Essentials of Anatomy and Physiology ( PDFDrive )_239-267.pdf

10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 221 Copyright © 2007 by F. A. Davis. CHAPTER 10 The Endocrine System 221 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 222 Copyright © 2007 by F. A. Davis....

10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 221 Copyright © 2007 by F. A. Davis. CHAPTER 10 The Endocrine System 221 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 222 Copyright © 2007 by F. A. Davis. CHAPTER 10 Chapter Outline Testes Chemistry of Hormones Testosterone Regulation of Hormone Secretion Inhibin The Pituitary Gland Other Hormones Posterior Pituitary Gland Prostaglandins Antidiuretic hormone Mechanisms of Hormone Action Oxytocin The Two-Messenger Mechanism—Protein Anterior Pituitary Gland Hormones Growth hormone Action of Steroid Hormones Thyroid-stimulating hormone Aging and the Endocrine System Adrenocorticotropic hormone Prolactin BOX 10–1 DISORDERS OF GROWTH HORMONE Follicle-stimulating hormone BOX 10–2 DISORDERS OF THYROXINE Luteinizing hormone BOX 10–3 DIABETES MELLITUS Thyroid Gland BOX 10–4 DISORDERS OF THE ADRENAL CORTEX Thyroxine and T3 Calcitonin Parathyroid Glands Student Objectives Parathyroid Hormone Name the endocrine glands and the hormones Pancreas secreted by each. Glucagon Explain how a negative feedback mechanism Insulin works. Adrenal Glands Explain how the hypothalamus is involved Adrenal Medulla in the secretion of hormones from the pos- Epinephrine and norepinephrine terior pituitary gland and anterior pituitary Adrenal Cortex gland. Aldosterone State the functions of oxytocin and antidiuretic Cortisol hormone, and explain the stimulus for secretion Ovaries of each. Estrogen State the functions of the hormones of the ante- Progesterone rior pituitary gland, and state the stimulus for Inhibin secretion of each. 222 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 223 Copyright © 2007 by F. A. Davis. The Endocrine System Student Objectives (Continued) Gluconeogenesis (GLOO-koh-nee-oh-JEN-i-sis) Glycogenesis (GLIGH-koh-JEN-i-sis) State the functions of thyroxine and T3, and Glycogenolysis (GLIGH-ko-jen-OL-i-sis) describe the stimulus for their secretion. Hypercalcemia (HIGH-per-kal-SEE-mee-ah) Explain how parathyroid hormone and calcitonin Hyperglycemia (HIGH-per-gligh-SEE-mee-ah) work as antagonists. Hypocalcemia (HIGH-poh-kal-SEE-mee-ah) Explain how insulin and glucagon work as antago- Hypoglycemia (HIGH-poh-gligh-SEE-mee-ah) nists. Hypophysis (high-POFF-e-sis) State the functions of epinephrine and norepineph- Islets of Langerhans (EYE-lets of LAHNG-er-hanz) rine, and explain their relationship to the sympa- thetic division of the autonomic nervous system. Prostaglandins (PRAHS-tah-GLAND-ins) Renin-angiotensin mechanism (REE-nin AN-jee- State the functions of aldosterone and cortisol, and oh-TEN-sin) describe the stimulus for secretion of each. Sympathomimetic (SIM-pah-tho-mi-MET-ik) State the functions of estrogen, progesterone, Target organ (TAR-get OR-gan) testosterone, and inhibin and state the stimulus for secretion of each. Explain what prostaglandins are made of, and state some of their functions. Related Clinical Terminology Explain how the protein hormones are believed to Acromegaly (AK-roh-MEG-ah-lee) exert their effects. Addison’s disease (ADD-i-sonz) Explain how the steroid hormones are believed to Cretinism (KREE-tin-izm) exert their effects. Cushing’s syndrome (KOOSH-ingz SIN-drohm) Diabetes mellitus (DYE-ah-BEE-tis mel-LYE-tus) Giantism (JIGH-an-tizm) New Terminology Goiter (GOY-ter) Alpha cells (AL-fah SELLS) Graves’ disease (GRAYVES) Beta cells (BAY-tah SELLS) Ketoacidosis (KEY-toh-ass-i-DOH-sis) Catecholamines (KAT-e-kohl-ah-MEENZ) Myxedema (MIK-suh-DEE-mah) Corpus luteum (KOR-pus LOO-tee-um) Pituitary dwarfism (pi-TOO-i-TER-ee DWORF- izm) Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547. 223 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 224 Copyright © 2007 by F. A. Davis. 224 The Endocrine System W e have already seen how the nervous system regulates body functions by means of nerve impulses glands that secrete chemicals called hormones. These glands, and the names of the hormones they secrete, and integration of information by the spinal cord and are shown in Fig. 10–1. brain. The other regulating system of the body is Endocrine glands are ductless; that is, they do not the endocrine system, which consists of endocrine have ducts to take their secretions to specific sites. PITUITARY (HYPOPHYSIS) GLAND HYPOTHALAMUS Anterior: GH, TSH, ACTH Releasing hormones FSH, LH, Prolactin for anterior pituitary Posterior: ADH, Oxytocin PINEAL GLAND Melatonin THYROID GLAND PARATHYROID GLANDS Thyroxine and T3 PTH Calcitonin THYMUS GLAND Immune hormones ADRENAL (SUPRARENAL) GLANDS Cortex: Aldosterone PANCREAS Cortisol Insulin Sex hormones Glucagon Medulla: Epinephrine Norepinephrine OVARIES Estrogen Progesterone Inhibin TESTES Testosterone Inhibin Figure 10–1. The endocrine system. Locations of many endocrine glands. Both male and female gonads (testes and ovaries) are shown. QUESTION: Why is the location of the thyroid gland not really important for its function? 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 225 Copyright © 2007 by F. A. Davis. The Endocrine System 225 Instead, hormones are secreted directly into capillaries mation they use to increase or decrease secretion of and circulate in the blood throughout the body. Each their own hormones. When a hormone brings about hormone then exerts very specific effects on certain its effects, the stimulus is reversed, and secretion of organs, called target organs or target tissues. Some the hormone decreases until the stimulus reoccurs. hormones, such as insulin and thyroxine, have many You may recall from Chapter 1 that this is a negative target organs. Other hormones, such as calcitonin and feedback mechanism, and the mechanism for thyrox- some pituitary gland hormones, have only one or a ine was depicted in Fig. 1–3. Let us use insulin as a dif- few target organs. ferent example here. In general, the endocrine system and its hormones Insulin is secreted by the pancreas when the blood help regulate growth, the use of foods to produce glucose level is high; that is, hyperglycemia is the energy, resistance to stress, the pH of body fluids and stimulus for secretion of insulin. Once circulating fluid balance, and reproduction. In this chapter we will in the blood, insulin enables cells to remove glucose discuss the specific functions of the hormones and from the blood so that it can be used for energy how each contributes to homeostasis. production and enables the liver to store glucose as glycogen. As a result of these actions of insulin, the blood glucose level decreases, reversing the stim- CHEMISTRY OF HORMONES ulus for secretion of insulin. Insulin secretion then decreases until the blood glucose level increases With respect to their chemical structure, hormones again. may be classified into three groups: amines, proteins, In any hormonal negative feedback mechanism, and steroids. information about the effects of the hormone is “fed back” to the gland, which then decreases its secretion 1. Amines—these simple hormones are structural of the hormone. This is why the mechanism is called variations of the amino acid tyrosine. This group “negative”: