MCAT Biology Review

Chapter 5: The Endocrine System

Conclusion

The endocrine system is unique because its organs are not anatomically related. Hormones are produced in a wide variety of locations and can have wide-reaching effects throughout the entire organism. The endocrine system allows for integration and execution of the homeostatic parameters that are necessary to ensure proper functioning of the body as a whole. For example, we learned that calcium is maintained within a narrow concentration range in the plasma by the antagonistic actions of calcitonin and parathyroid hormone (and vitamin D). Each hormone manipulates the steady state of the organism. As you continue your study of the human body, you will find that the endocrine system participates in every system of the body by regulating fuel metabolism, bloodflow, growth, and development.

In the last section of this chapter, there was a small note on erythropoietin, the hormone that stimulates the production of red blood cells in the bone marrow. Don’t interpret the small amount of real estate afforded to this hormone as an indication that it is unimportant to the functioning of the body. In fact, every cell of the body (except red blood cells themselves) needs a constant supply of oxygen to accomplish its function. Our bodies acquire this oxygen through the respiratory system and then carry the oxygen to all of the tissues using the circulatory system. In the next two chapters, we will explore each of these systems separately. Recognize, however, that this division is artificial; indeed, like the body as a whole, the respiratory and circulatory systems are indivisible as they serve a common function: providing oxygen for every working cell in the body.

Concept Summary

Mechanisms of Hormone Action

·        Endocrine signaling involves the secretion of hormones directly into the bloodstream. The hormones travel to distant target tissues, where they bind to receptors and induce a change in gene expression or cell function.

·        Peptide hormones are composed of amino acids and are derived from larger precursors that are cleaved during posttranslational modification.

o   Peptide hormones are polar and cannot pass through the plasma membrane.

o   These hormones bind to extracellular receptors, where they trigger the transmission of a second messenger.

o   Each step of the signaling cascade can demonstrate amplification of the signal.

o   Peptide hormones usually have rapid onset but are short-lived.

o   These hormones travel freely in the bloodstream and do not require a special carrier.

·        Steroid hormones are derived from cholesterol.

o   Steroid hormones are minimally polar and can pass through the plasma membrane.

o   These hormones bind to intracellular or intranuclear receptors, where they promote conformational change and bind to DNA, affecting the transcription of a particular gene.

o   Steroid hormones usually have slow onset but are long-lived.

o   These hormones cannot dissolve in the bloodstream and must be carried by specific proteins.

·        Amino acid-derivative hormones are modified amino acids.

o   Their chemistry shares some features with peptide hormones and some features with steroid hormones; different amino acid-derivative hormones share different features with these other hormone classes.

o   Common examples are epinephrine, norepinephrine, triiodothyronine, and thyroxine.

·        Hormones can be classified by their target tissues.

o   Direct hormones have major effects in non-endocrine tissues.

o   Tropic hormones have major effects in other endocrine tissues.

Endocrine Organs and Hormones

·        The hypothalamus is the bridge between the nervous and endocrine systems.

o   The release of hormones from the hypothalamus is mediated by a number of factors, including projections from other parts of the brain, chemo- and baroreceptors in the blood vessels, and negative feedback from other hormones.

o   In negative feedback, the final hormone (or product) of a pathway inhibits hormones (or enzymes) earlier in the pathway, maintaining homeostasis.

o   The hypothalamus stimulates the anterior pituitary gland through paracrine release of hormones into the hypophyseal portal system, which directly connects the two organs.

o   Gonadotropin-releasing hormone (GnRH) promotes the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

o   Growth hormone-releasing hormone (GHRH) promotes the release of growth hormone.

o   Thyroid-releasing hormone (TRH) promotes the release of thyroid-stimulating hormone (TSH).

o   Corticotropin-releasing factor (CRF) promotes the release of adreno- corticotropic hormone (ACTH).

o   Prolactin-inhibiting factor (PIF or dopamine) inhibits the release of prolactin.

o   Interactions with the posterior pituitary occur via the axons of nerves in the hypothalamus. Antidiuretic hormone (ADH or vasopressin) and oxytocin are synthesized in the hypothalamus and then travel down these axons to the posterior pituitary, where they are released into the bloodstream.

