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TRH from the hypothalamus stimulates the anterior pituitary gland to release TSH, which stimulates the thyroid gland to release hormones. These thyroid hormones reduce the secretion of TSH and TRH.

known that the hypothalamus secretes a gonadotropin-releasing hormone (GnRH). The hypothalamus does not secrete this hormone in significant amounts until puberty. Gonadotropins are virtually absent in the body fluids of infants and children.

D What is the function of ACTH?

^9 Describe the functions of FSH and LH in a male and in a female.

^9 What is a gonadotropin?

Posterior Pituitary Hormones

Unlike the anterior lobe of the pituitary gland, which is primarily composed of glandular epithelial cells, the posterior lobe largely consists of nerve fibers and neuroglial cells (pituicytes). The neuroglial cells support the nerve fibers that originate in the hypothalamus. The hypotha-lamic cells that give rise to these fibers are called neu-rosecretory cells because their secretions function not as neurotransmitters but as hormones.

Specialized neurons in the hypothalamus produce the two hormones associated with the posterior pituitary— antidiuretic (an"ti-di"u-ret'ik) hormone and oxytocin (ok"se-to'sin) (see fig. 13.10). These hormones travel

(Asp Oxytocin
Antidiuretic hormone

Figure 13.15

The structure of oxytocin differs from that of ADH by only two amino acids, yet they function differently.

down axons through the pituitary stalk to the posterior pituitary and are stored in vesicles (secretory granules) near the ends of the axons. The hormones are released into the blood in response to nerve impulses coming from the hypothalamus.

Antidiuretic hormone and oxytocin are short polypeptides with similar sequences (fig. 13.15). A diuretic is a substance that increases urine production. An antidiuretic, then, is a chemical that decreases urine formation. ADH produces its antidiuretic effect by causing the kidneys to reduce the volume of water they excrete. In this way, ADH plays an important role in regulating the concentration of body fluids (see chapter 20, p. 840).

Drinking alcohol is often followed by frequent and copious urination. This is because alcohol (ethyl alcohol) inhibits ADH secretion. A person must replace the lost body fluid to maintain normal water balance. Although it seems counterintuitive, drinking too much beer can lead to dehydration because the body loses more fluid than it replaces.

ADH present in sufficient concentrations contracts certain smooth muscles, including those in the walls of blood vessels. As a result, vascular resistance and blood pressure may increase. For this reason, ADH is also called vasopressin. Although ADH is seldom present in quantities sufficient to cause high blood pressure, its secretion increases following severe blood loss. Blood pressure may drop as a consequence of profuse bleeding. In this situation, ADH's vasopressor effect may help to minimize the drop and return blood pressure toward normal.

ADH's two effects—vasoconstriction and water retention—are possible because the hormone binds two different receptors on target cells. The binding of ADH to V1 receptors increases the concentration of the second messenger inositol triphosphate, which increases calcium ion concentration, leading to vasoconstriction. The second receptor, V2, is found on parts of the kidneys' microscopic tubules called collecting ducts. ADH binding there activates the cAMP second messenger system, which ultimately causes collecting duct cells to reabsorb water that would otherwise be excreted as urine.

The hypothalamus regulates secretion of ADH. Certain neurons in this part of the brain, called osmoreceptors, sense changes in the concentration of body fluids. For example, if a person is dehydrating due to a lack of water intake, the solutes in blood become more concentrated. The osmoreceptors, sensing the resulting increase in osmotic pressure, signal the posterior pituitary to release ADH, which causes the kidneys to retain water.

On the other hand, if a person drinks a large volume of water, body fluids become more dilute, which inhibits the release of ADH. Consequently, the kidneys excrete more dilute urine until the concentration of body fluids returns to normal.

Blood volume also affects ADH secretion. Increased blood volume stretches the walls of certain blood vessels, stimulating volume receptors that signal the hypothalamus to inhibit release of ADH. However, if hemorrhage decreases blood volume, these receptors are stretched less and therefore send fewer inhibiting impulses. As a result, ADH secretion increases, and as before, ADH causes the kidneys to conserve water. This helps prevent further volume loss.

Oxytocin also has an antidiuretic action, but less so than ADH. In addition, oxytocin can contract smooth muscles in the uterine wall, playing a role in the later stages of childbirth. The uterus becomes more sensitive to oxy-tocin's effects during pregnancy. Stretching of uterine and vaginal tissues late in pregnancy, caused by the growing fetus, initiates nerve impulses to the hypothalamus, which then signals the posterior pituitary to release oxytocin, which, in turn, stimulates the uterine contractions of labor.

In the breasts, oxytocin contracts certain cells near the milk-producing glands and their ducts. In lactating breasts, this action forces liquid from the milk glands into the milk ducts and ejects the milk.

The mechanical stimulation of suckling initiates nerve impulses that travel to the mother's hypothalamus, which responds by signaling the posterior pituitary to release oxytocin, which, in turn, stimulates milk release. Thus, milk is normally not ejected from the milk glands and ducts until the baby suckles. The fact that milk is ejected from both breasts in response to suckling is a reminder that all target cells respond to a hormone.

Oxytocin has no established function in males, although it is present in the male posterior pituitary. There is evidence that it may stimulate the movement of certain fluids in the male reproductive tract during sexual activity. Table 13.5 reviews the hormones of the pituitary gland.

If the uterus is not sufficiently contracting to expel a fully developed fetus, oxytocin is sometimes given intravenously to stimulate uterine contractions, thus inducing labor. Oxytocin is also administered to the mother following childbirth to ensure that the uterine muscles contract enough to squeeze broken blood vessels closed, minimizing the danger of hemorrhage.

A baby first displayed symptoms at five months of age—he drank huge volumes of water. By thirteen months, he had become severely dehydrated, although he drank nearly continuously. His parents were constantly changing his wet diapers. Doctors finally diagnosed a form of the condition diabetes insipidus, which impairs ADH regulation of water balance. The boy was drinking sufficient fluids, but his kidneys could not retain the water. ADH V2 receptors on the kidney collecting ducts were defective. The hormone could bind, but the receptor failed to trigger cAMP formation. The boy's ADH was still able to constrict blood vessels because the V1 receptors were unaffected. A high-calorie diet and providing lots of water preserved the boy's mental abilities, but he remained small for his age. Tumors and injury affecting the hypothalamus and posterior pituitary can also cause diabetes insipidus.

What is the function of ADH?

How is the secretion of ADH controlled?

What effects does oxytocin produce in females?

Thyroid Gland

The thyroid gland (thi'roid gland), as figure 13.16 shows, is a very vascular structure that consists of two large lateral lobes connected by a broad isthmus. It is located just below the larynx on either side and anterior to the trachea. It has a special ability to remove iodine from the blood.

Structure of the Gland

A capsule of connective tissue covers the thyroid gland, which is made up of many secretory parts called follicles. The cavities within these follicles are lined with a single layer of cuboidal epithelial cells, and are filled

Shier-Butler-Lewis: III. Integration and 13. Endocrine System © The McGraw-Hill

Human Anatomy and Coordination Companies, 2001

Physiology, Ninth Edition

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Pregnancy And Childbirth

Pregnancy And Childbirth

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