The Endocrine System CHAPTER 8 The endocrine system, interacts with the nervous system to coordinate and integrate the activity of body cells. -The NS regulates the activity of muscles and glands via electrochemical impulses and those organs respond within milliseconds. -The endocrine system influences metabolic activity by means of hormones that typically occur after a lag period of seconds or even days. But, once initiated, those responses tend to be much more prolonged than those induced by the nervous system. Hormonal targets Include most cells of the body, but mainly: -Reproduction; - Growth and development; - Mobilization of body defenses; - Maintenance of electrolyte, H2O, & nutrient balance of the blood; and -Regulation of cellular metabolism and energy balance. Endocrinology: scientific study of hormones and the endocrine organs Compared with other organs, those of the endocrine system are: - small and unimpressive. (to collect 1 kg of hormone- producing tissue, endocrine tissue from eight or nine adults would be needed) - no anatomical continuity typical of most organ systems,the endocrine organs are widely scattered about the body. The synthesis and release of most hormones are regulated by some type of negative feedback system In such a system, hormone secretion is triggered by some internal or external stimulus. As hormone levels rise, they cause target organ effects and inhibit further hormone release. As a result, blood levels of many hormones vary only within a narrow range. Control of Hormone Release Endocrine Organs The endocrine glands include the pituitary, thyroid, parathyroid, adrenal, pineal, and thymus glands. -In addition, the pancreas and gonads (ovaries and testes), are also major endocrine(mixed glands). -The hypothalamus, along with its neural functions, produces and releases hormones, so we can consider the hypothalamus a neuroendocrine organ. Endocrine function of hypothalamus: (a) regulates the hormonal output of the anterior pituitary via releasing and inhibiting hormones. (b) synthesizes two hormones (???????) that it exports to the posterior pituitary for storage and later release. Major Endocrine Organs I-The Pituitary Gland (Hypophysis) It hangs from the base of the brain by a stalk and is enclosed by bone(Turk`s saddle). It consists of a hormone-producing glandular portion (anterior pituitary, adenohypophysis ) and a neural portion (posterior pituitary, neurohypophysis). Four of the six anterior pituitary hormonesthyroid- stimulating hormone, adrenocorticotropic hormone, follicle-stimulating hormone, and luteinizing hormoneare tropins or tropic hormones (tropi = turn on, change), which are hormones that regulate the secretory action of other endocrine glands. 1-Growth hormone (GH) : - stimulates most body cells to increase in size and divide, mainly bones and skeletal muscles leading to long bone growth and increased muscle mass. - Secretion of GH is regulated chiefly by two antagonistic hypothalamic hormones: 1- Growth hormonereleasing hormone (GHRH) and 2- Growth hormone inhibiting hormone (GHIH), (also called somatostatin). Hormones of Pitutary gland 2-Prolactin (PRL) promotes milk production in humans. Its secretion is prompted by prolactin-releasing hormone (PRH) and inhibited by prolactin- inhibiting hormone (PIH) from the hypothalamus. 3-Adrenocorticotropic hormone (ACTH)or corticotropin stimulates the adrenal cortex to release corticosteroid hormones. ACTH release, elicited by hypothalamic corticotropin-releasing hormone (CRH). Hormones of Pitutary gland 4- Thyroid-stimulating hormone (TSH), or thyrotropin, is a tropic hormone that stimulates normal development and secretory activity of the thyroid gland. TSH release is triggered by the hypothalamic thyrotropin-releasing hormone (TRH). 5&6-Gonadotropins : Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) referred to collectively as gonadotropins, regulate the function of the gonads (ovaries and testes)In both sexes: -FSH stimulates gamete (sperm or egg) production -LH promotes production of gonadal hormones. * In females, LH works with FSH to cause maturation of an egg-containing ovarian follicle. LH then independently triggers ovulation and promotes synthesis and release of ovarian hormones. *In males, LH stimulates the interstitial cells of the testes to produce the male hormone testosterone. Gonadotropins are absent in prepubertal boys and girls. At puberty, gonadotropin begins to rise, causing the gonads to mature. In both sexes, gonadotropin release by the adenohypophysis is prompted by gonadotropin-releasing hormone (GnRH) produced by the hypothalamus. Gonadal hormones, feed back to suppress FSH and LH release. The Posterior Pituitary It stores antidiuretic hormone (ADH) and oxytocin that have been synthesized by hypothalamus. 1-Oxytocin is released in higher amounts during childbirth and breastfeeding women. As blood levels of oxytocin rise, the expulsive contractions of labor increases and finally end in birth. 