CHAPTER
42
HORMONES AND ENDOCRINE SYSTEMS
This chapter has been entirely rearranged from the previous edition. The categories of hormones are described as
are the biochemical activities elicited by the individual hormones. Each glands’ hormones are described
individually. Many endocrine-related disorders are described, including a detailed discussion of diabetes mellitus.
A Science Focus box discusses “Isolation of Insulin.”
Chapter Outline
42.1 Endocrine Glands
1. The endocrine system functions differently from the nervous system.
2. An endocrine system consists of glands that coordinate body activities through hormones.
3. In contrast to the exocrine glands, which have ducts leading to other organs or outside the body, the
endocrine glands secrete their products into the bloodstream, which delivers them throughout the body.
4. Chemical signals are used between individuals, between body parts, and between cells.
5. Both the nervous system and the endocrine system rely on negative feedback mechnisms.
A. Hormones Are Chemical Signals
1. Chemical signals are a means of communication between cells, body parts, or individuals.
2. In general, they affect the metabolism of the cells that have receptors for them.
3. Most hormones act at a distance between body parts, traveling through the bloodstream from the gland
to the target cell.
4. Some hormones (e.g., prostaglandins, growth factors) are local hormones—they affect only
neighboring cells.
5. Pheromones are environmental signals that act at a distance between individual organisms.
a. Axillary secretions of men and women may have some effect on other people; women may
synchronize their menstrual cycles with co-workers and some women may prefer the axillary odor
of men with a different plasma membrane protein.
B. The Action of Hormones
1. The Action of Peptide Hormones
a. The term peptide hormone is used to include hormones that are peptides, proteins, glycoproteins,
and modified amino acids.
b. Epinephrine is a peptide hormone that binds to a receptor protein in the target cell’s plasma
membrane; a relay system leads to conversion of ATP to cyclic AMP (cAMP).
c. Cyclic adenosine monophosphate (cAMP) is made from ATP; it has one phosphate group
attached to adenosine at two locations.
d. Peptide hormones are the first messenger; cAMP and calcium are the second messenger.
e. The peptide hormone does not enter the cell; the second messenger sets an enzyme cascade in
motion.
2. The Action of Steroid Hormones
a. Steroid hormones are lipids and cross cell membranes freely; they do not bind to plasma
membrane receptors.
b. Inside the cytoplasm or a nucleus, steroid hormones (e.g., estrogen, progesterone) bind to a
specific receptor.
c. The hormone-receptor complex binds to DNA, resulting in activation of genes (transcription) that
produce enzymes (translation).
d. Steroids act more slowly than peptide hormones because it takes more time to synthesize new
proteins than to activate enzymes already present in cells; however, their action lasts longer.
42.2 Hypothalamus and Pituitary Gland
1. The hypothalamus regulates the internal environment through the autonomic system.
a. It controls heartbeat, temperature, water balance, as well as glandular secretions of the pituitary
gland.
2. The pituitary gland (hypophysis) is connected to the hypothalamus by a stalklike structure.
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a. It is about 1 cm in diameter and lies just below the hypothalamus.
b. It is comprised of two portions: the posterior pituitary and the anterior pituitary.
A. Posterior Pituitary
1. Neurons in the hypothalamus called neurosecretory cells produce antidiuretic hormone (ADH) and
oxytocin, which pass through axon endings in the posterior pituitary and are stored until released.
2. Antidiuretic hormone (ADH) promotes reabsorption of water from the collecting ducts in the
kidneys.
a. Nerve cells in the hypothalamus determine when the blood is too concentrated; ADH is released
and the kidneys respond by reabsorbing water.
b. As the blood becomes dilute, ADH is no longer released; this is a case of negative feedback.
c. Inability to produce ADH causes diabetes insipidus (watery urine), in which the individual
produces copious amounts of urine and a resultant loss of ions from the blood.
