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Chapter 16 Endocrine System

Chapter 16 Endocrine System. Introduction. The endocrine and nervous systems function to achieve and maintain homeostasis (Table 16-1)

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Chapter 16 Endocrine System

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  1. Chapter 16Endocrine System

  2. Introduction • The endocrine and nervous systems function to achieve and maintain homeostasis (Table 16-1) • When the two systems work together as one system, referred to as the neuroendocrine system, they perform the same general functions: communication, integration, and control • In the endocrine system, secreting cells send hormone molecules via the blood to specific target cells contained in target tissues or target organs

  3. Introduction • Hormones—carried to almost every point in the body; can regulate most cells; effects work more slowly and last longer than those of neurotransmitters • Endocrine glands are “ductless glands”; many are made of glandular epithelium whose cells manufacture and secrete hormones; a few endocrine glands are made of neurosecretory tissue • Glands of the endocrine system are widely scattered throughout the body (Figure 16-2; Table 16-2)

  4. Hormones • Classification of hormones • Classification by general function • Tropic hormones—target other endocrine glands and stimulate their growth and secretion • Sex hormones—target reproductive tissues • Anabolic hormones—stimulate anabolism in target cells • Classification by chemical structure (Figure 16-3; Table 16-3) • Steroid hormones • Nonsteroid hormones

  5. Hormones • Classification of hormones (cont.) • Steroid hormones (Figure 16-4) • Synthesized from cholesterol (Figure 16-5) • Lipid-soluble and can easily pass through the phospholipid plasma membrane of target cells • Examples of steroid hormones: cortisol, aldosterone, estrogen, progesterone, and testosterone

  6. Hormones • Classification of hormones (cont.) • Nonsteroid hormones (Figure 16-6) • Synthesized primarily from amino acids • Protein hormones—long, folded chains of amino acids; e.g., insulin and parathyroid hormone • Glycoprotein hormones—protein hormones with carbohydrate groups attached to the amino acid chain • Peptide hormones—smaller than protein hormones; short chain of amino acids; e.g., oxytocin and antidiuretic hormone (ADH) • Amino acid derivative hormones—each is derived from a single amino acid molecule • Amine hormones—synthesized by modifying a single molecule of tyrosine; produced by neurosecretory cells and by neurons; e.g., epinephrine and norepinephrine • Amino acid derivatives produced by the thyroid gland; synthesized by adding iodine to tyrosine

  7. Hormones • How hormones work • General principles of hormone action • Hormones signal a cell by binding to the target cell’s specific receptors in a “lock-and-key” mechanism (Figure 16-7) • Different hormone-receptor interactions produce different regulatory changes within the target cell through chemical reactions • Combined hormone actions: • Synergism—combinations of hormones acting together have a greater effect on a target cell than the sum of the effects that each would have if acting alone • Permissiveness—when a small amount of one hormone permits, or enables,a second one to have its full effects on a target cell • Antagonism—one hormone produces the opposite effects of another hormone; used to “fine tune” the activity of target cells with great accuracy • Most hormones have primary effects that directly regulate target cells and many secondary effects that influence or modulate other regulatory mechanisms in target cells • Endocrine glands produce more hormone molecules than are actually needed; the unused hormones are quickly excreted by the kidneys or broken down by metabolic processes

  8. Hormones • How hormones work (cont.) • Mechanism of steroid hormone action (Figure 16-8) • Steroid hormones are lipid-soluble, and their receptors are normally found in the target cell’s cytosol • After a steroid hormone molecule has diffused into the target cell, it binds to a receptor molecule to form a hormone-receptor complex • Mobile-receptor model—hormone passes into nucleus, where it binds to mobile receptor and activates a certain gene sequence to begin transcription of mRNA; newly formed mRNA molecules move into the cytosol, associate with ribosomes, and begin synthesizing protein molecules that produce the effects of the hormone • Steroid hormones regulate cells by regulating production of certain critical proteins • The amount of steroid hormone present determines magnitude of a target cell’s response • Because transcription and protein synthesis take time, responses to steroid hormones are often slow

  9. Hormones • How hormones work (cont.) • Mechanisms of nonsteroid hormone action • The second messenger mechanism—also known as the fixed-membrane-receptor model (Figure 16-9) • A nonsteroid hormone molecule acts as a “first messenger” and delivers its chemical message to fixed receptors in the target cell’s plasma membrane • The “message” is then passed by way of a G protein into the cell, where a “second messenger” triggers the appropriate cellular changes • Second messenger mechanism—produces target cell effects that differ from steroid hormone effects in several important ways: • Effects of the hormone are amplified by the cascade of reactions • There are a variety of second messenger mechanisms—examples: IP3, GMP, calcium-calmodulin mechanisms (Figure 16-10) • The second messenger mechanism operates much more quickly than the steroid mechanism • The nuclear receptor mechanism—small iodinated amino acids (T4 and T3) enter the target cell and bind to receptors associated with a DNA molecule in the nucleus; this binding triggers transcription of mRNA and synthesis of new enzymes