·        The anterior pituitary releases hormones in response to stimulation from the hypothalamus. Four of these (FSH, LH, ACTH, and TSH) are tropic hormones, while three (prolactin, endorphins, and growth hormone) are direct hormones.

o   Follicle-stimulating hormone (FSH) promotes the development of ovarian follicles in females and spermatogenesis in males.

o   Luteinizing hormone (LH) promotes ovulation in females and testosterone production in males.

o   Adrenocorticotropic hormone (ACTH) promotes synthesis and release of glucocorticoids from the adrenal cortex.

o   Thyroid-stimulating hormone (TSH) promotes synthesis and release of triiodothyronine and thyroxine from the thyroid.

o   Prolactin promotes milk production.

o   Endorphins decrease perception of pain and can cause euphoria.

o   Growth hormone (GH) promotes growth of bone and muscle and shunts glucose to these tissues. It raises blood glucose concentrations.

·        The posterior pituitary releases two hormones produced in the hypothalamus.

o   Antidiuretic hormone (ADH or vasopressin) is secreted in response to low blood volume or increased blood osmolarity and increases reabsorption of water in the collecting duct of the nephron, increasing blood volume and decreasing blood osmolarity.

o   Oxytocin is secreted during childbirth and promotes uterine contractions. It also promotes milk ejection and may be involved in bonding behavior. It is unusual in that it has a positive feedback loop, not negative.

·        The thyroid is located at the base of the neck in front of the trachea; it produces three key hormones.

o   Triiodothyronine (T3) and thyroxine (T4) are produced by follicular cells and contain iodine. They increase basal metabolic rate and alter the utilization of glucose and fatty acids. Thyroid hormones are required for proper neurological and physical development in children.

o   Calcitonin is produced by parafollicular (Ccells. It decreases plasma calcium concentration by promoting calcium excretion in the kidneys, decreasing calcium absorption in the gut, and promoting calcium storage in bone.

·        The parathyroid glands release parathyroid hormone (PTH), which increases blood calcium concentration.

o   PTH decreases excretion of calcium by the kidneys and increases bone resorption directly to increase blood calcium concentration.

o   PTH activates vitamin D, which is necessary for calcium and phosphate absorption from the gut.

o   PTH promotes resorption of phosphate from bone and reduces reabsorption of phosphate in the kidney, but vitamin D promotes absorption of phosphate from the gut; these two effects on phosphate concentration somewhat cancel each other.

·        The adrenal cortex produces three classes of steroid hormones.

o   Glucocorticoids such as cortisol and cortisone increase blood glucose concentration, reduce protein synthesis, inhibit the immune system, and participate in the stress response. Glucocorticoid release is stimulated by ACTH.

o   Mineralocorticoids such as aldosterone promote sodium reabsorption in the distal convoluted tubule and collecting duct, thus increasing water reabsorption. Aldosterone also increases potassium and hydrogen ion excretion. It is regulated by the renin–angiotensin–aldosterone system, not ACTH.

o   Cortical sex hormones include androgens (like testosterone) and estrogens in both males and females.

·        The adrenal medulla is derived from the nervous system and secretes catecholamines into the bloodstream.

o   Catecholamines include epinephrine and norepinephrine, which are involved in the fight-or-flight (sympathetic) response.

o   These hormones promote glycogenolysis, increase the basal metabolic rate, increase heart rate, dilate the bronchi, and alter blood flow.

·        The endocrine pancreas produces hormones that regulate glucose homeostasis.

o   Glucagon is produced by α-cells and raises blood glucose levels by stimulating protein and fat degradation, glycogenolysis, and gluconeogenesis.

o   Insulin is produced by β-cells and lowers blood glucose levels by stimulating uptake by cells and anabolic processes, like glycogen, fat, and protein synthesis.

o   Somatostatin is produced by δ-cells and inhibits insulin and glucagon secretion.