3. Oxytocin is also made in the hypothalamus and stored in the posterior pituitary.
a. Oxytocin stimulates uterine muscle contraction in response to uterine wall nerve impulses.
b. It also stimulates the release of milk from mammary glands.
c. This positive feedback mechanism increases intensity; such positive feedback does not maintain
homeostasis.
d. Oxytocin also may play a role in the propulsion of semen through the male reproductive tract.
B. Anterior Pituitary
1. Stimulation by the hypothalamus controls the release of anterior pituitary hormones through a portal
system consisting of two capillary systems connected by a vein.
2. The hypothalamus produces hypothalamic-releasing and hypothalamic-inhibiting hormones which
pass to the anterior pituitary by this portal system.
a. Thyroid-releasing hormones released from the hypothalamus act on cells in the anterior pituitary
to stimulate the production and secretion of a specific hormone.
b. Thyroid-inhibiting hormones produced in and released from the hypothalamus act on cells in the
anterior pituitary to inhibit the production and secretion of a specific hormone.
3. The anterior pituitary produces six different hormones.
a. Three of these anterior pituitary hormones affect other glands.
1) The thyroid-stimulating hormone (TSH) stimulates the thyroid to produce and secrete
thyroxin.
2) Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to release cortisol.
3) Gonadotropic hormones (follicle-stimulating hormone [FSH] and luteinizing hormone
[LH]) act on the gonads (ovaries and testes) to secrete sex hormones.
b. The other three hormones do not affect other glands.
1) Prolactin (PRL) is produced in quantity only after childbirth.
a) Prolactin causes the mammary glands to produce milk.
b) It also plays a role in carbohydrate and fat metabolism.
2) Melanocyte-stimulating hormone (MSH) causes skin color changes in fishes, amphibians,
and reptiles with melanophores, special skin cells.
3) Growth hormone (GH or somatotropic hormone)
a) GH promotes skeletal and muscular growth.
b) GH acts to stimulate the transport of amino acids into cells and to increase the activity of
ribosomes.
c) GH promotes fat metabolism rather than glucose metabolism.
d) Too little GH during childhood makes an individual a pituitary dwarf.
e) Too much forms a giant; life expectancy is less because GH affects blood glucose levels
and promotes diabetes mellitus.
f) The overproduction of GH in adults results in acromegaly; since long bone growth is no
longer possible, only the feet, hands, and face grow.
42.3 Other Endocrine Glands and Hormones
A. Thyroid and Parathyroid Glands
1. The thyroid gland is in the neck and attached to the trachea just below the larynx.
2. The parathyroid glands are embedded in the posterior surface of the thyroid gland.
3. Thyroid Gland
a. The thyroid gland is the largest endocrine gland.
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b.
The two hormones produced by the many follicles of the thyroid gland both contain iodine.
1) Thyroxine (T4) contains four iodine atoms.
2) Triiodothyronine (T3) contains three iodine atoms.
c. Lack of iodine causes enlargement of the thyroid (simple goiter).
1) Simple goiter is easily prevented by supplementing iodine intake in salt.
d. Thyroid hormones increase the metabolic rate; there is no one target organ—all organs respond.
e. Cretinism (congenital hypothyroidism) occurs in individuals who have suffered from low thyroid
function since birth.
1) Affected individuals are short and stocky and have had hypothyroidism since infancy.
2) Thyroid treatment helps but unless it is begun in the first two months, mental retardation can
occur.
f.
Myxedema is hypothyroidism in adults; thyroid hormones can restore normal function.
g. Hyperthyroidism (Graves disease) occurs when the thyroid gland is enlarged or overactive.
1) The eyes protrude because of edema in the eye socket tissue; this is called exophthalmic
goiter.
2) Removal or destruction of some thyroid tissue by surgery or radiation often cures the
condition.
h. The thyroid gland also produces calcitonin.
1) Calcitonin lowers the calcium level in the blood and increases deposits in the bone by
reducing osteoclasts.
2) Calcitonin is also necessary for blood clotting.
3) If blood calcium is lowered to normal, the release of calcitonin is inhibited.
4) Too low calcium levels stimulate the release of parathyroid hormone (PTH) by the
parathyroid glands.