The effects of the hormone reverse the includes thyroxine from the thyroid gland and epi- stimulus and decrease the secretion of the hormone. nephrine and norepinephrine from the adrenal The secretion of many other hormones is regulated in medulla. a similar way. 2. Proteins—these hormones are chains of amino The hormones of the anterior pituitary gland are acids. Insulin from the pancreas, growth hormone secreted in response to releasing hormones (also from the anterior pituitary gland, and calcitonin called releasing factors) secreted by the hypothalamus. from the thyroid gland are all proteins. Short You may recall this from Chapter 8. Growth hormone, chains of amino acids may be called peptides. for example, is secreted in response to growth hor- Antidiuretic hormone and oxytocin, synthesized by mone–releasing hormone (GHRH) from the hypo- the hypothalamus, are peptide hormones. thalamus. As growth hormone exerts its effects, the 3. Steroids—cholesterol is the precursor for the secretion of GHRH decreases, which in turn decreases steroid hormones, which include cortisol and aldos- the secretion of growth hormone. This is another type terone from the adrenal cortex, estrogen and pro- of negative feedback mechanism. gesterone from the ovaries, and testosterone from For each of the hormones to be discussed in this the testes. chapter, the stimulus for its secretion will also be men- tioned. Some hormones function as an antagonistic pair to regulate a particular aspect of blood chemistry; REGULATION OF these mechanisms will also be covered. HORMONE SECRETION Hormones are secreted by endocrine glands when THE PITUITARY GLAND there is a need for them, that is, for their effects on their target organs. The cells of endocrine glands The pituitary gland (or hypophysis) hangs by a short respond to changes in the blood or perhaps to other stalk (infundibulum) from the hypothalamus and is hormones in the blood. These stimuli are the infor- enclosed by the sella turcica of the sphenoid bone. 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 226 Copyright © 2007 by F. A. Davis. 226 The Endocrine System Figure 10–2. Hormones of the pituitary gland and their target organs. QUESTION: Which pituitary hormones have other endocrine glands as their target organs? Despite its small size, the pituitary gland regulates which is separate glandular tissue. All of the hormones many body functions. Its two major portions are the of the pituitary gland and their target organs are posterior pituitary gland (neurohypophysis), which is shown in Fig. 10–2. It may be helpful for you to look an extension of the nerve tissue of the hypothalamus, at this summary picture before you begin reading the and the anterior pituitary gland (adenohypophysis), following sections. 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 227 Copyright © 2007 by F. A. Davis. The Endocrine System 227 POSTERIOR PITUITARY GLAND through sweating or diarrhea, for example, osmore- ceptors in the hypothalamus detect the increased The two hormones of the posterior pituitary gland “saltiness” of body fluids. The hypothalamus then are actually produced by the hypothalamus and simply transmits impulses to the posterior pituitary to stored in the posterior pituitary until needed. Their increase the secretion of ADH and decrease the loss of release is stimulated by nerve impulses from the hypo- more water in urine. thalamus (Fig. 10–3). Any type of dehydration stimulates the secretion of ADH to conserve body water. In the case of severe Antidiuretic Hormone hemorrhage, ADH is released in large amounts and Antidiuretic hormone (ADH, also called vaso- will also cause vasoconstriction, especially in arteri- pressin) increases the reabsorption of water by kidney oles, which will help to raise or at least maintain blood tubules, which decreases the amount of urine formed. pressure. This function gives ADH its other name, The water is reabsorbed into the blood, so as urinary vasopressin. output is decreased, blood volume is increased, which Ingestion of alcohol inhibits the secretion of ADH helps maintain normal blood pressure. ADH also and increases urinary output. If alcohol intake is exces- decreases sweating, but the amount of water con- sive and fluid is not replaced, a person will feel thirsty served is much less than that conserved by the kidneys. and dizzy the next morning. The thirst is due to the The stimulus for secretion of ADH is decreased loss of body water, and the dizziness is the result of water content of the body. If too much water is lost low blood pressure. Hypothalamus Hypothalamus Optic chiasma Releasing hormones Capillaries in hypothalamus Hypothalamic-hypophyseal tract Hypophyseal portal veins Superior hypophyseal Optic chiasma arteries Posterior pituitary Capillaries in anterior pituitary Inferior hypophyseal artery Hormones of anterior pituitary Hormones of posterior pituitary Lateral hypophyseal vein Posterior lobe vein A B Figure 10–3. Structural relationships of hypothalamus and pituitary gland. (A) Posterior pituitary stores hormones produced in the hypothalamus. (B) Releasing hormones of the hypothalamus circulate directly to the anterior pituitary and influence its secretions. Notice the two networks of capillaries. QUESTION: In part A, name the hormones of the posterior pituitary. In part B, what stim- ulates secretion of anterior pituitary hormones? 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 228 Copyright © 2007 by F. A. Davis. 228 The Endocrine System Oxytocin brain cells have receptors for vasopressin, and they seem to be involved in creating the bonds that sustain Oxytocin stimulates contraction of the uterus at the family life. Trust is part of many social encounters end of pregnancy and stimulates release of milk from such as friendship, school, sports and games, and buy- the mammary glands. ing and selling, as well as family life. These two small As labor begins, the cervix of the uterus is hormones seem to have some influence on us mentally stretched, which generates sensory impulses to the as well as physically. hypothalamus, which in turn stimulates the posterior pituitary to release oxytocin. Oxytocin then causes strong contractions of the smooth muscle (myo- ANTERIOR PITUITARY GLAND metrium) of the uterus to bring about delivery of the The hormones of the anterior pituitary gland regu- baby and the placenta. The secretion of oxytocin is late many body functions. They are in turn regulated one of the few positive feedback mechanisms within by releasing hormones from the hypothalamus. the body, and the external brake or shutoff of the These releasing hormones are secreted into capillaries feedback cycle is delivery of the baby and the placenta. in the hypothalamus and pass through the hypophy- It has been discovered that the placenta itself seal portal veins to another capillary network in the secretes oxytocin at the end of gestation and in an anterior pituitary gland. Here, the releasing hormones amount far higher than that from the posterior pitu- are absorbed and stimulate secretion of the anterior itary gland. Research is continuing to determine the pituitary hormones. This small but specialized path- exact mechanism and precise role of the placenta in way of circulation is shown in Fig. 10–3. This pathway labor. permits the releasing hormones to rapidly stimulate