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THE ENDOCRINE SYSTEM 1) consists of endocrine cells which are formed into glands which release secretions internally rather than externally like exocrine glands which release secretions onto epithelial tissue 2) ductless glands that secrete: a) prostoglandins- (local hormones which affect local cells) b) hormones-chemical messengers released in one tissue and carried to another tissue by the bloodstream 3) Twelve regions containing endocrine glands: hypothalamus, pineal gland, pituitary gland, thyroid gland, parathyroid glands, thymus, adrenal gland, heart, kidney, digestive tract, pancreas, gonads. THE STRUCTURE OF HORMONES Classified into three groups based on chemical structure: 1) Amino acid derivatives: epinephrine, norepinephrine, thyroid hormones, and melatonin 2) Peptide hormones: chains of amino acids, largest class of hormones, includes all hormones secreted by the hypothalamus, pituitary gland, heart, kidneys, thymus, digestive tract, and pancreas. 3) Lipid derivatives: two classes: a) Steroid hormones-secreted by reproductive glands and adrenal glands--structurally similar to cholesterol b) prostaglandins-fatty acid based,coordinate cellular activity and affects enzymatic processes (blood clotting, digestion....) HORMONES AND THE CELL MEMBRANE 1) Hormones have specific shapes that bind with specific receptors on target cells (hormones change the identity, activities or quantities of enzymes and structural proteins in target cells) in target organs. 2) Epinephrine, norepinephrine, and peptide hormones are not lipid soluble and too large to diffuse through pores in the membrane (channels). They are called first messengers and targets receptors on the cell membrane. After binding, they trigger the appearance of a second messenger (in the cytoplasm) which functions as an enzyme activator,or inhibitor resulting in a change in the metabolic rate of a cell) An example of a second messenger would be cyclic-AMP (cAMP) which forms after the binding hormone activates and enzyme called adenylate cyclase which converts ATP to cAMP. The presence of cAMP will have different effects in on target organ from another(response determined by enzymes already present in the target organ). Effects of cAMP is short-lived, it is quickly broken down by phospodiesterase Other common second messengers: calcium ions and cGMP. 3) Steroid hormones diffuse through lipid portion of membrane, bind to receptors in the cytoplasm or the nucleus producing a hormone receptor complex which binds to DNA and triggers the activation or inactivation of certain genes. example: alters the rate of transcription of mRNA. This can change the structure of function of the cell. Testosterone stimulates the production of enzymes and proteins in skeletal muscle fibers increasing muscle size and strength. 3) Thyroid hormones cross the cell membrane through diffusion or by carrier molecules, then bind to receptors in the nucleus or mitochondria, complexes in mitochondria increase ATP production; complexes in nucleus activate genes which changes enzyme production (types or quantities of enzymes are altered) CONTROL OF ENDOCRINE ACTIVITY 1) Negative feedback: The effector that is activated by a hormone opposes the stimulus and restores homeostasis due to the presence of another hormone. example: When calcium levels in the blood decline below normal the parathyroid glands will secrete PTH which stimulates target cells to increase blood calcium levels. When calcium levels arise to normal then calcitonin is released (from the thyroid gland) and the target cells respond by lowering calcium levels) THE ENDOCRINE GLANDS 1) The hypothalamus: a) controls endocrine cells in the adrenal medullae through sympathetic innervation b) hypothalamus secretes ADH (antidiuretic hormone) and oxytocin at posterior pituitary gland c) hypothalamus secretes regulatory hormones (RH -releasing hormone and IH-inhibiting hormone) at the anterior pituitary gland 2) The pituitary gland (hypophysis): a) found attached beneath the hypothalamus by the infidibulum within a cavity called the sella turcica in the sphenoid bone of the skull b) Nine hormones are secreted (all are peptide hormones which bind to membrane receptors and use AMP as a second messenger. c) has distinct anterior and posterior regions: Anterior Pituitary: ----contains part of the hypophyseal **portal system (blood vessels that link the two capillary networks together **Portal systems transport blood from one capillary network to another and is named after their distinations ----Hormones released by the anterior pituitary are controlled by RH and IH (**regulatory hormones from the hypothalamus) **regulatary hormones are also called tropins because they turn on other tissues or organs ---Seven hormones are secreted by the anterior pituitary: 1) TSH (thyroid stimulating hormone) stimulates the thyroid gland to secrete thyroxine, and involves negative feedback: a)RH (from the hypothalamus stimulates release of TSH from the anterior pituitary) b) TSH stimulates the release of thyroxine from the thyroid gland c) When thyroxin levels increase above normal it inhibits the release of RH which brings the level back to normal 2) ACTH (adrenocorticotropic hormone) : causes adrenal glands to release glucocorticoids which also involves negative feedback inhibiting RH hormones 3) FSH (follicle stimulating hormone) promotes egg development in the of ovaries women and stimulates the secretion of estrogen; promotes sperm production in testis of men (feedback mechanism same as those above) 4) HL (luetinizing hormone) promotes ovulation in women and secretion of estrogens progesterone by ovaries (preparing body for pregnancy) Promotes production of androgens in men(testosterrone). **LH and FSH are called gonadotropins (same feedback mechanism as the others above) 5) PRL (prolactin) targets mammary glands, stimulates the development of breasts and milk production in females) (other hormones involved but PRL is the dominant one) **PRL has no known effect on the human male Regulation of prolactin determined by interaction between RH and IH from the hypothalamus: RH stimulates the release of PRL from the anterior pituitary. As levels of PRL rise it inhibits RH and stimulates the release of IH(IH inhibits release of RH also) 6) GH (growth hormone or somatotropin): targets all body tissue-especially skeletal muscle cells and chondrocytes (stimulates cell growth and division by accelerating the rate of protein synthesis)