4. Parathyroid Glands
a. Parathyroid glands produce parathyroid hormone (PTH).
1) Under the influence of PTH, the calcium level in blood increases and the phosphate level
decreases.
2) PTH stimulates the absorption of Ca2+ by activating vitamin D, the retention of Ca2+ (and
excretion of phosphate) by the kidneys, and demineralization of bone by promoting the
activity of osteoclasts.
3) When the blood calcium level reaches the right level, the parathyroid glands no longer
produce PTH.
4) If PTH is not produced in response to low blood Ca 2+, tetany results because the Ca2+ plays
an important role in both nerve conduction and muscle contraction.
b. In tetany, the body shakes from continuous muscle contraction due to the increased excitability of
nerves that fire spontaneously and without rest; this condition may be due to hypoparathyroidism.
c. In hyperparathyroidism, the abnormally high blood calcium levels can cause the bones to be soft
and fragile, and the individual to be prone to kidney stones.
B. Adrenal Glands
1. Two adrenal glands sit atop the kidneys.
2. Each gland consists of two parts: an outer adrenal cortex and an inner adrenal medulla.
3. The cortex and medulla have no physiological connection between them.
4. The hypothalamus exerts control over both portions.
a. Nerve impulses travel via the brain stem to the spinal cord to sympathetic nerve fibers to the
medulla.
b. The hypothalamus uses ACTH-releasing hormone to control the anterior pituitary’s secretion of
ACTH.
5. Adrenal hormones increase during times of physical and emotional stress.
6. Both epinephrine and norepinephrine are produced by the adrenal medulla.
7. Both hormones bring about body changes corresponding to an emergency.
a. The blood glucose level rises and metabolic rate increases.
b. The bronchioles dilate and breathing rate increases.
c. Blood vessels to the digestive tract and skin constrict; those to the skeletal muscles dilate.
d. The cardiac muscle contracts more forcefully and the heart rate increases.
8. The adrenal cortex hormones provide a sustained response to stress.
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a.
The adrenal cortex secretes two types of hormones: glucocorticoids and mineralocorticoids.
1) Glucocorticoids help to regulate blood glucose levels.
2) Mineralocorticoids regulate the levels of minerals in the blood.
3) The adrenal cortex also secretes a small amount of both male and female sex hormones in
both sexes.
9. Glucocorticoids: Cortisol is a biologically significant glucocorticoid.
a. Cortisol promotes the breakdown of muscle protein into amino acids taken up by the liver from
the blood.
b. Cortisol breaks down fatty acids rather than carbohydrates; cortisol therefore raises blood glucose
levels.
c. Cortisol counteracts the inflammatory response; it helps medicate arthritis and bursitis.
10. Mineralocorticoids: Aldosterone is the most important of the mineralocorticoids.
a. The primary target organ is the kidney where it promotes the reabsorption of Na + and the excretion
of K+.
b. Mineralocorticoid secretion is controlled by the renin-angiotensin-aldosterone system
1) Under low blood volume and sodium levels, the kidneys secrete renin.
2) The enzyme renin converts the plasma protein angiotensinogen to angiotensin I; this becomes
angiotensin II by a converting enzyme in the lungs.
3) Angiotensin II stimulates the adrenal cortex to release aldosterone.
4) Angiotensin I constricts the arterioles directly; aldosterone causes the kidneys to absorb
calcium.
5) When the blood sodium rises, water is reabsorbed as the hypothalamus secretes ADH; blood
pressure then increases to normal.
c. Atrial natriuretic hormone (ANH) causes the excretion of sodium.
1) When the atria of the heart are stretched due to increased blood volume, cardiac cells release
ANH.
2) ANH inhibits the secretion of renin by the kidneys and the secretion of aldosterone from the
adrenal cortex.
3) When sodium is excreted, so is water; the blood volume and pressure then return to normal.
11. Malfunction of the Adrenal Cortex
a. Low levels of adrenal cortex hormones (hyposecretion) result in Addison disease.
