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Anatomy & Physiology Chapter 18: Endocrine System Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. Organization of the Endocrine System The endocrine and nervous systems function to achieve and maintain homeostasis (Table 18-1) When the two systems work together, 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 by way of the blood to specific target cells contained in target tissues or target organs Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 2 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 3 Organization of the Endocrine System 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 18-2; Table 18-2) Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 4 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 5 Hormones Classification of hormones Classification by general function Tropic hormones—hormones that target other endocrine glands and stimulate their growth and secretion Sex hormones—hormones that target reproductive tissues Anabolic hormones—hormones that stimulate anabolism in target cells Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 6 Hormones Classification of hormones (cont) Classification by chemical structure (Figure 18-3; Table 18-3) Steroid hormones Nonsteroid hormones Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 7 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 8 Hormones Classification of hormones (cont) Steroid hormones (Figure 18-4) Synthesized from cholesterol (Figure 18-5) Lipid soluble and can easily pass through the phospholipid plasma membrane of target cells Examples of steroid hormones: cortisol, aldosterone, estrogen, progesterone, and testosterone Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 9 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 10 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 11 Hormones Classification of hormones (cont) Nonsteroid hormones (Figure 18-6) Synthesized primarily from amino acids Protein hormones—long, folded chains of amino acids; e.g., insulin, 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, antidiuretic hormone (ADH) Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 12 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 13 Hormones Nonsteroid hormones (cont) Amino acid derivative hormones—each is derived from a single amino acid molecule Amine hormones—synthesized by modifying a single molecule of tyrosine or tryptophan; produced by neurosecretory cells and by neurons; e.g., epinephrine, norepinephrine Amino acid derivatives produced by the thyroid gland; synthesized by adding iodine to tyrosine Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 14 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 18-7) Different hormone-receptor interactions produce different regulatory changes within the target cell through chemical reactions Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 15 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 16 Hormones General principles of hormone action (cont) 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 allows 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 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 17 Hormones General principles of hormone action (cont) 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 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 18 Hormones How hormones work (cont) Mechanism of steroid hormone action (Figure 18-8) 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 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 19 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 20 Hormones Mechanism of steroid hormone action (cont) Mobile-receptor model—the hormone passes into the nucleus, where it binds to a 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 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 21 Hormones Mechanism of steroid hormone action (cont) The amount of steroid hormone present determines the magnitude of a target cell’s response Because transcription and protein synthesis take time, responses to steroid hormones are often slow Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 22 Hormones How hormones work (cont) Mechanism of nonsteroid hormone action The second messenger mechanism—also known as the fixed-membrane-receptor model (Figure 18-9) 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 Gprotein–coupled receptor (GPCR) into the cell where a “second messenger” triggers a G protein, which leads to the appropriate cellular changes Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 23 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 24 Hormones The second messenger mechanism (cont) Second messenger mechanism produces target cell effects that differ from steroid hormone effects in several important ways The effects of the hormone are amplified by the cascade of reactions There are a variety of second messenger mechanisms—e.g., IP3, cGMP, calciumcalmodulin mechanisms (Figure 18-10) Second messenger mechanism operates much more quickly than the steroid mechanism Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 25 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 26 Hormones Mechanism of nonsteroid hormone action 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 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 27 Hormones Regulation of hormone secretion Control of hormonal secretion is usually part of a negative feedback loop and is called endocrine reflexes (Figure 18-11) 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 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 28 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 29 Hormones Regulation of target cell sensitivity Sensitivity of target cell depends in part on number of receptors (Figure 18-12) 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 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 30 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 31 Prostaglandins 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 18-13; Table 18-4) 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 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 32 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 33 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 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 34 Prostaglandins Many tissues are known to secrete PGs PGs have diverse physiological effects Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 35 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 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 36