When a baby is breast-fed, the sucking of the baby the anterior pituitary, without having to pass through stimulates sensory impulses from the mother’s nipple general circulation. to the hypothalamus. Nerve impulses from the hypo- thalamus to the posterior pituitary cause the release of Growth Hormone oxytocin, which stimulates contraction of the smooth muscle cells around the mammary ducts. This release Growth hormone (GH) is also called somatotropin, of milk is sometimes called the “milk let-down” reflex. and it does indeed promote growth (see Fig. 10–4). The hormones of the posterior pituitary are summa- GH stimulates cells to produce insulin-like growth fac- rized in Table 10–1. tors (IGFs), intermediary molecules that bring about Both ADH and oxytocin are peptide hormones the functions of GH. Growth hormone increases the with similar structure, having nine amino acids each. transport of amino acids into cells, and increases the And both have been found to influence aspects of rate of protein synthesis. Amino acids cannot be stored behavior such as nurturing and trustfulness. Certain in the body, so when they are available, they must be Table 10–1 HORMONES OF THE POSTERIOR PITUITARY GLAND Hormone Function(s) Regulation of Secretion Antidiuretic hormone Increases water reabsorption Decreased water content in the body (alcohol (ADH or vasopressin) by the kidney tubules (water inhibits secretion) returns to the blood) Decreases sweating Causes vasoconstriction (in large amounts) Oxytocin Promotes contraction of Nerve impulses from hypothalamus, the result of myometrium of uterus (labor) stretching of cervix or stimulation of nipple Promotes release of milk from Secretion from placenta at end of gestation— mammary glands stimulus unknown 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 229 Copyright © 2007 by F. A. Davis. The Endocrine System 229 Bone and Increases muscle Anterior mitosis pituitary GH Liver and other viscera Increases protein synthesis ATP Increases use of fats for energy Figure 10–4. Functions of growth hormone. QUESTION: Which functions of growth hormone directly help bones and muscles to grow? used in protein synthesis. Excess amino acids are and the use of fats for energy production. This is changed to carbohydrates or fat, for energy storage. important any time we go for extended periods with- Growth hormone ensures that amino acids will be used out eating, no matter what our ages. for whatever protein synthesis is necessary, before the The secretion of GH is regulated by two releasing amino acids can be changed to carbohydrates. Growth hormones from the hypothalamus. Growth hormone– hormone also stimulates cell division in those tissues releasing hormone (GHRH), which increases the capable of mitosis. These functions contribute to the secretion of GH, is produced during hypoglycemia growth of the body during childhood, especially and during exercise. Another stimulus for GHRH is a growth of bones and muscles. high blood level of amino acids; the GH then secreted You may now be wondering if GH is secreted in will ensure the conversion of these amino acids into adults, and the answer is yes. The use of amino acids protein. Somatostatin may also be called growth hor- for the synthesis of proteins is still necessary. Even if mone inhibiting hormone (GHIH), and, as its name the body is not growing in height, some tissues will tells us, it decreases the secretion of GH. Somatostatin require new proteins for repair or replacement. GH is produced during hyperglycemia. Disorders of GH also stimulates the release of fat from adipose tissue secretion are discussed in Box 10–1. 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 230 Copyright © 2007 by F. A. Davis. 230 The Endocrine System BOX 10–1 DISORDERS OF GROWTH HORMONE A deficiency or excess of growth hormone (GH) not have this condition; they are tall as a result of during childhood will have marked effects on the their genetic makeup and good nutrition. growth of a child. Hyposecretion of GH results in In an adult, hypersecretion of GH is caused by a pituitary dwarfism, in which the person may pituitary tumor, and results in acromegaly. The attain a final height of only 3 to 4 feet but will have long bones cannot grow because the epiphyseal normal body proportions. GH can now be pro- discs are closed, but the growth of other bones is duced using genetic engineering and may be used stimulated. The jaw and other facial bones become to stimulate growth in children with this disorder. disproportionately large, as do the bones of the GH will not increase growth of children with the hands and feet. The skin becomes thicker, and the genetic potential for short stature. Reports that GH tongue also grows and may protrude. Other conse- will reverse the effects of aging are simply not true. quences include compression of nerves by abnor- Hypersecretion of GH results in giantism (or mally growing bones and growth of the articular gigantism), in which the long bones grow exces- cartilages, which then erode and bring on arthritis. sively and the person may attain a height of 8 feet. Treatment of acromegaly requires surgical removal Most very tall people, such as basketball players, do of the tumor or its destruction by radiation. Thyroid-Stimulating Hormone terone, which are secreted in large amounts by the placenta during pregnancy. Then, after delivery of the Thyroid-stimulating hormone (TSH) is also called baby, prolactin secretion increases and milk is pro- thyrotropin, and its target organ is the thyroid gland. duced. If the mother continues to breast-feed, pro- TSH stimulates the normal growth of the thyroid and lactin levels remain high. the secretion of thyroxine (T4) and triiodothyronine (T3). The functions of these thyroid hormones will be covered later in this chapter. Follicle-Stimulating Hormone The secretion of TSH is stimulated by thyrotropin- Follicle-stimulating hormone (FSH) is one of the releasing hormone (TRH) from the hypothalamus. gonadotropic hormones; that is, it has its effects on When metabolic rate (energy production) decreases, the gonads: the ovaries or testes. FSH is named for TRH is produced. one of its functions in women. Within the ovaries are ovarian follicles that contain potential ova (egg cells). Adrenocorticotropic Hormone FSH stimulates the growth of ovarian follicles; that is, Adrenocorticotropic hormone (ACTH) stimulates it initiates egg development in cycles of approximately the secretion of cortisol and other hormones by the 28 days. FSH also stimulates secretion of estrogen by adrenal cortex. Secretion of ACTH is increased by the follicle cells. In men, FSH initiates sperm produc- corticotropin-releasing hormone (CRH) from the tion within the testes. hypothalamus. CRH is produced in any type of phys- The secretion of FSH is stimulated by the hypo- iological stress situation such as injury, disease, exer- thalamus, which produces gonadotropin-releasing cise, or hypoglycemia (being hungry is stressful). hormone (GnRH). FSH secretion is decreased by inhibin, a hormone produced by the ovaries or testes. Prolactin Luteinizing Hormone Prolactin, as its name suggests, is responsible for lac- tation. More precisely, prolactin initiates and main- Luteinizing hormone (LH) is another gonadotropic tains milk production by the mammary glands. The hormone. In women, LH is responsible for ovulation, regulation of secretion of prolactin is complex, involv- the release of a mature ovum from an ovarian follicle. ing both prolactin-releasing hormone (PRH) and LH then stimulates that follicle to develop into the prolactin-inhibiting hormone (PIH) from the hypo- corpus luteum, which secretes progesterone, also thalamus. The mammary glands must first be acted under the influence of LH. In men, LH stimulates the upon by other hormones such as estrogen and proges- interstitial cells of the testes to secrete testosterone. 