1) When ACTH is in excess, like MSH, it can lead to the buildup of melanin and a bronzing of
the skin.
2) The lack of cortisol results in low glucose levels; a stressed person has insufficient energy.
3) The lack of aldosterone drops blood sodium levels; a person then has low blood pressure and
dehydration.
4) Left untreated, Addison disease can be fatal.
b. High levels of adrenal cortex hormones from hypersecretion result in Cushing syndrome.
1) Excess cortisol causes a tendency toward diabetes mellitus.
2) Muscular protein then decreases and subcutaneous fat forms an obese trunk but normal arms
and legs.
C. Pancreas
1. The pancreas lies transversely in the abdomen between the kidneys and near the duodenum.
2. The pancreas is composed of two types of tissue.
a. Exocrine tissue produces and secretes digestive juices into the small intestine by way of ducts.
b. Endocrine tissues called pancreatic islets (islets of Langerhans) produce insulin and glucagon.
3. All body cells utilize glucose; therefore its level must be closely regulated.
4. Insulin is secreted when the blood glucose level is high after eating; insulin has three actions.
a. Insulin stimulates liver, fat, and muscle cells to take up glucose.
b. Insulin stimulates the liver and muscles to store glucose as glycogen.
c. Insulin promotes buildup of fats and proteins and inhibits their use as an energy source.
5. Glucagon is secreted between meals in response to low blood glucose level.
a. Liver and adipose tissue are the main targets.
b. Adipose tissue cells break the fat into glycerol and fatty acids.
c. The liver uses glycerol and fatty acids as substrates for glucose, raising the blood glucose levels.
6. Diabetes Mellitus
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a.
Diabetes mellitus is a fairly common disease where the body cells do not take up or metabolize
sugar.
b. Blood glucose level becomes high enough for the kidneys to excrete glucose; therefore this is
detected by a urine test.
c. The liver is not storing glucose as glycogen and cells are not utilizing glucose for energy.
d. Since carbohydrate is not being metabolized, the body breaks down protein and fat for energy.
e. Ketones then build up in blood; the resulting reduced blood volume and acidosis can lead to coma
and death.
f.
In type 1 (insulin-dependent) diabetes, the pancreas does not produce insulin.
1) A viral infection can cause cytotoxic T cells to destroy pancreatic islets.
2) This is treated with a daily administration of insulin; an overdose or lack of eating results in
hypoglycemia.
3) The brain also has constant sugar requirements; low blood sugar can result in
unconsciousness.
4) An immediate intake of sugar is a simple and effective treatment.
7. Of 18 million diabetics in the U.S., most have type 2 (noninsulin-dependent) diabetes.
a. This form of diabetes usually occurs in obese and inactive individuals of any age.
b. The pancreas does produce insulin but live muscle cells do not respond to it.
c. Initially, this is a result of cells lacking the receptors for insulin.
d. Untreated, type II diabetes can have serious symptoms: blindness, kidney disease, circulatory
disorders, strokes, etc.
e. A low fat diet and regular exercise help; oral drugs can make cells more sensitive to insulin or
stimulate higher levels of insulin production by the pancreas.
D. Testes and Ovaries
1. The testes located in the scrotum function as gonads and produce androgens (e.g., testosterone).
a. Testosterone is the male sex hormone.
b. It stimulates the development of male secondary sex characteristics: large vocal cords, pubic hair,
etc.
c. Testosterone is largely responsible for the sex drive.
d. Anabolic steroids are supplemental testosterone or similar chemicals with serious side effects.
e. Testosterone also affects sweat glands, expression of baldness genes, and other effects.
2. The ovaries, located in the pelvic cavity, produce the female sex hormones estrogens and
progesterone.
a. Estrogens secreted at puberty stimulate the maturation of ovaries and other sexual organs.
b. Estrogen is necessary for oocyte development.
c. It is responsible for the development of female secondary sex characteristics: a layer of fat beneath
the skin, a larger pelvic girdle, etc.
d. Estrogen and progesterone are required for breast development and the regulation of the uterine
cycle.