·        The gonads produce hormones that are involved in development and maintenance of the reproductive systems and secondary sex characteristics.

o   The testes secrete testosterone.

o   The ovaries secrete estrogen and progesterone.

·        The pineal gland releases melatonin, which helps to regulate circadian rhythms.

·        Other organs may release hormones, even if they are not primarily considered part of the endocrine system.

o   Cells in the stomach and intestine produce hormones like secretingastrin, and cholecystokinin.

o   The kidneys secrete erythropoietin, which stimulates bone marrow to produce erythrocytes (red blood cells) in response to low oxygen levels in blood.

o   The atria of the heart secrete atrial natriuretic peptide (ANP), which promotes excretion of salt and water in the kidneys in response to stretching of the atria (high blood volume).

o   The thymus secretes thymosin, which is important for proper T-cell development and differentiation.

Answers to Concept Checks

·        5.1

1.     

Criterion

Peptide Hormones

Steroid Hormones

Chemical precursor

Amino acids (polypeptides)

Cholesterol

Location of receptor

Extracellular (cell membrane)

Intracellular or intranuclear

Mechanism of action

Stimulates a receptor (usually a G protein-coupled receptor), affecting levels of second messengers (commonly cAMP). Initiates a signal cascade.

Binds to a receptor, induces conformational change, and regulates transcription at the level of the DNA.

Method of travel in the bloodstream

Dissolves and travels freely

Binds to a carrier protein

Rapidness of onset

Quick

Slow

Duration of action

Short-lived

Long-lived

2.    Amino acid-derivative hormones are made by modifying amino acids, such as the addition of iodine to tyrosine (in thyroid hormone production).

3.    Direct hormones are secreted into the bloodstream and travel to a target tissue, where they have direct effects. Tropic hormones cause secretion of another hormone that then travels to the target tissue to cause an effect.

·        5.2

1.     

Hypothalamic Releasing Hormone

Hormone(s) from Anterior Pituitary

Target Organ

Hormone(s) Released by Target Organ

*Note that a decrease in dopamine from the hypothalamus promotes prolactin secretion.

Gonadotropin-releasing hormone (GnRH)

Follicle-stimulating hormone (FSH) and luteinizing hormone (LH)

Gonads (testes or ovaries)

Testosterone (testes) or estrogen and progesterone (ovaries)

Corticotropin-releasing factor (CRF)

Adrenocorticotropic hormone (ACTH)

Adrenal cortex

Glucocorticoids (cortisol and cortisone)

Thyroid-releasing hormone (TRH)

Thyroid-stimulating hormone (TSH)

Thyroid

Triiodothyronine (T3), thyroxine (T4)

Dopamine

Prolactin*

Breast tissue

N/A

Growth hormone-releasing hormone (GHRH)

Growth hormone

Bone, muscle

N/A

2.    Calcitonin from the parafollicular (C) cells of the thyroid decreases blood calcium concentration. Parathyroid hormone from the parathyroid glands increases blood calcium concentration.

3.    The adrenal medulla synthesizes catecholamines, including epinephrine and norepinephrine.

4.    Glucagon from the α-cells of the pancreas increases blood glucose concentration. Insulin from the β-cells of the pancreas decreases blood glucose concentration.

5.    Antidiuretic hormone (ADH or vasopressin) from the hypothalamus (released by the posterior pituitary) increases blood volume and decreases blood osmolarity. Aldosterone from the adrenal cortex increases blood volume with no effect on blood osmolarity. Atrial natriuretic peptide (ANP) from the heart decreases blood volume with no effect on blood osmolarity.

Shared Concepts

·        Behavioral Sciences Chapter 5

o   Motivation, Emotion, and Stress

·        Biochemistry Chapter 3

o   Nonenzymatic Protein Function and Protein Analysis

·        Biochemistry Chapter 8

o   Biological Membranes

·        Biochemistry Chapter 12

o   Bioenergetics and Regulation of Metabolism

·        Biology Chapter 4

o   The Nervous System

·        Biology Chapter 10

o   Homeostasis