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 231 Copyright © 2007 by F. A. Davis. The Endocrine System 231 (LH is also called ICSH, interstitial cell stimulating The third hormone produced by the thyroid hormone.) gland is calcitonin, which is secreted by parafol- Secretion of LH is also regulated by GnRH from licular cells. Its function is very different from those the hypothalamus. We will return to FSH and LH, as of thyroxine and T3, which you may recall from well as a discussion of the sex hormones, in Chapter Chapter 6. 20. The hormones of the anterior pituitary are summa- THYROXINE AND T3 rized in Table 10–2. Thyroxine (T4) and T3 have the same functions: regu- lation of energy production and protein synthesis, which contribute to growth of the body and to normal THYROID GLAND body functioning throughout life (Fig. 10–5). Thyrox- ine and T3 increase cell respiration of all food types The thyroid gland is located on the front and sides of (carbohydrates, fats, and excess amino acids) and the trachea just below the larynx. Its two lobes are thereby increase energy and heat production. They connected by a middle piece called the isthmus. The also increase the rate of protein synthesis within cells. structural units of the thyroid gland are thyroid folli- Normal production of thyroxine and T3 is essential for cles, which produce thyroxine (T4) and triiodothy- physical growth, normal mental development, and ronine (T3). Iodine is necessary for the synthesis of maturation of the reproductive system. These hor- these hormones; thyroxine contains four atoms of mones are the most important day-to-day regulators iodine, and T3 contains three atoms of iodine. of metabolic rate; their activity is reflected in the func- Table 10–2 HORMONES OF THE ANTERIOR PITUITARY GLAND Hormone Function(s) Regulation of Secretion Growth hormone (GH) Increases rate of mitosis GHRH (hypothalamus) stimulates secretion Increases amino acid transport into cells GHIH—somatostatin (hypothalamus) Increases rate of protein synthesis inhibits secretion Increases use of fats for energy Thyroid-stimulating Increases secretion of thyroxine and T3 TRH (hypothalamus) hormone (TSH) by thyroid gland Adrenocorticotropic Increases secretion of cortisol by the CRH (hypothalamus) hormone (ACTH) adrenal cortex Prolactin Stimulates milk production by the mam- PRH (hypothalamus) stimulates secretion mary glands PIH (hypothalamus) inhibits secretion Follicle-stimulating In women: GnRH (hypothalamus) stimulates secretion hormone (FSH) Initiates growth of ova in ovarian follicles Inhibin (ovaries or testes) inhibits secretion Increases secretion of estrogen by follicle cells In men: Initiates sperm production in the testes Luteinizing hormone In women: GnRH (hypothalamus) (LH) (ICSH) Causes ovulation Causes the ruptured ovarian follicle to become the corpus luteum Increases secretion of progesterone by the corpus luteum In men: Increases secretion of testosterone by the interstitial cells of the testes 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 232 Copyright © 2007 by F. A. Davis. 232 The Endocrine System Bone and muscle Increase protein synthesis T4 and T3 Thyroid gland Increase cell respiration of all foods Liver and viscera Glucose Brain Fats ATP Excess amino acids Reproductive organs Figure 10–5. Functions of thyroxine and T3. QUESTION: Which functions of thyroxine help bones and muscles to grow and maintain their own functions? tioning of the brain, muscles, heart, and virtually all metabolic rate (energy production) decreases, this other organs. Although thyroxine and T3 are not vital change is detected by the hypothalamus, which hormones, in that they are not crucial to survival, their secretes thyrotropin releasing hormone (TRH). TRH absence greatly diminishes physical and mental stimulates the anterior pituitary to secrete TSH, growth and abilities (see Box 10–2: Disorders of which stimulates the thyroid to release thyroxine and Thyroxine). T3, which raise the metabolic rate by increasing Secretion of thyroxine and T3 is stimulated by energy production. This negative feedback mecha- thyroid-stimulating hormone (TSH) from the ante- nism then shuts off TRH from the hypothalamus until rior pituitary gland (see also Fig. 1–3). When the the metabolic rate decreases again. 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 233 Copyright © 2007 by F. A. Davis. The Endocrine System 233 Table 10–3 HORMONES OF THE THYROID GLAND Regulation Hormone Function(s) of Secretion Thyroxine (T4) and Increase energy production from all food types TSH (anterior pituitary) triiodothyronine (T3) Increase rate of protein synthesis Calcitonin Decreases the reabsorption of calcium and phos- Hypercalcemia phate from bones to blood CALCITONIN raises the blood levels of these minerals. In the kid- neys, PTH stimulates the activation of vitamin D and Calcitonin decreases the reabsorption of calcium increases the reabsorption of calcium and the excre- and phosphate from the bones to the blood, thereby tion of phosphate (more than is obtained from bones). lowering blood levels of these minerals. This function Therefore, the overall effect of PTH is to raise the of calcitonin helps maintain normal blood levels blood calcium level and lower the blood phosphate of calcium and phosphate and also helps maintain level. The functions of PTH are summarized in Table a stable, strong bone matrix. It is believed that 10–4. calcitonin exerts its most important effects during Secretion of PTH is stimulated by hypocalcemia, childhood, when bones are growing. A form of calci- a low blood calcium level, and inhibited by hypercal- tonin obtained from salmon is used to help treat cemia. The antagonistic effects of PTH and calcitonin osteoporosis. are shown in Fig. 10–6. Together, these hormones The stimulus for secretion of calcitonin is hyper- maintain blood calcium within a normal range. calcemia, that is, a high blood calcium level. When Calcium in the blood is essential for the process of blood calcium is high, calcitonin ensures that no more blood clotting and for normal activity of neurons and calcium will be removed from bones until there is a muscle cells. real need for more calcium in the blood (Fig. 10–6). As you might expect, a sustained hypersecretion of The hormones of the thyroid gland are summarized in PTH, such as is caused by a parathyroid tumor, would Table 10–3. remove calcium from bones and weaken them. It has been found, however, that an intermittent, brief excess of PTH, such as can occur by injection, will stimulate PARATHYROID GLANDS the formation of more bone matrix, rather than matrix reabsorption. This may seem very strange—the oppo- There are four parathyroid glands: two on the back site of what we would expect—but it shows how much of each lobe of the