E. Pineal Gland
1. The pineal gland, located in the brain, produces melatonin, primarily at night.
2. The pineal gland and melatonin help establish circadian rhythms, 24-hour physiological cycles.
3. The pineal gland may also be involved in human sexual development; children in whom a brain tumor
has destroyed the pineal gland experience puberty earlier.
F. Thymus Gland
1. The thymus gland is a lobular gland that lies just beneath the sternum in the upper thoracic cavity.
2. It reaches its largest size and is most active during childhood; with age, it shrinks and becomes fatty.
3. Some lymphocytes that originate in the bone marrow pass through the thymus and change into T
lymphocytes.
4. The thymus produces thymosins, which aid in the differentiation of T cells and may stimulate immune
cells.
G. Other Hormones
1. Leptin is a peptide hormone secreted by adipose tissue.
a. Its main function is its role in the feedback control of appetite; it can suppress appetite.
2. Erythropoietin (EPO) is a peptide hormone produced in the kidneys.
a. It stimulates the production of red blood cells and speeds up their maturation.
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3.
Local hormones such as prostaglandins are produced by certain cells and act on neighboring cells.
a. Prostaglandins are potent chemical signals which have several known functions.
b. They cause the contraction of uterine muscle, and they mediate the effects of pyrogens (chemicals
believed to affect the temperature regulatory center of the brain).
c. Certain prostaglandins reduce gastric secretions, others lower blood pressure.
d. Aspirin reduces temperature and controls pain because of its effect on prostaglandins.
Ninth Edition Changes
This chapter now begins with an overview of the endocrine system, which includes a contrast between hormone and
nervous signaling. An in-depth look at hormone signaling follows. The review of endocrine glands and their
hormones includes an updated discussion of diabetes mellitus.
Lecture Enrichment Ideas
Experience Base: Many students will have an interest in this section due to hormone involvement in sexual
development, and awareness of some of the abnormalities that result from faulty hormone activity. Abuse of
steroids and “blood doping” may be understood by many students. It is likely, though, that there will be more than a
few misconceptions concerning the pineal gland, melatonin and circadian rhythms.
1.
Describe the differences in actions of nonsteroid hormones versus steroid hormones as a function of the
kind of chemicals involved, with peptides unable to pass easily through the plasma membrane, while lipidbased steroids easily diffuse through the plasma membrane.
2.
Explain how nonsteroid and steroid hormones activate proteins present in the cytoplasm, as opposed to
stimulating the production of new hormones. This affects the length of time required for the response and
the stability of the response.
3.
Discuss the mechanism of control of some kinds of endocrine functions (i.e., receptors in the hypothalamus
respond to the presence or absence of the final hormone in the sequence, thus turning on or off the
regulatory hormones that stimulate the anterior pituitary to release stimulatory hormones).
4.
Discuss how the nervous and endocrine systems interact in regulating the homeostatic mechanisms of the
body. The innervation of the adrenal medulla and the action of the brain as a gland serve to blur the lines
between what were once considered quite different control systems.
5.
The quantity and duration of hormones, as well as the time for feedback mechanisms to work, are
important considerations. Lecture question: Does a blood recipient get a dosage of hormones in a blood
transfusion? A male who receives a blood transfusion from a female donor does get some additional
estrogen in the plasma but the quantity of blood is a small proportion of his total blood volume, and this
diluted hormone level is not sustained because there are no ovaries producing more estrogen. His own
adrenal glands would provide more estrogens over time in his blood stream naturally.
6.
Blood-borne hormones wash throughout the body and are recognized by target tissues only as they “pass
by.” Stress that there is no intent or “purposefulness” in the movement of chemicals by physical properties.
However, the close placement and related capillary circulation of some endocrine organs to target tissues
can speed the process.
7.
Classes generally cover organ systems organ-by-organ as if they were distinct operators on an assembly
line. However, many organs must coordinate with others and hormones are a central part of this
coordination. Lecture question: What would happen if the gall bladder didn’t “know” when fat was
moving onward from the stomach, or the production of breast milk was not in some way associated with
the contractions of the uterus in childbirth?
