air from the lungs.

Quick Quiz #13

Fill in the blanks and check the appropriate boxes:

1. Most of the carbon dioxide in the blood is carried as _____________ and _______________________________.

2. If your blood is too acidic, your pH is [ higher lower ] and you will breathe [ slower faster ].

3. The system that can change pH more quickly is the [ respiratory renal ] system.

4. The primary muscle of respiration is the ______________________.

5. Breath rate is adjusted by the ______________________________.

6. When the diaphragm contracts it [ curves upward flattens downward ].

7. Normal blood pH is approximately __________________________.

8. When the chest cavity gets smaller, air in the lungs rushes [ inward outward ].

Correct answers can be found in Chapter 15.


We”ve already seen how the body (the body”s cells, really) needs glucose to run cellular respiration. We know also that the body”s cells need amino acids to make proteins (enzymes, etc.) and fats to make cell membranes and other things. How does the body get these things?

These things are acquired by the consumption of other animals or plants. The carbohydrates, proteins, and fats in the food are broken down into glucose, amino acids, and fats, and these building blocks are used to run cellular respiration or build the specific proteins and fats the body needs. The process of eating (ingesting), breaking down (digesting), and taking up (absorbing) food is managed by the digestive system.

The organs of the digestive system can be divided into two major groups: the alimentary canal and the accessory organs.

The Alimentary Canal

The alimentary canal is a long, muscular tube that begins at the mouth and ends at the anus. The numbers on the diagram on the following page indicate the order in which food travels through the organs of the alimentary canal.

Food enters the mouth and we swallow it. It enters the esophagus, and the esophagus begins a series of rhythmic, wavelike contractions that push the food down to the stomach. These rhythmic contractions are referred to as peristalsis.

From the stomach, the food enters the small intestine, then the large intestine (the colon), and finally the rectum. Indigestible material—feces—is eliminated through the anus. All of the organs in the alimentary canal perform peristalsis to keep the food moving through the tube.

The Accessory Organs

Organs that play a role in digestion, but are NOT part of the long tube previously described, are known as accessory organs. The accessory organs of the digestive system include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

The above illustration shows only a small portion of the pancreas. That”s because the pancreas is tucked into a loop of small intestine on its left and extends to the right, behind the stomach where (in this picture) we can”t see it. If we were to remove the stomach, we”d see the pancreas tucked into a loop of small intestine like this:

One last note before we look at the functions of the digestive system: Many of the organs are involved in exocrine secretion. We”ve already talked about endocrine secretion. That”s the secretion of hormones into the blood (endo = inside). Exocrine secretion is secretion that occurs outside the blood (exo = outside). Exocrine secretions are released into body cavities (such as the mouth, the stomach, or the intestines) or onto the body surface (the skin). Some examples of exocrine secretions are digestive enzymes, saliva, mucus, tears, and sweat.

Putting It All Together

Now that we”ve seen the organs of the digestive system, let”s take a trip through the alimentary canal to see how it all works, both the alimentary organs and the accessory organs.

The Mouth

The mouth is where it all starts. Ingestion, or the intake of food into the system, is accomplished here. The teeth and the tongue are accessory organs that help grind the food and form it into a lump called a bolus. Accessory organs called salivary glands secrete saliva. Saliva is made up mostly of water and mucus, and it helps to moisten the food and clump it together into the bolus.

Saliva also contains a digestive enzyme called amylase. Amylase catalyzes a reaction that breaks long carbohydrate molecules (starch) into little pieces. Amylase helps digest starch.

When you see “amylase,” think

digestive enzyme

contained in saliva, which is secreted in the mouth

helps digest starch, which is a carbohydrate

When food leaves the mouth, it”s in the form of a bolus, and some of its carbohydrate has been partially digested. It now moves through the esophagus and into the stomach.

The Stomach

The stomach is acidic. As you know, acidity is measured on a pH scale, and the lower the number, the more acidic something is. The stomach has a pH between 1 and 2, which is pretty acidic. The stomach is acidic because gastric glands located in the walls of the stomach secrete hydrochloric acid (HCl). This acid helps kill germs that have been swallowed with the food and also helps to break up the food further.

The gastric glands of the stomach secrete not only acid but an enzyme that helps break protein into amino acids (in other words, the enzyme helps digest protein). The enzyme is called pepsin or gastric protease. To break down protein, pepsin works best in an acidic environment. When HCl is secreted, it lowers the pH of the stomach and activates pepsinogen into pepsin to digest proteins. The stomach also secretes mucus, which protects the stomach lining from the acidic juices. Finally, HCl kills most bacteria.

When food leaves the stomach it is really a pile of mush, and this mush is called chyme. The chyme enters the small intestine.

The Small Intestine

The small intestine is the site of the most digestion and absorption. The chyme that enters the small intestine is subjected to bile from the liver and gallbladder and many enzymes from the pancreas.

Bile is produced by the liver and stored and concentrated in the gallbladder. It is important to remember that the gallbladder doesn”t actually make the bile, it just stores (and concentrates) the bile that”s made in the liver. Bile is released into the small intestine to help break down fats contained in chyme.

So how exactly does bile do this? Well, before we answer this, we”ll tell you what bile does NOT do: It does NOT digest fat. Bile does not digest fat because it is not an enzyme. What it does is emulsify fat. Emulsify means “to break up.” The problem with fats is that they do not mix well with the chyme, which is very hydrophilic. Fats are hydrophobic, and they separate from the chyme like oil separates from water. Bile emulsifies the fats and allows them to better mix in with the chyme. This allows the real fat-digesting enzymes (the lipases) easier access to the fat.