  10. Hormones • Regulation of hormone secretion • Control of hormonal secretion is usually part of a negative feedback loop and is called endocrine reflexes (Figure 16-11) • Simplest mechanism—when an endocrine gland is sensitive to the physiological changes produced by its target cells • Endocrine gland secretion may also be regulated by a hormone produced by another gland • Endocrine gland secretions may be influenced by nervous system input; this fact emphasizes the close functional relationship between the two systems

  11. Hormones • Regulation of target cell sensitivity • Sensitivity of target cell depends in part on number of receptors (Figure 16-12) • Up-regulation—increased number of hormone receptors increases sensitivity • Down-regulation—decreased number of hormone receptors decreases sensitivity • Sensitivity of target cell may also be regulated by factors that affect signal transcription or gene transcription

  12. Prostaglandins (PGs) • Unique group of lipid hormones (20-carbon fatty acid with 5-carbon ring) that serve important and widespread integrative functions in the body but do not meet the usual definition of a hormone (Figure 16-13; Table 16-4) • Called tissue hormones because the secretion is produced in a tissue and diffuses only a short distance to other cells within the same tissue; PGs tend to integrate activities of neighboring cells

  13. Prostaglandins • Many structural classes of prostaglandins have been isolated and identified: • Prostaglandin A (PGA)—intraarterial infusion resulting in an immediate fall in blood pressure accompanied by an increase in regional blood flow to several areas • Prostaglandin E (PGE)—vascular effects: regulation of red blood cell deformability and platelet aggregation; inflammation (which can be blocked with drugs that inhibit PG-producing enzymes such as COX-1 and COX-2), gastrointestinal effects: regulates hydrochloric acid secretion • Prostaglandin F (PGF)—especially important in reproductive system, causing uterine contractions; also affects intestinal motility and is required for normal peristalsis • Many tissues are known to secrete PGs • PGs have diverse physiological effects

  14. Pituitary Gland • Structure of the pituitary gland • Formerly known as hypophysis • Size: 1.2 to 1.5 cm (about 1⁄2 inch) across; weight: 0.5 g (1⁄60 ounce) • Located on the ventral surface of the brain within the skull (Figure 16-14) • Infundibulum—stemlike stalk that connects pituitary to the hypothalamus • Made up of two separate glands, the adenohypophysis (anterior pituitary gland) and the neurohypophysis (posterior pituitary gland)

  15. Pituitary Gland • Adenohypophysis (anterior pituitary) • Divided into two parts: • Pars anterior—forms the major portion of adenohypophysis • Pars intermedia • Tissue is composed of irregular clumps of secretory cells supported by fine connective tissue fibers and surrounded by a rich vascular network • Three types of cells can be identified according to their affinity for certain stains (Figure 16-15): • Chromophobes—do not stain • Acidophils—stain with acid stains • Basophils—stain with basic stains

  16. Pituitary Gland • Adenohypophysis (anterior pituitary) (cont.) • Five functional types of secretory cells exist: • Somatotrophs—secrete GH • Corticotrophs—secrete ACTH • Thyrotrophs—secrete TSH • Lactotrophs—secrete prolactin (PRL) • Gonadotrophs—secrete LH and FSH

  17. Pituitary Gland • Adenohypophysis (anterior pituitary) (cont.) • Growth hormone (GH) (Figure 16-16; Table 16-6) • Also known as somatotropin (STH) • Promotes growth of bone, muscle, and other tissues by accelerating amino acid transport into the cells • Stimulates fat metabolism by mobilizing lipids from storage in adipose cells and speeding up catabolism of the lipids after they have entered another cell • GH tends to shift cell chemistry away from glucose catabolism and toward lipid catabolism as an energy source; this leads to increased blood glucose levels • GH functions as an insulin antagonist and is vital to maintaining homeostasis of blood glucose levels

  18. Pituitary Gland • Adenohypophysis (anterior pituitary) (cont.) • Prolactin (PRL) (Table 16-6) • Produced by acidophils in the pars anterior • Also known as lactogenic hormone • During pregnancy, PRL promotes development of the breasts, anticipating milk secretion; after the baby is born, PRL stimulates the mother’s mammary glands to produce milk