thyroid gland (Fig. 10–7). The hor- we have yet to learn about the body. PTH is being mone they produce is called parathyroid hormone. investigated as a possible way to help prevent osteo- porosis. PARATHYROID HORMONE Parathyroid hormone (PTH) is an antagonist to cal- citonin and is important for the maintenance of nor- PANCREAS mal blood levels of calcium and phosphate. The target organs of PTH are the bones, small intestine, and kid- The pancreas is located in the upper left quadrant of neys. the abdominal cavity, extending from the curve of the PTH increases the reabsorption of calcium and duodenum to the spleen. Although the pancreas is phosphate from bones to the blood, thereby raising both an exocrine (digestive) gland as well as an their blood levels. Absorption of calcium and phos- endocrine gland, only its endocrine function will phate from food in the small intestine, which also be discussed here. The hormone-producing cells of requires vitamin D, is increased by PTH. This too the pancreas are called islets of Langerhans (pancre- 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 234 Copyright © 2007 by F. A. Davis. 234 The Endocrine System Bones Kidneys (vitamin D activated) Hypocalcemia (low blood calcium) Ca+2 is retained in bone matrix Parathyroids PTH Small intestine Accelerates calcium absorption by bones Bones Calcitonin Inhibits Thyroid Hypercalcemia (high blood calcium) Reabsorb Ca+2 to the blood Figure 10–6. Calcitonin and parathyroid hormone (PTH) and their functions related to the maintenance of the blood calcium level. QUESTION: Which hormone helps keep calcium in bones? What vitamin does PTH help activate, and where? Table 10–4 HORMONE OF THE PARATHYROID GLANDS Regulation Hormone Functions of Secretion Parathyroid hormone Increases the reabsorption of calcium and phosphate Hypocalcemia stimulates secre- (PTH) from bone to blood tion. Hypercalcemia inhibits Increases absorption of calcium and phosphate by secretion. the small intestine Increases the reabsorption of calcium and the excre- tion of phosphate by the kidneys; activates vitamin D 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 235 Copyright © 2007 by F. A. Davis. The Endocrine System 235 BOX 10–2 DISORDERS OF THYROXINE Iodine is an essential component of thyroxine (and (energy production) decreases, resulting in lethargy, T3), and a dietary deficiency of iodine causes goi- muscular weakness, slow heart rate, a feeling of ter. In an attempt to produce more thyroxine, the cold, weight gain, and a characteristic puffiness of thyroid cells become enlarged, and hence the thy- the face. The administration of thyroid hormones roid gland enlarges and becomes visible on the will return the metabolic rate to normal. front of the neck. The use of iodized salt has made Graves’ disease is an autoimmune disorder goiter a rare condition in many parts of the world. that causes hypersecretion of thyroxine. The Hyposecretion of thyroxine in a newborn has autoantibodies seem to bind to TSH receptors on devastating effects on the growth of the child. the thyroid cells and stimulate secretion of excess Without thyroxine, physical growth is diminished, thyroxine. The symptoms are those that would be as is mental development. This condition is called expected when the metabolic rate is abnormally cretinism, characterized by severe physical and elevated: weight loss accompanied by increased mental retardation. If the thyroxine deficiency is appetite, increased sweating, fast heart rate, feeling detected shortly after birth, the child may be of warmth, and fatigue. Also present may be goiter treated with thyroid hormones to promote normal and exophthalmos, which is protrusion of the eyes. development. Treatment is aimed at decreasing the secretion of Hyposecretion of thyroxine in an adult is called thyroxine by the thyroid, and medications or myxedema. Without thyroxine, the metabolic rate radioactive iodine may be used to accomplish this. atic islets; see Fig. 16–7); they contain alpha cells that produce glucagon and beta cells that produce insulin. GLUCAGON Larynx Glucagon stimulates the liver to change glycogen to glucose (this process is called glycogenolysis, which literally means “glycogen breakdown”) and to increase the use of fats and excess amino acids for energy pro- duction. The process of gluconeogenesis (literally, “making new glucose”) is the conversion of excess amino acids into simple carbohydrates that may enter the reactions of cell respiration. The overall effect of glucagon, therefore, is to raise the blood glucose level Thyroid and to make all types of food available for energy production. The secretion of glucagon is stimulated by hypo- Parathyroid glycemia, a low blood glucose level. Such a state may glands occur between meals or during physiological stress sit- uations such as exercise (Fig. 10–8). Trachea INSULIN Insulin increases the transport of glucose from the blood into cells by increasing the permeability of Figure 10–7. Parathyroid glands in posterior view, on cell membranes to glucose. (Brain, liver, and kidney lobes of the thyroid gland. cells, however, are not dependent on insulin for QUESTION: Which of the target organs of PTH may be glucose intake.) Once inside cells, glucose is used in called a reservoir, and what do they store? 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 236 Copyright © 2007 by F. A. Davis. 236 The Endocrine System Hyperglycemia Glucagon Liver (High blood glucose) Liver and skeletal Liver changes muscles change glycogen to glucose to glycogen glucose and converts amino Liver acids to carbohydrates Pancreas Skeletal muscles Figure 10–8. Insulin and glucagon and their functions Insulin related to the maintenance of the blood glucose level. Hypoglycemia (Low blood glucose) QUESTION: Which hormone enables cells to use glucose Cells for energy production? What Cells use glucose is the stimulus for secretion of for energy production this hormone? cell respiration to produce energy. The liver and skele- You will also notice in Fig. 16–7 the cells called tal muscles also change glucose to glycogen (glycoge- delta cells. These produce the hormone somatostatin, nesis, which means “glycogen production”) to be which is identical to growth hormone–inhibiting hor- stored for later use. Insulin is also important in the mone from the hypothalamus. Pancreatic somato- metabolism of other food types; it enables cells to take statin acts locally to inhibit the secretion of insulin and in fatty acids and amino acids to use in the synthesis of glucagon, and it seems to slow the absorption of the lipids and proteins (not energy production). Without end products of digestion in the small intestine. The insulin, blood levels of lipids tend to rise and cells accu- hormones of the pancreas are summarized in Table mulate excess fatty acids. With respect to blood glu- 10–5. cose, insulin decreases its level by promoting the use of glucose for energy production. The antagonistic func- tions of insulin and glucagon are shown in Fig. 10–8. ADRENAL GLANDS Insulin is a vital hormone; we cannot survive for very long without it. A deficiency of insulin or in its The two adrenal glands are located one on top of functioning is called diabetes mellitus, which is dis- each kidney, which gives them their other name of cussed in Box 10–3: Diabetes Mellitus. suprarenal