Digestive Processes

The digestive tract carries out the following six processes:

1) Ingestion—bringing food into the system

2) Movement (Peristalsis)—moving food along the system

3) Digestion—mechanical and chemical processes that break down food

4) Secretion—release of enzymes and bile into the digestive tract

5) Absorption—moving food molecules from the digestive tract into the blood

6) Defecation—eliminating solid waste from the large intestine

Bile works the same way as soap. Imagine that you”ve been eating butter-covered popcorn and you want to get the butter off your hands. So you go to the sink and rinse your hands off, but the butter stays. Water all by itself cannot remove the butter, because the butter is hydrophobic and the water is hydrophilic, and they do not mix well. However, if you add some soap, the butter breaks up into tiny pieces that DO mix well with the water. We say that the soap emulsifies the fat on your hands. In exactly the same way, bile emulsifies the fat in your intestines. (In fact, bile is sometimes referred to as “intestinal soap.”)

One more thing about the liver—it does a lot more than just make bile. The liver also stores glycogen, produces glucose, metabolizes fats, produces blood proteins, stores vitamins, and detoxifies the blood.

The actual enzymes in the small intestine come mostly from the pancreas. The pancreas secretes at least one enzyme for each type of food that needs to be digested. In other words, the pancreas secretes amylase (for carbohydrates), it secretes lipases (for fats), and it secretes proteases (for proteins). The pancreas also secretes bicarbonate, which is a base. The chyme that comes out of the stomach is very acidic, and the enzymes from the pancreas cannot work well in an acidic environment. So the pancreas secretes bicarbonate—a base—to neutralize the acid from the stomach. As a result, the pH in the intestines is close to neutral.

The small intestine is very long—in fact, it”s the longest part of the alimentary canal. It”s about 23 feet long in an average man. It also has many internal folds and villi that drastically increase the surface area inside the tube. The length and the relatively large surface area of the small intestine increase the amount of absorption that takes place quite a bit. Think about it this way: If you spilled a cup of water on the floor, would you use a square of toilet paper to mop it up, or would you grab a couple of paper towels? The paper towels, of course, and why? Because they have a greater surface area and therefore absorb more. It”s the same deal with the small intestine—the greater the surface area, the more food molecules can be absorbed. So absorption occurs at a fairly high rate in this part of the alimentary canal. The nutrient molecules are absorbed into the blood, which transports them to the liver for processing. The capillaries from the intestines merge to form special veins called portal veins, which divide into capillaries again when they reach the liver. This system—called the hepatic portal system—is designed to directly deliver nutrients from the intestines to the liver.

Where Does Digestion Really Happen?

One more thing about the stomach and small intestine: When most people think of the stomach, they think “digestion.” But the fact of the matter is that the stomach doesn”t do much digestion at all. It is mostly a storage tank for food, and it helps grind food up. Very little digestion and absorption occur in the stomach.

Almost all digestion and absorption occur in the small intestine.

From the small intestine, the chyme moves into the large intestine. Not too much happens in the large intestine. The large intestine is also called the colon, and it is responsible for reabsorbing water from the chyme. No further digestion takes place in the large intestine. As the water is reabsorbed, the chyme becomes more solid and is referred to as feces. Feces is just indigestible, solid waste. It is stored in the large intestine and egested from the body through the rectum and anus.

The large intestine also contains a large population of bacteria, E. coli. These nonpathogenic (not harmful) bacteria help keep pathogenic (harmful) bacteria from growing, and they also supply us with practically all the vitamin K we need.

The Alimentary Canal: A Quick Review

Mouth: The mouth grinds and moistens food, begins starch digestion.

Esophagus: The esophagus moves food to stomach.

Stomach: The stomach is responsible for grinding, protein digestion by pepsin, and food storage.

Small intestine: Chyme is subjected to bile from the liver (emulsifies fats) and digestive enzymes from the pancreas (digests chyme). This is the longest and most extensively folded part of the canal and is the site of almost all the absorption and digestion.

Large intestine: No further digestion occurs here, but water is reabsorbed from the chyme, leaving behind a solid, indigestible waste product: the feces. Bacteria in the large intestine provide us with vitamin K.

Not Just for Humans

Fish, amphibians, turtles, and snakes and lizards all have a complete digestive tract. They have organs as we do, such as a mouth, pharynx, esophagus, stomach, intestines, liver, pancreas, and anus. Crocodiles, alligators, and birds have those organs listed above but additionally they have a crop to store the food and a gizzard for grinding it in their complete digestive tract.


There are 6 kinds of nutrients: Carbohydrates, Lipids, Proteins, Vitamins, Minerals, and Water. Vitamins, Minerals, and Water are all small molecules and require no digesting. Carbohydrates, Lipids, and Proteins are large molecules and need to be digested. Digestion is the breaking of large complex molecules into molecules small enough to cross cell membranes and enter the bloodstream. These are known as the end products of digestion. The process of digestion requires water when breaking down these molecules. This is also known as hydrolysis as described in Chapter 3.

Vitamins and Minerals

Because we”re talking about digestion and nutrition, now is a good time to mention vitamins and minerals. Many vitamins function as coenzymes, and we”ve already seen how a few minerals are important (iron and iodine, for example). Here are some important things to know about vitamins and minerals.

Quick Quiz #14

Fill in the blanks and check the appropriate boxes:

1. The pancreas is [ part of the alimentary canal an accessory organ ].

2. The order of the organs in the alimentary canal is _______________, _________________, ___________________, ________________, _________________________.

3. The stomach [ does does not ] secrete a digestive enzyme.

4. The salivary glands secrete an enzyme called [ pepsin amylase ] that helps in the digestion of [ carbohydrates proteins ].

5. The colon is the [ small large ] intestine.

6. The vitamin needed for blood clotting is vitamin __________, and it is made by bacteria in the [ small large ] intestine.