  19. Pituitary Gland • Adenohypophysis (anterior pituitary) (cont.) • Tropic hormones—have a stimulating effect on other endocrine glands; four principal tropic hormones are produced and secreted by the basophils of the pars anterior (Table 16-6): • Thyroid-stimulating hormone (TSH), or thyrotropin—promotes and maintains growth and development of thyroid; also causes thyroid to secrete its hormones • Adrenocorticotropic hormone (ACTH), or adrenocorticotropin—promotes and maintains normal growth and development of cortex of adrenal gland; also stimulates adrenal cortex to secrete some of its hormones • Follicle-stimulating hormone (FSH)—in female, stimulates primary graafian follicles to grow toward maturity; also stimulates follicle cells to secrete estrogens; in male, FSH stimulates development of seminiferous tubules of testes and maintains spermatogenesis • Luteinizing hormone (LH)—in female, stimulates formation and activity of corpus luteum of ovary; corpus luteum secretes progesterone and estrogens when stimulated by LH; LH also supports FSH in stimulating maturation of follicles; in male, LH stimulates interstitial cells in testes to develop and secrete testosterone; FSH and LH are called gonadotropins because they stimulate growth and maintenance of gonads

  20. Pituitary Gland • Adenohypophysis (anterior pituitary) (cont.) • Control of secretion in the adenohypophysis • Hypothalamus secretes releasing hormones into the blood, which are then carried to hypophyseal portal system (Figure 16-17; Table 16-5) • Hypophyseal portal system carries blood from hypothalamus directly to adenohypophysis where target cells of releasing hormones are located (Figure 16-18) • Releasing hormones influence secretion of hormones by acidophils and basophils • Through negative feedback, hypothalamus adjusts secretions of adenohypophysis, which then adjusts secretions of target glands that, in turn, adjust activity of their target tissues (Figure 16-19) • Minute-by-minute variations in hormone secretion can exhibit occasional large peaks, caused by pulse in releasing hormone secretion by hypothalamus (Figure 16-20) • In stress, hypothalamus translates nerve impulses into hormone secretions by endocrine glands, basically creating a mind-body link

  21. Adrenal Glands • Adrenal cortex (cont.) • Mineralocorticoids • Have an important role in regulatory process of sodium in the body • Aldosterone • Only physiologically important mineralocorticoid in the human; primary function is maintenance of sodium homeostasis in the blood by increasing sodium reabsorption in the kidneys • Aldosterone also increases water retention and promotes loss of potassium and hydrogen ions • Aldosterone secretion is controlled by the renin-angiotensin-aldosterone system (RAAS) and by blood potassium concentration (Figure 16-32)

  22. Adrenal Glands • Adrenal cortex (cont.) • Glucocorticoids • Main glucocorticoids secreted by the zona fasciculata are cortisol, cortisone, and corticosterone, with cortisol the only one secreted in significant quantities • Affect every cell in the body • Are protein-mobilizing, gluconeogenic, and hyperglycemic • Tend to cause a shift from carbohydrate catabolism to lipid catabolism as an energy source • Essential for maintaining normal blood pressure by aiding norepinephrine and epinephrine to have their full effect, causing vasoconstriction

  23. Adrenal Glands • Glucocorticoids (cont.) • High blood concentration causes eosinopenia and marked atrophy of lymphatic tissues • Act with epinephrine to bring about normal recovery from injury produced by inflammatory agents • Secretion increases in response to stress • Except during stress response, secretion is mainly controlled by a negative feedback mechanism involving ACTH from the adenohypophysis • Secretion is characterized by several large pulses of increased hormone levels throughout the day—the largest occurring just before waking (Figure 16-33) • Gonadocorticoids—sex hormones (androgens) that are released from the adrenal cortex

  24. Adrenal Glands • Adrenal medulla • Neurosecretory tissue—composed of neurons specialized to secrete their products into the blood • Adrenal medulla secretes two important hormones—epinephrine and norepinephrine; they are part of the class of nonsteroid hormones called catecholamines • Both hormones bind to the receptors of sympathetic effectors to prolong and enhance the effects of sympathetic stimulation by the ANS (Figure 16-34)

  25. Pancreatic Islets • Structure of the pancreatic islets (Figure 16-35) • Elongated gland, weighing approximately 100 g (3.5 ounces); its head lies in the duodenum, extends horizontally behind the stomach, and then touches the spleen • Composed of endocrine and exocrine tissues • Pancreatic islets (islets of Langerhans)—endocrine portion • Acini—exocrine portion—secretes a serous fluid containing digestive enzymes into ducts draining into the small intestine