glands. Each adrenal gland consists of two Secretion of insulin is stimulated by hyper- parts: an inner adrenal medulla and an outer adrenal glycemia, a high blood glucose level. This state cortex. The hormones produced by each part have occurs after eating, especially of meals high in carbo- very different functions. hydrates. As glucose is absorbed from the small intes- ADRENAL MEDULLA tine into the blood, insulin is secreted to enable cells to use the glucose for immediate energy. At the same The cells of the adrenal medulla secrete epinephrine time, any excess glucose will be stored in the liver and and norepinephrine, which collectively are called cat- muscles as glycogen. echolamines and are sympathomimetic. The secre- 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 237 Copyright © 2007 by F. A. Davis. The Endocrine System 237 Table 10–5 HORMONES OF THE PANCREAS Regulation Hormone Functions of Secretion Glucagon Increases conversion of glycogen to glucose in the liver Hypoglycemia (alpha cells) Increases the use of excess amino acids and of fats for energy Insulin (beta cells) Increases glucose transport into cells and the use of glucose for Hyperglycemia energy production Increases the conversion of excess glucose to glycogen in the liver and muscles Increases amino acid and fatty acid transport into cells, and their use in synthesis reactions Somatostatin Decreases secretion of insulin and glucagon Rising levels of insulin (delta cells) Slows absorption of nutrients and glucagon tion of both hormones is stimulated by sympathetic decreases peristalsis, stimulates the liver to change impulses from the hypothalamus, and their functions glycogen to glucose, increases the use of fats for duplicate and prolong those of the sympathetic divi- energy, and increases the rate of cell respiration. Many sion of the autonomic nervous system (mimetic means of these effects do indeed seem to be an echo of sym- “to mimic”). pathetic responses, don’t they? Responding to stress is so important that the body acts redundantly (that is, Epinephrine and Norepinephrine exceeds what is necessary, or repeats itself) and has Epinephrine (Adrenalin) and norepinephrine (nor- both a nervous mechanism and a hormonal mecha- adrenalin) are both secreted in stress situations and nism. Epinephrine is actually more effective than sym- help prepare the body for “fight or flight.” Norepi- pathetic stimulation, however, because the hormone nephrine is secreted in small amounts, and its most increases energy production and cardiac output to a significant function is to cause vasoconstriction in the greater extent. The hormones of the adrenal medulla skin, viscera, and skeletal muscles (that is, throughout are summarized in Table 10–6, and their functions are the body), which raises blood pressure. shown in Fig. 10–9. Epinephrine, secreted in larger amounts, increases the heart rate and force of contraction and stimulates ADRENAL CORTEX vasoconstriction in skin and viscera and vasodilation in The adrenal cortex secretes three types of steroid skeletal muscles. It also dilates the bronchioles, hormones: mineralocorticoids, glucocorticoids, and Table 10–6 HORMONES OF THE ADRENAL MEDULLA Regulation Hormone Function(s) of Secretion Norepinephrine Causes vasoconstriction in skin, viscera, and skeletal muscles Epinephrine Increases heart rate and force of contraction Sympathetic impulses Dilates bronchioles from the hypo- Decreases peristalsis thalamus in stress Increases conversion of glycogen to glucose in the liver situations Causes vasodilation in skeletal muscles Causes vasoconstriction in skin and viscera Increases use of fats for energy Increases the rate of cell respiration 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 238 Copyright © 2007 by F. A. Davis. 238 The Endocrine System BOX 10–3 DIABETES MELLITUS There are two types of diabetes mellitus: Type 1 more water is lost as well, symptoms include is called insulin-dependent diabetes and its onset is greater urinary output (polyuria) and thirst (poly- usually in childhood (juvenile onset). Type 2 dipsia). is called non–insulin-dependent diabetes, and its The long-term effects of hyperglycemia produce onset is usually later in life (maturity onset). distinctive vascular changes. The capillary walls Type 1 diabetes is characterized by destruction thicken, and exchange of gases and nutrients of the beta cells of the islets of Langerhans and a diminishes. The most damaging effects are seen in complete lack of insulin (see Box Figure 10–A); the skin (especially of the feet), the retina (diabetic onset is usually abrupt. Destruction of the beta cells retinopathy), and the kidneys. Poorly controlled is an autoimmune response, perhaps triggered by a diabetes may lead to dry gangrene, blindness, and virus. There may be a genetic predisposition, severe kidney damage. Atherosclerosis is common, because certain HLA types are found more fre- because faulty triglyceride metabolism is linked to quently in type 1 diabetics than in other children faulty glucose metabolism. Neuropathy (damage to (see Box 11–5: HLA). Insulin by injection (inhaled nerves) leads to impaired cutaneous sensation and insulin is undergoing clinical trials) is essential to difficulty with fine movements, such as buttoning a control type 1 diabetes. Research is continuing on shirt. It is now possible for diabetics to prevent the use of immunosuppressant medications to try much of this tissue damage by precise monitoring to preserve some beta cells (if diagnosis is early), of the blood glucose level and more frequent and also on the transplantation of stem cells to administration of insulin. Insulin pumps are able to replace lost beta cells. more closely mimic the natural secretion of insulin. In type 2 diabetes, insulin is produced but can- A very serious potential problem for the type 1 not exert its effects on cells because of a loss of diabetic is ketoacidosis. When glucose cannot be insulin receptors on cell membranes (see Box Figure used for energy, the body turns to fats and proteins, 10–A). Onset of type 2 diabetes is usually gradual, which are converted by the liver to ketones. and risk factors include a family history of diabetes Ketones are organic acids (acetone, acetoacetic and being overweight. Control may not require acid) that can be used in cell respiration, but cells insulin, but rather medications that enable insulin are not able to utilize them rapidly so ketones to react with the remaining membrane receptors. accumulate in the blood. Ketones are acids, and For those with a family history of diabetes, a low-fat lower the pH of the blood as they accumulate. The diet and regular exercise reduce the risk of devel- kidneys excrete excess ketones, but in doing so oping the disease. The commitment to exercise excrete more water as well, which leads to dehy- must be lifelong but is well worth the effort, dration and worsens the acidosis. Without adminis- because diabetes is very destructive. tration of insulin to permit the use of glucose, and Without insulin (or its effects) blood glucose level IV fluids to restore blood volume to normal, ketoaci- remains high, and glucose is lost in urine. Since dosis will progress to coma and death. Glucose Insulin Insulin Receptor A Normal B Type 1 C Type 2 Box Figure 10–A (A) Cell membrane in normal state, with insulin receptors and insulin to regulate glucose intake. (B) Cell membrane in type 1 diabetes: insulin not present, glucose remains outside cell. (C) Cell membrane in type 2 diabetes: without insulin receptors, glucose remains outside cell. 