7. What does the liver produce?

(A) Pepsin

(B) Collagen

(C) Bicarbonate

(D) Bile

(E) Fats

8. The products of digestion are absorbed through the walls of the

(A) small intestine

(B) large intestine

(C) liver

(D) kidney

(E) pancreas

9. The stomach is [ alkaline acidic ], which means that its pH is [ low high ].

10. The pancreas produces [ bicarbonate bile ].

11. The function of the large intestine is to _______________________.

12. Vitamin C is necessary to make [ collagen retinal ].

13. Pepsin is an ____________________, secreted by the ___________. It helps in the digestion of _______________________.

14. Bile [ digests emulsifies ] fats.

15. The _________________________ produces blood proteins and regulates glycogen metabolism.

Correct answers can be found in Chapter 15.


The digestive system brings nutrients into the body and breaks them down so that the body”s cells have the substances they need to function properly. As the cells run their reactions, waste products are formed and released into the blood. Two of the waste products, carbon dioxide and water, are eliminated by the respiratory system, but what about the others?

Other waste products are filtered from the blood by the kidneys and eliminated as urine. The three main waste products found in urine are urea (from breakdown of amino acids), uric acid (from breakdown of nucleic acids), and creatinine (a waste product from muscle metabolism).

The functional unit of the kidney is the nephron. Kidneys contain about a million nephrons each. Here”s what they look like:

Here”s how the nephron sits within the kidney:

Generally speaking, blood enters the kidney through renal arteries that branch into capillaries. The glomerulus is a tiny “knot” of capillaries at the beginning of the nephron. It sits inside a cuplike structure called Bowman”s capsule. These capillaries have pores in their walls and act as tiny sieves. Blood pressure forces the fluid portion of blood (the plasma) through the pores, but because the cells and proteins are too large to fit through, they remain behind, in the capillaries. The plasma enters Bowman”s capsule and is now known as filtrate. The filtrate begins traveling along the tubules of the nephron, and, along its path, it is modified. Substances that the body needs (like glucose, amino acids, and water) are returned to the blood. Substances that the body wants to eliminate are left in the tubules. When it”s all said and done, the filtrate is called urine.

Where does the urine go next? From the kidneys, it travels down the ureters to be stored in the bladder. From the bladder it is eliminated from the body through the urethra.

Urine Formation—Modifying the Filtrate

There are three processes the nephron uses to make urine. We”ve already described the first: filtration. Filtration just refers to blood pressure forcing plasma out of the capillaries and into Bowman”s capsule.

The second process is called reabsorption. Reabsorption is the process of taking substances out of the filtrate and returning them to the blood. Some substances, such as glucose and amino acids, are always reabsorbed. Under normal conditions, all the glucose and amino acids that are filtered are reabsorbed. But for some other substances, the amount reabsorbed is regulated according to the body”s needs. For example, more or less water is reabsorbed, depending on how well hydrated the body is. The reabsorption of some ions, such as sodium, potassium, and calcium, is also regulated depending on body needs.

The third process is called secretion. Secretion is the process of taking substances out of the blood and adding them to the filtrate. Creatinine is always secreted. Some ions, drugs, and toxins are also always secreted.

So, by reabsorption and secretion, the filtrate is modified as it travels through the nephron. Now let”s take a look at the specific functions of different regions of the nephron.

The Nephron

The first portion of the nephron, after Bowman”s capsule, is the proximal convoluted tubule. Proximal means “close to.” Convoluted means “twisted up.” A tubule is just a small tube. So the proximal convoluted tubule is just a small, twisted-up tube that”s close to the glomerulus. The proximal convoluted tubule is where most reabsorption and secretion take place.

The next portion of the nephron is the loop of Henle, in which a lot of water is reabsorbed. Additionally here, a fair amount of salt is transported out of the filtrate and into the tissues of the kidney. This helps to establish a concentration gradient in the kidney. The inner portions of the kidney (the medulla) are “saltier” (more concentrated) than are the outer portions of the kidney (the cortex). This gradient is important for water reabsorption elsewhere in the nephron. The longer the loop of Henle, the greater the concentration gradient, and the more water that can be reabsorbed. For example, some animals, such as desert rodents and lizards, have very long loops of Henle and reabsorb a lot of water. Their urine is very concentrated. These animals have a great need to conserve water because they live in very dry environments.

After the loop of Henle is the distal convoluted tubule (distal = farther away), the small, twisted-up tube that”s farther from the glomerulus. Reabsorption and secretion occur here; however, it tends to be more specialized and regulated. You can think of it as “urine fine-tuning.” This is the last chance to modify the filtrate before it enters the collecting duct. The distal tubule is where the hormone aldosterone has its effect. Aldosterone increases the amount of sodium that”s reabsorbed into the blood by the distal tubule. When sodium is reabsorbed, water follows, so water is reabsorbed as well.

Filtration Rate

The rate at which healthy
kidneys filter blood is
approximately 125 mL/min,
and the average adult
blood volume is about 5 L.
This means that the total
blood volume can be
filtered in about 40
minutes! About 99%
of the filtered volume
is reabsorbed, making
reabsorption critical to the
maintenance of normal
blood volume.

The final portion of the nephron is the collecting duct. The collecting duct can actually receive filtrate from several nephrons. This is the final location for urine concentration and is where water reabsorption is regulated (all of the water reabsorption we”ve been talking about so far has been unregulated—the tubules of the kidney have just reabsorbed as much as possible). The collecting duct is where the hormone ADH (antidiuretic hormone) has its effect. ADH causes the walls of the collecting duct to become permeable to water. (In the absence of ADH, the walls are impermeable to water, so water cannot leave the tube—notice that this is one of the few areas of the body that is impermeable to water.) The collecting duct travels inward, toward the center of the kidney along the large concentration gradient established by the loop of Henle. In effect, the duct enters regions that are becoming increasingly “saltier.” How does this affect water reabsorption and urine concentration?