  26. Pancreatic Islets • Structure of the pancreatic islets (cont.) • Pancreatic islets—each islet contains four primary types of endocrine glands joined by gap junctions • Alpha cells (A cells)—secrete glucagon (Figure 16-36) • Beta cells (B cells)—secrete insulin; account for up to 75% of all pancreatic islet cells • Delta cells (D cells)—secrete somatostatin • Pancreatic polypeptide cells (F, or PP, cells)—secrete pancreatic polypeptides

  27. Pancreatic Islets • Pancreatic hormones (Table 16-9)—work collaboratively to maintain homeostasis of food molecules (Figure 16-37) • Glucagon—produced by alpha cells; tends to increase blood glucose levels; stimulates gluconeogenesis in liver cells • Insulin—produced by beta cells; lowers blood concentration of glucose, amino acids, and fatty acids and promotes their metabolism by tissue cells • Somatostatin—produced by delta cells; primary role is regulating the other endocrine cells of the pancreatic islets • Pancreatic polypeptide—produced by F (PP) cells; influences the digestion and distribution of food molecules to some degree

  28. Gonads • Testes (Figure 16-2; Table 16-10) • Paired organs within the scrotum in the male • Composed of seminiferous tubules and a scattering of interstitial cells • Testosterone is produced by the interstitial cells and is responsible for the growth and maintenance of male sexual characteristics • Testosterone secretion is mainly regulated by gonadotropin levels in the blood

  29. Gonads • Ovaries (Figure 16-2; Table 16-10) • Primary sex organs in the female • Set of paired glands in the pelvis that produce several types of sex hormones • Estrogens—steroid hormones secreted by ovarian follicles; promote development and maintenance of female sexual characteristics • Progesterone—secreted by corpus luteum; maintains the lining of the uterus necessary for successful pregnancy • Ovarian hormone secretion depends on the changing levels of FSH and LH from adenohypophysis

  30. Placenta • Tissues that form on the lining of the uterus as a connection between the circulatory systems of the mother and the developing child • Serves as a temporary endocrine gland that produces human chorionic gonadotropin, estrogens, and progesterone (Table 16-10)

  31. Thymus (Figure 16-2) • Gland located in the mediastinum just beneath the sternum • Thymus is large in children, begins to atrophy at puberty, and, by old age, the gland is a vestige of fat and fibrous tissue • Considered to be primarily a lymphatic organ, but the hormone thymosin has been isolated from thymus tissue (Table 16-10) • Thymosin—stimulates development of T cells

  32. Gastric and Intestinal Mucosa • The mucous lining of the GI tract contains cells that produce both endocrine and exocrine secretions (Table 16-10) • GI hormones such as gastrin, secretin, and cholecystokinin (CCK) play regulatory roles in coordinating the secretory and motor activities involved in the digestive process • Ghrelin—hormone secreted by endocrine cells in gastric mucosa; stimulates hypothalamus to boost appetite; slows metabolism and fat burning; may be a contributor to obesity

  33. Heart • The heart has a secondary endocrine role • Hormone-producing cells produce several atrial natriuretic peptides (ANPs), including atrial natriuretic hormone (ANH) (Table 16-10) • ANH’s primary effect is to oppose increases in blood volume or blood pressure; also an antagonist to ADH and aldosterone

  34. Other Endocrine Glands and Organs (Table 16-10) • Major endocrine glands produce more hormones that are outlined in this book (e.g., inhibin secreted by the ovaries) • Many tissues (perhaps all tissues) produce hormones, most of which are beyond the scope of this book (e.g., leptin and resistin secreted by adipose tissue)

  35. Cycle of Life: Endocrine System • Endocrine regulation begins in the womb • Many hormones are active from gestational period • Evidence that a hormonal signal from fetus to mother signals the onset of labor • Hormones related to reproduction begin at puberty • Secretion of male reproductive hormones—continuous production from puberty, slight decline in late adulthood • Secretion of female reproductive hormones declines suddenly and completely in middle adulthood

  36. The Big Picture: The Endocrine System and the Whole Body • Nearly every process in the human organism is kept in balance by the intricate interaction of different nervous and endocrine regulatory chemicals • The endocrine system operates with the nervous system to finely adjust the many processes they regulate • Neuroendocrine system adjusts nutrient supply • Calcitonin, parathyroid hormone, and vitamin D balance calcium ion use • The nervous system and hormones regulate reproduction

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