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 239 Copyright © 2007 by F. A. Davis. The Endocrine System 239 Adrenal medulla Increases rate and Norepinephrine Epinephrine force of contraction Vasoconstriction in skin Dilates bronchioles Vasoconstriction in viscera Increases conversion of Decreases glycogen to glucose peristalsis Vasoconstriction in skeletal muscle Increases use of fats for energy Vasodilation in skeletal muscle Increases cell respiration Figure 10–9. Functions of epinephrine and norepinephrine. QUESTION: Do epinephrine and norepinephrine have the same effect on skeletal muscle? Explain your answer. sex hormones. The sex hormones, “female” estrogens rapid body growth during early puberty. They may and “male” androgens (similar to testosterone), are also be important in supplying estrogen to women produced in very small amounts, and their importance after menopause and to men throughout life (see the is not known with certainty. They may contribute to “Estrogen” section later in this chapter). 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 240 Copyright © 2007 by F. A. Davis. 240 The Endocrine System The functions of the other adrenal cortical hor- potassium, and contributes to the maintenance of nor- mones are well known, however, and these are consid- mal blood pH, blood volume, and blood pressure. ered vital hormones. A number of factors stimulate the secretion of aldosterone. These are a deficiency of sodium, loss of Aldosterone blood or dehydration that lowers blood pressure, or an elevated blood level of potassium. Low blood pressure Aldosterone is the most abundant of the mineralo- or blood volume activates the renin-angiotensin corticoids, and we will use it as a representative of this mechanism of the kidneys. This mechanism is dis- group of hormones. The target organs of aldosterone cussed in Chapters 13 and 18, so we will say for now are the kidneys, but there are important secondary that the process culminates in the formation of a effects as well. Aldosterone increases the reabsorption chemical called angiotensin II. Angiotensin II causes of sodium and the excretion of potassium by the kid- vasoconstriction and stimulates the secretion of aldos- ney tubules. Sodium ions (Na⫹) are returned to the terone by the adrenal cortex. Aldosterone then blood, and potassium ions (K⫹) are excreted in urine. increases sodium and water retention by the kidneys Look at Fig. 10–10 as you read the following. to help restore blood volume and blood pressure to As Na⫹ ions are reabsorbed, hydrogen ions (H⫹) normal. may be excreted in exchange. This is one mechanism to prevent the accumulation of excess H⫹ ions, which Cortisol would cause acidosis of body fluids. Also, as Na⫹ ions are reabsorbed, negative ions such as chloride (Cl⫺) We will use cortisol as a representative of the group and bicarbonate (HCO3–) follow the Na⫹ ions back to of hormones called glucocorticoids, because it is the blood, and water follows by osmosis. This indirect responsible for most of the actions of this group effect of aldosterone, the reabsorption of water by the (Fig. 10–11). Cortisol increases the use of fats and kidneys, is very important to maintain normal blood excess amino acids (gluconeogenesis) for energy and volume and blood pressure. In summary, then, aldos- decreases the use of glucose. This is called the glucose- terone maintains normal blood levels of sodium and sparing effect, and it is important because it conserves Adrenal cortex Na+ ions reabsorbed HCO3– ions reabsorbed Aldosterone H2O reabsorbed H+ ions excreted K+ ions excreted Blood volume, blood pressure, and pH are maintained Figure 10–10. Functions of aldosterone. Direct and indirect functions are shown. QUESTION: What ions does aldosterone have a direct effect on, and what is the effect? 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 241 Copyright © 2007 by F. A. Davis. The Endocrine System 241 Adipose tissue Increases use ATP of fats Most tissues ATP Amino acids Adrenal cortex Increases use of excess amino acids Cortisol Liver Glycogen Glucose Conserves glucose Brain Limits inflammation Figure 10–11. Functions of cortisol. QUESTION: Which food types will be used for energy by most tissues? Which food type may be stored? glucose for use by the brain. Cortisol is secreted in any the effects of histamine and stabilizes lysosomal mem- type of physiological stress situation: disease, physical branes, preventing excessive tissue destruction. injury, hemorrhage, fear or anger, exercise, and Inflammation is a beneficial process up to a point, and hunger. Although most body cells easily use fatty acids is an essential first step if tissue repair is to take place. and excess amino acids in cell respiration, brain cells It may, however, become a vicious cycle of damage, do not, so they must have glucose. By enabling other inflammation, more damage, more inflammation, cells to use the alternative energy sources, cortisol and so on—a positive feedback mechanism. Normal ensures that whatever glucose is present will be avail- cortisol secretion seems to be the brake, to limit able to the brain. the inflammation process to what is useful for tissue Cortisol also has an anti-inflammatory effect. repair, and to prevent excessive tissue destruction. During inflammation, histamine from damaged tis- Too much cortisol, however, decreases the immune sues makes capillaries more permeable, and the lyso- response, leaving the body susceptible to infection somes of damaged cells release their enzymes, which and significantly slowing the healing of damaged help break down damaged tissue but may also cause tissue (see Box 10–4: Disorders of the Adrenal destruction of nearby healthy tissue. Cortisol blocks Cortex). 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 242 Copyright © 2007 by F. A. Davis. 242 The Endocrine System BOX 10–4 DISORDERS OF THE ADRENAL CORTEX Addison’s disease is the result of hyposecretion of The cause may be a pituitary tumor that increases the adrenol cortical hormones. Most cases are idio- ACTH secretion or a tumor of the adrenal cortex pathic, that is, of unknown cause; atrophy of the itself. adrenal cortex decreases both cortisol and aldos- Excessive cortisol promotes fat deposition in the terone secretion. trunk of the body, while the extremities remain Deficiency of cortisol is characterized by hypo- thin. The skin becomes thin and fragile, and healing glycemia, decreased gluconeogenesis, and deple- after injury is slow. The bones also become fragile tion of glycogen in the liver. Consequences are because osteoporosis is accelerated. Also character- muscle weakness and the inability to resist physio- istic of this syndrome is the rounded appearance of logical stress. Aldosterone deficiency leads to reten- the face. Treatment is aimed at removal of the cause tion of potassium and excretion of sodium and of the hypersecretion, whether it be a pituitary or water in urine. The result is severe dehydration, low adrenal tumor. blood volume, and low blood pressure. Without Cushing’s syndrome may also be seen in people treatment, circulatory shock and death