If the walls of the duct are permeable to water (if ADH is present), then water can move out of the duct by osmosis and be taken up by the blood. As water leaves the duct, the urine becomes more concentrated. If the walls are impermeable to water (if ADH is absent), water cannot move out of the duct and it must stay in the urine, so the urine stays dilute.

ADH levels are high when the body is dehydrated—for example, after a person has run a marathon. This allows the body to retain water by concentrating the urine.

ADH levels are low when the body is well hydrated—for example, after drinking a full water bottle at a meal. This allows the body to eliminate excess water by keeping the urine dilute.

Blood Pressure Regulation by the Kidneys

If your kidneys suddenly stopped filtering your blood properly, you would only live for about two days. People whose kidneys have failed must go to the hospital three or four times a week to have their blood filtered artificially through dialysis. Because filtration is dependent only on blood pressure, it makes sense that the kidneys would play a role in monitoring and maintaining blood pressure.

The kidneys regulate blood pressure primarily by releasing a substance into the blood called renin, when blood pressure is low. Renin is an enzyme that, through a series of reactions, causes the production of a chemical that constricts blood vessels throughout the body. This chemical is called angiotensin II. The constriction of the blood vessels causes the blood pressure to go up. Additionally, angiotensin II increases the secretion of aldosterone by the adrenal cortex. Aldosterone causes increased reabsorption of sodium, which causes increased reabsorption of water, which increases the blood volume, which in turn increases the blood pressure. This is an example of positive feedback.

Not Just for Humans

Most other animals have kidneys to maintain water balance and waste disposal in the body; however, the form in which they eliminate this nitrogen as waste is different. Fish excrete nitrogen wastes in the form of ammonia. Adult amphibians and turtles, as well as humans and other land mammals, excrete waste in the form of urea. Most reptiles and birds excrete waste in the form of uric acid. Worms and insects don”t have kidneys, but they still need to eliminate nitrogen wastes. Worms use structures called metanephridia to do this, while insects use structures calledmalphigian tubules.

Quick Quiz #15

Fill in the blanks and check the appropriate boxes:

1. The three processes used by the nephron to make urine are ________, _________________________, and ________________________.

2. The [ loop of Henle collecting duct ] sets up a concentration gradient in the medulla.

3. [ Secretion Reabsorption ] means taking a substance from the urine and returning it to the blood.

4. Glucose is always [ secreted reabsorbed ].

5. The kidney helps to regulate blood pressure by releasing [ aldosterone renin ].

6. The [ ureter urethra ] carries urine from the bladder to the outside of the body.

7. Filtration occurs at the ______________________________.

8. ADH increases the amount of [ water sodium ] reabsorbed from filtrate.

9. Most reabsorption and secretion occur in the ___________________.

10. ADH levels are [ high low ] when the body is dehydrated, and this causes the urine to be [ dilute concentrated ].

11. The three main waste products found in urine are _______________, ________________________, and _________________________.

Correct answers can be found in Chapter 15.


Many organisms have skeletons. The skeleton is responsible for holding the body together in some recognizable shape. Without a skeleton, an organism”s body would be, basically, a pile of mush. So when you see the word skeleton, think shape and support.

Some animals, like human beings, have skeletons made of bones, and which are located inside the body. When a skeleton is found inside the body, it”s called an endoskeleton (remember, endo means “inside”). When it comes to endoskeletons, remember this fact:

All vertebrates (animals with backbones) have endoskeletons.

Fish, amphibians, mammals, reptiles, and birds are all vertebrates, and they all have endoskeletons. In fact, having an endoskeleton is what classifies them as vertebrates.

Some animals, on the other hand, have skeletons outside their bodies. Their skeletons are not made of bones but of hard crusty shells that contain a substance called chitin. Because they”re found outside the body, they”re called exoskeletons (remember, exo means “outside”). For the SAT Biology E/M Subject Test, you should know that the arthropods (phylum arthropoda) are organisms with exoskeletons. This includes insects, arachnids, and crustaceans.

Skeletal Tissues

There are two main types of tissue found in the skeletal system—bone and cartilage. Bone is a rigid substance made up of cells embedded in a solid calcium-phosphate matrix. It is among the hardest tissues in the body.

Bones have several functions:

• support the body

• protect soft organs

• produce blood cells

• store minerals

We”ve already mentioned that the bones provide internal support for the body. They also provide protection for many organs; for example, the brain and spinal cord are almost completely encased in bone. In the same way, the rib cage protects the heart, lungs, and other organs in the chest cavity. Bone marrow is the site of the production of blood cells. Last, the bones are a supply of calcium that can be dissolved and released into the plasma if the body needs it. (Likewise, if calcium is in excess, it can be stored as bone.)

The other primary skeletal tissue is cartilage. Cartilage is also a relatively rigid substance, although it is considerably more flexible than bone. Cartilage is found on the ends of all the bones and at the joints, where it acts as a shock absorber to protect the bone ends from rubbing together. Some structures, such as the end of the nose, the external ear, and the anterior portions of the ribs, are completely made of cartilage. This allows these structures greater flexibility, while still providing them with a defined shape.

One last comment about bones: They are held to other bones (at joints) by ligaments. Generally speaking, the more ligaments a joint contains, the stronger the joint is.

The skeletons of other vertebrates include a skull and backbone with vertebrae. Most vertebrae are attached to sets of paired limbs, whether they are wings, legs, or fins. Muscles are attached by tendons to the bones, which move the skeleton, allowing animals to swim, walk, or fly.