2-Antidiuretic Hormone (ADH) It targets the kidney tubules that respond by reabsorbing more water to the bloodstream. As a result, less urine is produced and blood volume increases. As the solute concentration of the blood declines, the ADH release stops. II-The Thyroid Gland Butterfly-shaped located just inferior to the larynx and is composed of: -The follicles cells largest pure endocrine gland in the body. It which produce thyroid hormones. -The parafollicular cells, produce calcitonin, an entirely different hormone. 1-Thyroid Hormone (TH) Is the bodys major metabolic hormone.TH is actually two : thyroxine, or T 4, and triiodothyronine or T 3. The difference is the number of bound iodine atoms. TH affects virtually every cell in the body by ncreasing basal metabolic rate and body heat production.It is especially critical for normal skeletal, nervous system development and maturation and for reproductive capabilities. 2-Calcitonin Is produced by the parafollicular cells of the thyroid gland. It is a direct antagonist of parathyroid hormone. Calcitonin targets the skeleton,where it stimulates Ca 2+ uptake and incorporation into bone matrix. Thus, calcitonin has a bone-sparing effect. Excessive blood levels of Ca 2+ act as a stimulus for calcitonin release, whereas declining blood Ca 2+ levels inhibit its release. Calcitonin appears to be important only in childhood, when the skeleton grows quickly and the bones are changing dramatically in mass, size, and shape. III-The Parathyroid Glands The tiny, yellow-brown parathyroid glands are in the posterior aspect of the thyroid gland.There are usually four of these glands, but the precise number varies( may be eight, and some may be located in the neck or even in the thorax). Discovery of the parathyroid glands was after partial thyroid gland removal, when some patients suffered uncontrolled muscle spasms and severe pain, and subsequently died. III-The Parathyroid Glands The tiny, yellow-brown,four parathyroid glands are in the posterior aspect of the thyroid gland. Discovery of the parathyroid glands was after partial thyroid gland removal, when some patients suffered uncontrolled muscle spasms and severe pain, and subsequently died. Parathyroid hormone (PTH), or parathormone, is the single most important hormone controlling the calcium balance of the blood. PTH increases Ca 2+ levels in blood Leading to hypercalcemia. *Because plasma calcium is essential for nerve impulses, muscle contraction, and blood clotting, precise control of Ca 2+ levels is critical. IV-The Adrenal (Suprarenal) Glands The paired adrenal glands are pyramid-shaped organs atop the kidneys.Each adrenal gland is structurally and functionally two endocrine glands. -The inner adrenal medulla, more like a knot of the sympathetic nervous system. -The outer adrenal cortex, encapsulating the medullary region and forming the bulk of the gland. *Each region produces its own set of hormones but all adrenal hormones help us cope with stressful situations. A-The Adrenal Cortex Its Steroid hormones( all are called corticosteroids), are synthesized from cholesterol. The cortical cells are arranged in three layers or zones: -The superficial zone produce mineralocorticoids. -The middle zone produce the metabolic hormones called glucocorticoids. -The innermost zone produce small amounts of adrenal sex hormones, or gonadocorticoids. 1-Mineralocorticoids Although there are many mineralocorticoids, aldosterone accounts for more than 95% of them. Aldosterone reduces excretion of Na from the kidney tubules followed by water. Aldosterone secretion is stimulated by low blood levels of Na +, and decreasing blood volume and blood pressure 2- Glucocorticoids(cortisol) Essential to life, they : -influence the energy metabolism of most body cells - help us to resist stressors as hemorrhage, infection, or physical or emotional trauma. -keep blood glucose levels fairly constant (hyperglycemic) by breaking down of fats and even proteins(gluconeogenesis). 3-Gonadocorticoids (Sex Hormones) The bulk of it are weak androgens, or male sex hormones(it also makes small amounts of female hormones). It is insignificant compared with the amounts made by the gonads during late puberty and adulthood. B-The Adrenal Medulla It is considered as part of the autonomic nervous system. They are modified ganglionic sympathetic neurons that synthesize the catecholamines epinephrine and norepinephrine (NE). When the sympathetic nervous system is mobilized, blood glucose levels rise, blood vessels constrict and the heart beats faster (together raising the blood pressure), blood is diverted from temporarily nonessential organs to the heart and skeletal muscles, and pre- ganglionic sympathetic nerve endings to the adrenal medulla signal for release of catecholamines, which reinforce and prolong the fight-or-flight response. *Unlike the adrenocortical hormones, which promote long-lasting body responses to stressors, catecholamines cause fairly brief responses. VI-Pancreatic islets (islets of Langerhans) The pancreas is a mixed gland. Scattered among the enzyme-producing cells are approximately a million pancreatic islets,tiny cell clusters that produce pancreatic hormones.The islets contain : -the glucagon-synthesizing alpha () cells and -the more numerous insulin-producing beta () cells. Insulin and glucagon effects are antagonistic: Insulin is a hypoglycemic hormone, whereas glucagon is a hyperglycemic hormone. 