will follow. who receive corticosteroids for medical reasons. Treatment involves administration of hydrocorti- Transplant recipients or people with rheumatoid sone; in high doses this will also compensate for the arthritis or severe asthma who must take cortico- aldosterone deficiency. steroids may exhibit any of the above symptoms. In Cushing’s syndrome is the result of hyper- such cases, the disadvantages of this medication secretion of the adrenal cortex, primarily cortisol. must be weighed against the benefits provided. The direct stimulus for cortisol secretion is ACTH from the anterior pituitary gland, which in turn is OVARIES stimulated by corticotropin releasing hormone (CRH) from the hypothalamus. CRH is produced in the The ovaries are located in the pelvic cavity, one on physiological stress situations mentioned earlier. each side of the uterus. The hormones produced by Although we often think of epinephrine as a hormone the ovaries are the steroids estrogen and progesterone, important in stress, cortisol is also important. The and the protein inhibin. Although their functions are hormones of the adrenal cortex are summarized in an integral part of Chapters 20 and 21, we will briefly Table 10–7. discuss some of them here. Table 10–7 HORMONES OF THE ADRENAL CORTEX Hormone Functions Regulation of Secretion Aldosterone Increases reabsorption of Na⫹ ions by Low blood Na⫹ level the kidneys to the blood Low blood volume or blood pressure Increases excretion of K⫹ ions by the High blood K⫹ level kidneys in urine Cortisol Increases use of fats and excess amino ACTH (anterior pituitary) during acids for energy physiological stress Decreases use of glucose for energy (except for the brain) Increases conversion of glucose to glycogen in the liver Anti-inflammatory effect: stabilizes lyso- somes and blocks the effects of histamine 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 243 Copyright © 2007 by F. A. Davis. The Endocrine System 243 ESTROGEN by the anterior pituitary gland, and GnRH by the hypothalamus. Estrogen is secreted by the follicle cells of the ovary; secretion is stimulated by FSH from the anterior pitu- itary gland. Estrogen promotes the maturation of the TESTES ovum in the ovarian follicle and stimulates the growth of blood vessels in the endometrium (lining) of the The testes are located in the scrotum, a sac of skin uterus in preparation for a possible fertilized egg. between the upper thighs. Two hormones, testos- The secondary sex characteristics in women also terone and inhibin, are secreted by the testes. develop in response to estrogen. These include growth of the duct system of the mammary glands, TESTOSTERONE growth of the uterus, and the deposition of fat subcu- taneously in the hips and thighs. The closure of the Testosterone is a steroid hormone secreted by the epiphyseal discs in long bones is brought about by interstitial cells of the testes; the stimulus for secretion estrogen, and growth in height stops. Estrogen is also is LH from the anterior pituitary gland. believed to lower blood levels of cholesterol and tri- Testosterone promotes maturation of sperm in the glycerides. For women before the age of menopause seminiferous tubules of the testes; this process begins this is beneficial in that it decreases the risk of athero- at puberty and continues throughout life. At puberty, sclerosis and coronary artery disease. testosterone stimulates development of the male sec- Research suggests that estrogen no longer be con- ondary sex characteristics. These include growth of sidered only a “female” hormone. Estrogen seems to all the reproductive organs, growth of facial and body have effects on many organs, including the brain, the hair, growth of the larynx and deepening of the voice, heart, and blood vessels. In the brain, testosterone and growth (protein synthesis) of the skeletal muscles. from the testes or the adrenal cortex can be converted Testosterone also brings about closure of the epiphy- to estrogen, which may be important for memory, ses of the long bones. especially for older people. Estrogen seems to have non-reproductive functions in both men and women, INHIBIN although we cannot yet be as specific as we can be The hormone inhibin is secreted by the sustentacular with the reproductive functions in women, mentioned cells of the testes; the stimulus for secretion is previously. increased testosterone. The function of inhibin is to decrease the secretion of FSH by the anterior pituitary PROGESTERONE gland. The interaction of inhibin, testosterone, and When a mature ovarian follicle releases an ovum, the the anterior pituitary hormones maintains spermato- follicle becomes the corpus luteum and begins to genesis at a constant rate. secrete progesterone in addition to estrogen. This is stimulated by LH from the anterior pituitary gland. Progesterone promotes the storage of glycogen OTHER HORMONES and the further growth of blood vessels in the endo- Melatonin is a hormone produced by the pineal metrium, which thus becomes a potential placenta. gland, which is located at the back of the third ventri- The secretory cells of the mammary glands also cle of the brain. The secretion of melatonin is greatest develop under the influence of progesterone. during darkness and decreases when light enters the Both progesterone and estrogen are secreted by the eye and the retina signals the hypothalamus. A recent placenta during pregnancy; these functions are cov- discovery is that the retina also produces melatonin, ered in Chapter 21. which seems to indicate that the eyes and pineal gland work with the biological clock of the hypothalamus. In INHIBIN other mammals, melatonin helps regulate seasonal The corpus luteum secretes another hormone, called reproductive cycles. For people, melatonin definitely inhibin. Inhibin helps decrease the secretion of FSH stimulates the onset of sleep and increases its duration. 10Scanlon(p3)-ch10 8/17/06 10:50 AM Page 244 Copyright © 2007 by F. A. Davis. 244 The Endocrine System Other claims, such as that melatonin strengthens the A hormone must first bond to a receptor for it on immune system or prevents cellular damage and or in the target cell. Cells respond to certain hor- aging, are without evidence as yet. mones and not to others because of the presence of There are other organs that produce hormones that specific receptors, which are proteins. These receptor have only one or a few target organs. For example, the proteins may be part of the cell membrane or within stomach and duodenum produce hormones that regu- the cytoplasm or nucleus of the target cells. A hor- late aspects of digestion and appetite. Adipose tissue mone will affect only those cells that have its specific produces the appetite-suppressing hormone leptin. receptors. Liver cells, for example, have cell mem- The thymus gland produces hormones necessary for brane receptors for insulin, glucagon, growth hor- the normal functioning of the immune system, and the mone, and epinephrine; bone cells have receptors for kidneys produce a hormone that stimulates red blood growth hormone, PTH, and calcitonin. Cells of the cell production. All of these will be discussed in later ovaries and testes do not have receptors for PTH and chapters. calcitonin, but do have receptors for FSH and LH, which bone cells and liver cells do not have. Once a