The muscular system works together with the skeletal system to support, protect, and move the body. The muscles pull against the bones to move them around. Postural muscles, such as those around the back and abdominal areas, help support the body in an upright position, and some internal regions of the body—most noticeably the abdominal region—are protected entirely by muscle.

There are actually three different types of muscle tissue in the body: skeletal muscle, cardiac muscle, and smooth muscle. We”ve already seen two of the types. Cardiac muscle is found only in the heart. Smooth muscle is found in the walls of hollow organs such as the stomach, intestines, and bladder. Cardiac and smooth muscle are both involuntary muscle, meaning that you do NOT have conscious control over their contraction. Additionally, cardiac muscle is self-excitatory, meaning that it can initiate its own contraction.

Skeletal muscle is attached to the bones; it moves your body around. Skeletal muscle is a voluntary tissue—you have conscious control over its contraction. Skeletal muscle cells are very long and have many nuclei. They are described as being multinucleate. Let”s take a look at how a skeletal muscle is put together.

Building a Muscle

Muscle tissue is mostly made up of proteins—two types in particular: actin and myosin. Actin molecules form long, thin chains, while myosin molecules bundle together to form thick fibers. In a muscle cell, actin and myosin are arranged in structures called sarcomeres.

actin and myosinsarcomere

Many sarcomeres line up end to end to form a threadlike structure called a myofibril. A myofibril is just a string of sarcomeres.

actin and myosinsarcomeremyofibril

Many myofibrils bundle together with cytoplasm, organelles, nuclei, and a cell membrane. This is a muscle cell (also called a muscle fiber). A muscle cell is just a bundle of myofibrils and organelles surrounded by membrane.

actin and myosinsarcomeremyofibrilmuscle cell

Muscle cells are organized into groups called fascicles.

actin and myosinsarcomeremyofibrilmuscle cellfascicle

Fascicles are grouped together to form the whole muscle.

actin and myosinsarcomeremyofibrilmuscle cellfasciclewhole muscle

When a muscle contracts, it gets shorter. The first component of the muscle that actually contracts is the sarcomere. Actin and myosin fibers do NOT change length during muscle contraction. Let”s take a look; here are a couple of sarcomeres:

The ends of the sarcomeres are called Z-lines. Actin is attached to the Z-lines and extends inward, toward the center of the sarcomere. Myosin is found in between the actin filaments, overlapping with the ends, but myosin does not touch the Z-lines. During muscle contraction, myosin binds to the actin and drags it inward, toward the center of the sarcomere. A fully contracted sarcomere would look like this:

The actin filaments essentially slide over the myosin filaments, dragging the Z-lines with them, and when the sarcomere is completely contracted, the actin filaments actually overlap in the center. Because the sarcomeres get shorter, the myofibrils get shorter, which means that the muscle cell gets shorter, and ultimately the entire muscle gets shorter. The process of contracting, because the filaments appear to slide over one another, is called the sliding filament theory.

Miscellaneous Muscle Facts

1. Skeletal muscle is described as being striated, or striped. When observed under a microscope, the cells appear to be striped. This comes from the regular arrangement of protein filaments into sarcomers.

2. Skeletal muscles are attached to bones by tendons.

3. Skeletal muscles are stimulated by neurons that release neurotransmitters at special synapses called motor end plates, or neuromuscular junctions. The neurotransmitter used is acetylcholine.

4. Cardiac muscle is also striated. (What does that tell you about its structure?)

5. Muscle contraction requires calcium.


The skin and all of its associated structures, such as hair, nails, sweat glands, oil glands, and sensory receptors, are collectively considered an organ. Your skin is actually the largest organ in your body. It has a surface area of about 1.5 to 2 square meters and makes up about 7 percent of your total body weight!

The skin is made of three layers of tissue: the epidermis, the dermis, and the hypodermis. The epidermis is a thin layer of cells at the body surface, most of which are dead. The dermis is a relatively thick layer of connective tissue underneath the epidermis that contains blood vessels, nerves, hair follicles, and glands. The hypodermis is a deep layer of fat that helps protect and insulate the body. The hypodermis varies in thickness from person to person.

The skin”s primary job is to protect the body from:

• abrasion (friction)

• heat loss

• water loss

• infection

• UV radiation

Other functions of the skin include vitamin D production, sensation, and thermoregulation (body temperature control). Let”s talk about thermoregulation.

Too Hot or Too Cold

Most organisms are unable to regulate their own body temperatures; they are cold-blooded. Other terms for cold-blooded include ectothermic or poikilothermic. The body temperatures of these organisms change with changing external temperature. For example, it”s common to see reptiles basking in the sun to raise their body temperatures.

Birds and mammals, however, are warm-blooded, or endothermic. They are able to maintain a constant body temperature regardless of external temperature. This requires a large expenditure of energy, and birds and mammals have evolved mechanisms to retain heat, such as insulating fat, feathers, and hair. Also, there are various changes that occur in the body when the internal temperature changes.

When your body temperature rises, receptors in your skin and body core monitor the change. They send information to your brain, and your brain sends messages to your body that cause it to cool. Specifically, this is what happens:

1. Blood vessels in the dermis dilate, allowing more blood to come close to the surface of the skin. This allows more heat to leave the body. (It also produces the characteristic red flush of an overheated person.)

2. Sweat glands in the skin become active, secreting sweat. As the sweat evaporates, the body temperature lowers.

When your body temperature falls, again, receptors in the skin and body core monitor the change and send the information to the brain, and the brain sends commands to the body to increase warming. These commands are essentially the reverse of the body-cooling activity. This is what happens:

1. Blood vessels in the dermis constrict, keeping the blood from the surface of the body. This retains more heat near the body core. (It also produces the characteristic bluish color of a very cold person.)