1-Glucagon Is an extremely potent hyperglycemic agent by breakdown of glycogen in the liver to glucose (glycogenolysis). -Insulin Its main effect is to lower blood glucose levels by : 1-enhancing membrane transport of glucose (and other simple sugars) into body cells, especially muscle and fat cells. (It does not accelerate glucose entry into liver, kidney, and brain tissue, all of which have easy access to blood glucose regardless of insulin levels.) 2-Inhibition of the breakdown of glycogen to glucose and the conversion of amino acids or fats to glucose; thus, it counters any metabolic activity that would increase plasma levels of glucose. VII-The Gonads The male and female gonads produce steroid sex hormones, identical to those produced by adrenal cortical cells. The major distinction is the source and relative amounts produced. Female gonads: -The paired ovaries are small, oval organs located in the females abdomino-pelvic cavity. Besides producing ova, the ovaries produce several hormones, most importantly estrogens and progesterone. -the estrogens are responsible for maturation of the reproductive organs and the appearance of the secondary sex characteristics of females at puberty. Acting with progesterone, estrogens promote breast development and the menstrual cycle. -Progesterone is the hormone of pregnancy Male gonads: The male testes, located in an extra-abdominal skin pouch called the scrotum, produce sperm and male sex hormones( androgens), primarily testosterone -During puberty, testosterone initiates the maturation of the male reproductive organs and the appearance of secondary sex characteristics and sex drive. In addition, testosterone is necessary for normal sperm production and maintains the reproductive organs in their mature functional state in adult males. The release of gonadal hormones is regulated by gonadotropins(FSH and LH). The tiny, pineal gland hangs from the roof of the third ventricle in the diencephalon. The endocrine function of the pineal gland is still somewhat of a mystery, its only major secretory product is melatonin. Peak levels occur during the night and make us drowsy; lowest levels occur around noon. VIII-pineal gland. IX-The Thymus Located deep to the sternum,Large in infants and children, the thymus diminishes in size throughout adulthood. By old age, it is composed largely of adipose and fibrous connective tissues. The major hormonal products appear to be essential for the normal development of T lymphocytes and the immune response Other Hormone-Producing Structures Other hormone-producing cells occur in various organs of the body, including the following : 1. Heart. The atria contain some specialized cardiac muscle cells that secrete atrial natriuretic peptide. By signaling the kidneys to increase their production of salty urine and by inhibiting aldosterone release by the adrenal cortex, ANP reduces blood volume, blood pressure, and blood sodium concentration. 2. Gastrointestinal tract. In the mucosa of the GI tract, there are scattered cells release several peptide hormones that help regulate a wide variety of digestive functions. Gastrin, secretin and cholecystokinin are examples of these hormones. 3. Placenta. Besides sustaining the fetus during pregnancy, the placenta secretes several steroid and protein hormones that influence the course of pregnancy. Placental hormones include estrogens and progesterone (hormones more often associated with the ovary), and human chorionic gonadotropin (hCG). 4. Kidneys. Interstitial cells in the kidneys secrete erythropoietin,a protein hormone that signals the bone marrow to increase production of red blood cells. The kidneys also release renin, the hormone that initiates the renin-angiotensin mechanism. 5. Skin. The skin produces cholecalciferol, an inactive form of vitamin D 3 which is then activated in liver and kidneys. Prostglandins Are locally acting hormones derived from fatty acid molecules secreted from plasma membranes of virtually all body cells after stimulation by local irritants or hormones, etc. There are several groups known till now indicated by the letters A, B, C, till I (PGA-PGI) They have many targets, but act locally at site of release. Examples of effects include: -increase blood pressure by acting as vasoconstrictor -constriction of respiratory passageways -stimulate uterine muscles, promoting labor -enhance blood clotting -promote inflammation and pain -increase gastric secretions -cause fever