8.
Why is the hypothalamus and pituitary buried so deep and protected at the base of the brain? Why are so
many critical functions controlled from this one center?
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9.
Syntocin is a synthetic oxytocin; note how this is a very useful hormone when a physician finds that a
pregnancy is overdue and problems of delayed childbirth (e.g. pre-eclampsia) begin to escalate.
10.
Students may have seen huge laboratory rats with growth hormone irregularities. There are a growing
number of U.S. students and professors who are heavier than average, but otherwise “normal.” What
symptoms would be expected if this obesity was hormonal?
11.
A network newscaster reporting immediately after the Chernobyl nuclear plant disaster reported that
citizens in northern Europe were taking iodine tablets “to prevent radioactive iodine fallout from entering
their body.” The newscaster got the reason wrong; students who understand the use of iodine by the
thyroid for synthesis of hormones, and the limited number of receptor sites, and the excretion of surplus
substances by the kidney, should be able to identify this error.
12.
Case histories about various endocrine abnormalities can be presented to a class to allow them to recognize
abnormalities and discuss the cause and treatment of the disease.
13.
Embryologically, the adrenal medulla is differentiated from tissues similar to nerves. Lecture question:
Does this make sense, considering the function of the adrenal medulla hormones? Yes, since the action is
much faster than most hormones. Also, the modern perspective presented in this text, where hormones are
part of a continuum of environmental signals, puts this in a correct light.
14.
Lecture question: Cortisone has anti-inflammatory action and is useful for reducing deep inflammation, as
in Bell palsy, etc. Yet the dosage is given rapidly and for only a brief time. Why would a physician not
want to boost and maintain cortisone levels over a longer time?
15.
Students sometimes deduce wrongly that eating lots of sugar makes a normal person diabetic. Lead
students through a review of the compensating mechanisms involved while cavalierly eating several candy
bars. It is also possible to carry in a urine sample and test for sugar in urine; although test results are visual
but not easily projected on a screen, they can be reported by eyesight. Adding sugar to a normal sample
can convert the test. Lecture question: Why would the medical lab want a patient to abstain from eating
eight or more hours before a blood sample is taken?
Critical Thinking
Question 1. If cortisol is good at reducing inflammation, especially of deep tissues, why isn’t it a commonly used
over-the-counter long-term drug for the many inflammation problems people have every day?
Answer: Cortisol is far more than an anti-inflammatory medication. It promotes hydrolysis of muscle proteins to
amino acids that enter the blood. It increases blood glucose levels and shifts metabolism to fatty acids rather than
carbohydrates. Therefore, dosages used for reducing inflammation are carefully monitored and usually of short
duration.
Question 2. Not only does the adrenal cortex secrete some estrogens, the normal fat tissue also produces some
estrogen. Why might a female Olympic athlete cease having a menstrual cycle during strenuous training before a
major event?
Answer: The hypothalamus, pituitary, and uterus are responding to the overall threshold level of estrogen in the
system. This is provided by a base level from the adrenal cortex and fat tissue, with the cycling level of estrogen
from the ovaries providing an additional level that is detected by these feedback tissues. When fat is dramatically
reduced, the total level of estrogens–now coming only from the ovaries–is too low and never cycles to high enough
concentrations to trigger the tissues involved in the menstrual cycle.
Question 3. Biological controls are promoted as a replacement for pesticides because insects eventually develop
strains that are resistant. Hormone traps are one form of biological control where the sex pheromone is used to lure
the males (usually) to a trap that removes them from the mating population. There is natural variation in the male
response to pheromones. Is this biological control immune from the resistance problem that pesticides faced?
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Answer: No. Individuals that are less-attracted will avoid the traps and be the only ones to mate; therefore this
lower attractiveness will soon predominate and be a strain “resistant” to pheromone trapping. A pheromone may
have many components; only one is usually purified and used in traps.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
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