2. Sweat glands in the skin are inactivated.

3. Shivering is initiated. Shivering is rapid, involuntary muscular contractions. Muscle contraction generates a lot of heat, so shivering is an excellent way to raise body temperature.

Quick Quiz #16

Fill in the blanks and check the appropriate boxes:

1. Smooth muscle is found in the _____________________________.

2. The deepest layer of the skin is the [ dermis epidermis hypodermis ].

3. Spiders have [ exoskeletons endoskeletons ].

4. The two proteins found in muscle cell sarcomeres are _____________and __________________________.

5. When the body gets too warm, dermal blood vessels [ constrict dilate ], and shivering [ is initiated stops ].

6. Muscles are attached to bones by [ ligaments tendons ].

7. In a sarcomere, [ actin myosin ] attaches to the Z-lines.

8. The neurotransmitter used to stimulate muscle contraction is _________________________.

9. [ Cardiac Skeletal ] muscle is voluntary (under conscious control).

10. Bones are attached to other bones by [ ligaments tendons ].

11. Cardiac muscle is found in [ the heart only both the heart and the blood vessels ].

Correct answers can be found in Chapter 15.


Let”s start reproduction and development by talking about the male reproductive system. Because we”ve already covered a fair amount of the important stuff (spermatogenesis in Chapter 7 and hormones earlier in this chapter), and because you”re more likely to see questions about the female system on the SAT Biology E/M Subject Test, we won”t go into too much detail in this section.

Remember from Chapter 7 that the testes are the male gonads, meaning that they are the organs that produce gametes (sperm) for the male. The testes also produce testosterone (the primary male sex hormone) in response to luteinizing hormones (LH) from the pituitary. Testosterone is important for normal sperm development. It also causes the maturation of the sexual organs during puberty and maintains the male sexual characteristics in adulthood. Testes are found outside the body cavity in a sac called the scrotum. This keeps them a couple of degrees cooler than normal body temperature, which is also necessary for normal sperm development.

The sperm are produced inside the testes in small tubes—the seminiferous tubules—in response to follicle-stimulating hormones (FSH) from the pituitary gland. These tubules merge to form a large duct—the vas deferens. The vas deferens ultimately connects with the urethra to carry the sperm out of the body. Along the way, several glands secrete fluid (semen) that carries and provides nutrients for the sperm.


The female system is a little more complex than the male system. Not only does the female reproductive system have to produce gametes, it also has to prepare itself for pregnancy, because the female system (in humans) nurtures developing offspring. That means we have to consider two organs: the ovaries (which produce the gametes) and the uterus (which sustains a pregnancy).

The Menstrual Cycle

Generally speaking, the ovaries are controlled by hormones from the pituitary gland (FSH and LH), and the uterus is controlled by hormones from the ovaries (estrogen and progesterone). The average menstrual cycle lasts 28 days and affects both organs. The changes that occur in the ovary in response to FSH and LH are referred to as the ovarian cycle, and the changes that occur in the uterus in response to estrogen and progesterone are referred to as the uterine cycle. Because more people are familiar with the uterine cycle, let”s talk about that first.

The uterine cycle has three phases: menstruation, the proliferative phase, and the secretory phase. Menstruation is the shedding of the old uterine lining, the endometrium, and is commonly referred to as a woman”s period. The first day of menstruation is considered to be Day 1 of the menstrual cycle. Estrogen and progesterone levels are relatively low during this phase, which lasts about five days.

During the proliferative phase, a new uterine lining is built. A new endometrium grows on the inside of the uterus. This is under the control of estrogen, which is secreted from the ovary during this phase. The proliferative phase lasts from Day 6 to about Day 13 of the menstrual cycle.

During the secretory phase, the new uterine lining is maintained and enhanced in preparation for a possible pregnancy. New blood vessels are added, and glucose and glycogen are secreted into the lining to make it rich and nourishing. This is under the control of progesterone, which is secreted from the ovaries during this time period. The secretory phase lasts from Day 14 to the end of the cycle, Day 28. It ends when progesterone levels fall, and menstrual bleeding begins again, marking the onset of a new cycle.

The ovarian cycle also has three phases: the follicular phase, ovulation, and the luteal phase. The follicular phase begins on Day 1 of the cycle and lasts about 13 days. During this phase, FSH from the anterior pituitary gland causes the development of a follicle in the ovary. A follicle is just a maturing oocyte and its surrounding cells. As the surrounding cells divide and grow, they secrete estrogen, and as the estrogen level rises, it has its effect on the uterus, as we saw earlier.

Ovulation is the release of the oocyte from the follicle into the uterine (Fallopian) tube. It occurs on or about Day 14, and is caused by a large surge of LH from the anterior pituitary.

Some of the follicle stays behind in the ovary and, under the control of LH, matures into a structure called the corpus luteum. This marks the onset of the luteal phase. The corpus luteum secretes mostly progesterone (and some estrogen), which then has its effect on the uterus (described earlier). The corpus luteum has a natural life span of about two weeks, after which it degenerates. When it degenerates, the progesterone and estrogen levels fall, and the uterine lining degenerates and sheds off, marking the beginning of the next cycle.

But What If …

… the ovum released on Day 14 of the cycle gets fertilized and implants? Then the resulting embryo secretes a hormone called human chorionic gonadotropin (hCG). hCG has the effect of prolonging the life of the corpus luteum. If the corpus luteum lives longer, it secretes more progesterone; progesterone levels never fall, and the uterine lining never sheds off. This is commonly known as a “missed period” and is usually the first sign of a pregnancy. hCG levels stay high for the first three to four months of a pregnancy, until the placenta can take over production of progesterone.

The Menstrual Cycle: A Quick Review

• The menstrual cycle begins on Day 1, with the onset of bleeding (menstruation). Estrogen and progesterone levels are low. FSH from the anterior pituitary stimulates the growth of a follicle in the ovary (the follicular phase), which, as it grows, secretes estrogen.

