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The endocrine system, like the nervous system, adjusts and correlates the activities of the various body systems. Endocrine integration is brought about by hormones. Hormones are chemical messengers produced by endocrine glands and endocrine cells in the organs and tissues of the body. Some of the hormones are amines and others are amino acids, polypeptides, proteins and steroids. In general, the hormones regulate the processes of metabolism, growth and development, water and electrolyte balance, reproduction and behavior. The principal mechanisms by which hormones exert their intracellular effects are signal transduction pathways initiated by receptors. There are two important pathways by which most hormones function: (1) activation of second messenger systems (cAMP, IP3, DAG, and Ca2+) through membrane receptors acting via G-proteins or (2) regulation of gene transcription through intracellular receptors and transcription factors. Hormone secretion is usually controlled by negative and positive feedback mechanisms that ensure a proper level of hormone activity at the target tissue. This can display periodic variations. The major endocrine glands and their hormones are discussed in this chapter:
The pituitary and hypothalamus are an anatomical and functional unit. Six important hormones are secreted by the anterior pituitary, which plays major roles in the control of metabolic functions throughout the body, thus: (1) GH promotes growth of the entire body by enhancing protein formation, cell multiplication and differentiation; (2) ACTH controls the section of some of adrenocortical hormones in particular cortisol; (3) TSH causes the thyroid gland to secrete T3 and T4; (4) PRL stimulates development of breasts and secretion of milk; (5) FSH and LH control growth of the gonads as well as their hormonal secretions and reproduction. Secretion by the anterior pituitary is controlled by the hypothalamic releasing and inhibitory hormones secreted within the hypothalamus. Thus, TRH causes release of TSH; CRH, ACTH; GHRH, GH; GHIH inhibits release of GH; GnRH causes release of LH and FSH; PIH and PRH causes inhibition and release of PRL respectively. In contrast to the anterior pituitary, the posterior pituitary is controlled by nerve signals that originate in the hypothalamus. Their axons terminate in the posterior pituitary and release ADH or OXT into the bloodstream. ADH promotes water retention whereas OXT helps milk ejection and contraction of the pregnant uterus.
The thyroid secretes large amounts of two hormones, T4 and T3. It also secretes calcitonin. T4 and T3 are synthesized in the colloid by iodination and condensation of tyrosine molecules bound in peptide linkage in thyroglobulin. Physiologic effects of thyroid hormones are the increase in O2 consumption by metabolically active tissues, stimulation of lipolysis and protein breakdown, increasing carbohydrate absorption, promoting bone normal growth and brain development, and having chronotropic and inotropic action on the heart, and so on. The absence of thyroid hormones causes mental and physical slowing, poor resistance to cold and, in children, mental retardation and dwarfism. The rate of thyroid secretion is regulated primarily by specific feedback mechanisms, which operate through the hypothalamus (TRH) and anterior pituitary (TSH) to thyroid. The transient inhibition of thyroid hormone synthesis is also caused by the Wolff-Chaikoff effect.
There are two endocrine organs in the adrenal gland: medulla and cortex. The main secretions of the adrenal medulla are the catecholamines epinephrine, norepinephrine and dopamine. The adrenal cortex mainly secretes the glucocorticoid cortisol, the mineralocorticoid aldosterone and some sex hormones. Functions of cortisol are (1) elevatation of blood glucose concentration by stimulation of gluconeogenesis and decreased glucose utilization in the cell; (2) reduction of cellular protein by decreasing protein synthesis and increasing catabolism of protein; (3) increase oxidation and use of fatty acids and development of a peculiar obesity; (4) resistance to stress and inhibition of the inflammatory response; (5) decreases in the number of eosinophils and lymphocytes and increased the production of red blood cells; (6) permissive action for glucagon and catecholamine to exert their effects. Both basal secretions of glucocorticoids and the increase of secretion provoked by stress are dependent upon the feedback control of the hypothalamus (CRH)-anterior pituitary (ACTH)-adrenal cortex system. Effects of epinephrine and norepinephrine are (1) mimicking the effects of noradrenergic nervous discharge, especially on the heart; (2) exerting metabolic effects that include glycogenolysis in liver and skeletal muscle, mobilization of FFA and stimulation of the metabolic rate; and (3) being responsible for the "emergency function of the sympathoadrenal system".
The human pancreas has islets which secrete insulin and glucagons. Insulin is a small protein synthesized in the beta cells of islets. The insulin decreases glucose in the blood by increasing the uptake and utilization of glucose by the body's tissues, promoting glycogen synthesis, preventing breakdown of the glycogen, inhibiting gluconeogenesis,; Insulin also promotes fat and protein synthesis and storage. The factors stimulating insulin secretion are increased blood glucose, free fatty acids and amino acids, gastrointestinal hormones, glucagon, GH, cortisol, parasympathetic stimulation and obesity; The inhibiting factor for insulin secretion are decreased blood glucose, fasting, somatostatin. Human glucagon is produced by a cells of the pancreatic islets, when the blood glucose concentration falls. Its functions are opposed to those of insulin. Most important of these functions is to increase the blood glucose concentration by glycogenolysis and glyconeogenesis. Glucagon also increases ketone body formation and stimulates the secretion of GH, insulin, and pancreatic somatostatin. The factors stimulating glucagon secretion are amino acids, CCK, gastrin, cortisol, exercise, infection and physiological stresses; factors causing inhibition of glucagons release include glucose, somatostatin, secretin, FFA, ketones, insulin and GABA.
Parathyroid hormone (PTH), 1,25-(OH)2D3 and calcitonin (CT) are primarily concerned with the regulation of calcium metabolism of the body. PTH is secreted by the parathyroid glands. The action of PTH is to increase the plasma Ca2+ and depress the plasma phosphate by mobilization of Ca2+ from bone, increased urinary phosphate excretion, promotion of the formation of 1,25-(OH)2D3 and Ca2+ absorption from the intestine. The secretion of PTH is inhibited by circulating Ca2+ and 1,25-(OH)2-D3. Increased plasma phosphate stimulates PTH secretion by lowering plasma Ca2+. 1,25-(OH)2D3 is formed in the kidneys. Its actions are to increase Ca2+ absorption from the intestine and Ca2+reabsorption in the kidneys and to mobilize Ca2+ and PO43- by increasing the number of mature osteoclasts. Formation of 1, 25-(OH)2D3 is regulated in a feedback fashion by plasma Ca2+ and PO43-, and facilitated by PTH, GH and CT. CT secreted by thyroid gland lowers the circulating calcium and phosphate levels by inhibiting bone Ca2+ absorption and increasing uric Ca2+ excretion. Plasma CT is directly proportionate to plasma Ca2++, gastrin, CCK, glucagon, and secretin.
The pineal glands secrete melatonin and have function as a timing device to control circadian rhythm. Prostaglandin has been found in many tissues of the body, in addition to the prostate with a wide range of physiological effects.
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