• After Day 5, the rising estrogen levels stimulate the uterus to grow a new inner lining (the proliferative phase).

• By Day 14, the lining is thick. A surge in LH from the anterior pituitary gland causes the release of the oocyte and some of the follicular cells from the ovary (ovulation). LH also causes the remaining follicular cells to become the corpus luteum. The corpus luteum secretes progesterone and estrogen (the luteal phase), which further enhance the lining of the uterus (the secretory phase).

• If fertilization and implantation do not occur, the corpus luteum degenerates after about 14 days, and the drop in progesterone and estrogen causes the lining to degrade and shed off, starting the next cycle.

Quick Quiz #17

Fill in the blanks and check the appropriate boxes:

1. Progesterone is secreted during the [ secretory luteal ] phase of the ovarian cycle.

2. Sperm are produced in the

(A) vas deferens

(B) semen

(C) seminiferous tubules

(D) corpus luteum

(E) endometrium

3. The ovary is controlled by [ FSH and LH estrogen and progesterone ] from the anterior pituitary, and it secretes [ FSH and LH estrogen and progesterone ] that affect the uterus.

4. _________________________ prolongs the life of the corpus luteum if fertilization and implantation occur.

5. The _________________________ is a large duct that conducts sperm from the testes to the urethra.

6. A surge in [ FSH LH ] causes ovulation.

7. The remnants of a follicle after ovulation become the ____________.

8. [ Estrogen Progesterone ] causes the uterine lining to grow during the proliferative phase.

9. _________________________ is a nourishing fluid that carries sperm. It is secreted by glands in the male reproductive system.

10. Estrogen causes growth of the uterine lining during the [ proliferative follicular ] phase of the uterine cycle.

11. What is the lining of the uterus called?

(A) Estrogen

(B) Progesterone

(C) Follicle

(D) Endometrium

(E) Corpus luteum

Correct answers can be found in Chapter 15.


There is a lot of information concerning the development of a new human. Fortunately, you don”t need to know too much of it. Here are the basic steps and events:

1. Gametes are formed. We”ve already discussed spermatogenesis and oogenesis, so we won”t repeat that here. Remember that gametes (sperm and ovum) are haploid cells, so they contain only half the number of chromosomes in a normal, somatic cell.

2. The egg (ovum) is fertilized. The ovulated egg travels down the uterine tube. The sperm released during copulation swim up through the cervix and the uterus and meet up with the egg in the Fallopian tube. This is where fertilization occurs. At the top of the sperm is a region called the acrosome. The acrosome contains digestive enzymes that help the sperm penetrate the barriers surrounding the ovum.

3. A zygote is formed. Once the sperm has penetrated the egg plasma membrane and the sperm and egg nuclei have fused, the resulting cell is called a zygote. The zygote has a full set of chromosomes from the sperm and a full set from the ovum, so it is a diploid cell.

4. Cleavage. This is rapid mitosis with no accompanying growth. The zygote starts dividing and dividing and dividing. The first division occurs within 24 to 36 hours of fertilization. As it divides, it continues traveling down the uterine tube toward the uterus. Ultimately it becomes a solid ball of cells called a morula.

5. Implantation in the uterus. Once the morula reaches the uterus, it bumps around for a bit, then implants in the lining of the uterus. While it is bumping around, it continues to divide and starts hollowing out. The resulting structure is called a blastocyst. The blastocyst has a mass of cells on one side called the inner cell mass. This group of cells ultimately forms the embryo and all embryonic structures and membranes (like the umbilical cord, and the amniotic sac). The outer ring of cells ultimately forms part of the placenta.

6. The embryonic stage. Once the blastocyst has implanted, it enters the embryonic stage, which lasts until the eighth week of development. The embryonic stage can be divided into two phases: gastrulation and neurulation.

a) Gastrulation. During gastrulation (the first half of the embryonic stage), the inner cell mass divides into three layers, called the primary germ layers. They are the endoderm, the mesoderm, and the ectoderm, and each is responsible for producing different body structures.

Endoderm: gives rise to the inner linings of the respiratory system, the digestive system, the reproductive system, and the urinary system; forms glandular organs (such as the liver, pancreas, and salivary glands).

Mesoderm: gives rise to the “middle” structures, such as bones, blood vessels, muscles, the heart, and non-glandular organs (the kidneys, the ureters, the gonads, etc.)

Ectoderm: forms external structures (skin, hair, nails, etc.), the linings of the mouth and anus, and all nervous system structures (brain, cord, nerves, eyes, etc.)

b) Neurulation. During neurulation (the second half of the embryonic stage) the organs of the nervous system are formed. To call it neurulation, however, is a little misleading. Not only does the nervous system develop, but every other organ in the body is formed during this period. This is called organogenesis. By the time the embryonic period is finished, all the organs and structures that are supposed to be there are formed. From this point on, the organs and structures simply mature and grow larger.

7. The fetal stage. This stage lasts from the end of the embryonic stage (about eight weeks of development) until birth. During the fetal stage, the baby simply grows and matures. No new organs are formed during this stage.

Here”s a summary of the stages of human development:

GametesFertilizationZygoteCleavageMorulaBlastocystImplantation in the uterusGastrulationNeurulationFetusBirth

Not Just for Humans

Other vertebrates reproduce sexually as well. Fish and amphibians, for the most part, have external fertilization. The embryo develops in a nonwaterproof egg in an aqueous environment. Thousands of offspring are produced, and parental care is not common with fish or amphibians.

Turtles, snakes, and lizards have internal fertilization and lay eggs that are protected by a watertight shell. There is little parental care with these animals.

Crocodiles, alligators, and birds have internal fertilization and lay eggs with a hard, waterproof shell. There is some degree of parental care with these animals. Most mammals exhibit a prolonged period of parental care.

The Extraembryonic Membranes

“Extraembryonic” simply means “outside the embryo.” There are four extraembryonic membranes: the yolk sac, the amnion, the allantois, and the chorion. For clarity, as we discuss the human membranes, we will compare them to a chick embryo.

The yolk sac surrounds the yolk of an egg. Egg yolk is essentially food for a developing embryo. Human eggs have very little yolk, because human embryos (and all placental mammals) develop inside the mother”s body and receive their nutrition from a placenta. However, bird and reptile embryos develop outside the mother”s body in an egg, and the nutrition that supports their entire embryonic development must be contained within that egg. Consequently, the eggs of birds and reptiles contain a lot of yolk. Human yolk is the source of the first blood cells.

The amnion is a clear membrane that surrounds the developing embryo and is filled with a clear, watery fluid (the amniotic fluid). This fluid acts as a shock absorber to protect the embryo from physical damage.

The allantois, in humans, ultimately becomes the umbilical cord, which connects the embryo to the placenta. In birds and reptiles, the allantois forms a disposal site for solid wastes.

The chorion is the outermost membrane, and in humans it forms the embryo”s part of the placenta. It encloses all the other membranes. In birds and reptiles, the chorion lines the inside of the shell and still encloses all the other membranes.

Quick Quiz #18

Fill in the blanks and check the appropriate boxes:

1. Fertilization takes place in the

(A) uterus

(B) fallopian tube

(C) ovary

(D) blastocyst

(E) morula

2. The developmental stage marked by a series of rapid mitotic divisions is called

(A) gametes

(B) fertilization

(C) zygote

(D) cleavage

(E) morula

3. The human eye develops from [ mesoderm ectoderm ].

4. Blood vessels develop from [ endoderm mesoderm ].

5. The first eight weeks of development are called the

(A) fetal stage

(B) gastrulation

(C) embryonic stage

(D) zygote stage

(E) gamete stage

6. The membrane that most directly surrounds the embryo is the [ chorion amnion ].

7. Organogenesis occurs during [ neurulation gastrulation ].

8. Implantation occurs [ before after ] cleavage.

9. A blastocyst forms [ before after ] a morula.

10. Neurulation occurs [ before after ] the fetal stage.

11. The _________________________ is the region at the top of the sperm that contains digestive enzymes to help the sperm penetrate the ovum.

12. The nervous system develops from [ endoderm ectoderm ].

13. The kidneys develop from [ mesoderm endoderm ].

Correct answers can be found in Chapter 15.

Key Words

nervous system







resting membrane potential

sodium-potassium pump


leak channels

voltage-gated channels

threshold potential

sodium voltage-gated channels

potassium voltage-gated channels

action potential



Schwann cells

myelin sheath

nodes of Ranvier

saltatory conduction

refractory period




synaptic cleft




central nervous system

peripheral nervous system

sensory neurons

motor neurons


spinal cord





somatic nervous system

autonomic nervous system

sympathetic division

fight or flight


parasympathetic division

resting and digesting

vertebrate group




endocrine system


peptide hormones

steroid hormones

pituitary gland

anterior pituitary gland

growth hormone (GH)

cell-turnover rate

thyroid-stimulating hormone (TSH)

adrenocorticotropic hormone (ACTH)

follicle-stimulating hormone (FSH)

luteinizing hormone (LH)


posterior pituitary gland


antidiuretic hormone (ADH)







parathyroid hormone


adrenal glands

adrenal medulla

adrenal cortex





sex steroids


islet cells














red blood cells



white blood cells




helper T-cells

killer T-cells





universal recipient

universal donor

blood vessels






lymphatic system

lymph nodes





pulmonary circuit

systemic circuit

anterior vena cava

posterior vena cava

pulmonary artery

pulmonary veins


atrioventricular valves

semilunar valves

sinoatrial (SA) node



systolic pressure

diastolic pressure



gas exchange

conduction zone




right and left primary bronchi



passive diffusion





alimentary canal

accessory organs





salivary glands






small intestine








portal veins

hepatic portal system

large intestine





vitamin A

vitamin B

vitamin C

vitamin D

vitamin E

vitamin K






uric acid




Bowman”s capsule





proximal convoluted tubule

loop of Henle

distal convoluted tubule


collecting duct


angiotensin II






cardiac muscle

smooth muscle

skeletal muscle






muscle cell

muscle fiber



sliding filament theory











seminiferous tubules

vas deferens


uterine cycle



proliferation phase

secretory phase

ovarian cycle

follicular phase



corpus luteum

luteal phase

human chorionic gonadotrophin








embryonic stage







fetal stage

yolk sac





• The human body is composed of eleven different organ systems. Each organ system has its own particular function.

• The nervous system carries impulses between body parts.

• The endocrine system controls the body through the use of hormones.

• The circulatory system transports oxygen, carbon dioxide, glucose, hormones, waste products, and other materials around the body. It consists of the heart, a series of blood vessels, and blood.

• The lymphatic system recaptures and filters fluids from the tissues and returns it to the blood stream.

• The respiratory system works to exchange oxygen and carbon dioxide with the blood. It also helps regulate body pH.

• The digestive system manages the process of eating, digesting, and absorbing food.

• The urinary system eliminates nitrogenous waste products from food digestion. It also helps in water and electrolyte balance and blood pressure regulation.

• The skeletal system holds a body together in some regular shape, protects various organs, is a mineral storage site, and produces blood cells.

• The muscular system works with the skeletal system to support, protect, and move the body.

• The skin supports and protects the organs in the body and helps in thermoregulation.

• The reproductive system is responsible for the passing of genetic information to future generations.