Two mechanisms are involved : a) Liver cells respond by synthesizing and releasing somatomedins (IGF or insulin-like growth factors) Somatomedins bind to receptor site of cells and increase the rate of diffusion of amino acids for protein synthesis (important after eating) b) In epithelial and connective tissues: they stimulate stem cell division and differentiation **metabolic effects: Stimulates the breakdown of stored fat in adipose tissue and release of fatty acids into the blood Stimulates the breakdown of glycogen into glucose in the liver, and glucose is released into the blood 7) MSH (Melanocyte-stimulating hormone): stimulates melanocytes of the skin of fishes, amphibians, reptiles, and many mammals to produce color in hair and skin **MSH cells in humans nonfunctional for most of the time MSH is secreted by human pituitary.... ---during fetal development--- ---in very young children--- ---in pregnant women--- ---in some disease states---- ***functions of MSH in humans are not known Posterior Pituitary: a) Hypothalamus produces two hormones: ADH and oxytocin-transported along axons to posterior pituitary gland into the hypophseal portal system b) ADH (antidiuretic hormone): released in response to ----a rise in concentration of electrolytes in the blood-- ----a fall in blood volume or blood pressure--- functions ---decrease in the amount of water lost by the kidneys (decreases electrolyte concentration) ---causes constriction of peripheral blood vessels (increases blood pressure ) ***ADH is inhibited by alcohol and results in an increase in fluid excretion following consumption c) Oxytocin: In females.... stimulates smooth muscle cells in the uterus in later stages of pregnancy to maintain normal childbirth and labor ***until last stages of pregnancy uterine muscles are insensitive to oxytocin stimulates contractile cells of the mammary glands and is released in response to suckling ("mil letdown" reflex), results in the release of milk into large collecting chambers in mammary glands In males.... stimulates the smooth muscle contraction in the walls of the prostrate gland for the emission (ejection of prostrate secretions, spermatozoa, and other glandular secretions into male reproductive tract before ejaculation. 3) The thyroid gland: a) contains thyroid follicles (spheres surrounded by simple cuboidal epithelium) (cavity within each follicle contains a viscous colloid-a fluid containing proteins) b) thyroid hormones stored within each follicle released into the circulatory system by the epidermal cells when they are stimulated by TSH c) Most of the thyroid hormones become bound with carrier proteins in the blood and are unavailable for target cells, but represent a storage of thyroid hormone--about a week’s supply. Unbound hormone are free to diffuse to target cells. As unbound hormones are depleted, then carrier proteins release additional bound hormones. d) The thyroid hormones: -----thyroxine: (composed of the amino acid tyrosine with four iodine atoms attached, also called Tx or T4 (tetra- iodothyronine) accounts for 90% of thyroid secretions FUNCTIONS: a) affects almost all cells in the body (diffuses directly through cell membrane to receptor sites on mitochondria and in the nucleus. b) on mitochondria it stimulates an increase in ATP production and in the nucleus it activates genes for the synthesis of enzymes for energy production which results in an increase in cellular metablolism and oxygen consumption (called the calorigenic effect) --more heat generated and the body temperature rises. c) primarily affected are skeletal muscle,the liver, the kidneys. d) over or under active thyroid gland can cause problems (hyper and hypothryroidism) e) goiter results from lack of iodine in diet (thyroid follicles become distended with non functional secretions -----T3 ( triiodothyronine) tyrosine with three iodine atoms -----calcitonin: secreted by C cells in the thyroid gland (C cells found between follicle and basement membrane) FUNCTIONS: a) helps regulate calcium ion concentration in body fluids**calcitonis released when calcium levels in blood are above normal---bones and kidneys are target organs. ---calcitonin reduces calcium levels by inhibiting osteoclasts stimulation of calcium secretion by the kidneys resulting in reduction of Ca levels---turns off the C cells b) Ca is important in muscular contractions, and nerve activities (affects sodium permeability of axons) c) calcitonin is seldom released--most homeostatic regulators control "low" calcium levels which results in an increase in sodium permeability, exciting muscle cells more, convulsions or muscular spasms occur (parathyroid glands take care of this problem) 4) The parathyroid glands: a) two pairs enbedded in posterior surface of the thyroid gland b) two types of cells: chief cells which produce PTH (other cell type has an unknown function) c) PTH (parathyroid hormone) is secreted during low calcium levels, stimulate osteoclasts to release Ca, and promotes absorption of Ca by intestines, reduces urinary excretion of Ca until Ca content of blood returns to normal 5) The thymus: a) located just behind the sternum in the thoracic cavity b) much larger in infants (from base of the neck to the heart) reaches a maximum size just before puberty, decreases in size to age 50 (40 grams to 12 grams) c) secretes a group of hormones called thymosins which develop and maintain normal immunological defenses (decrease in thymus may make elderly more susceptible to disease) 6) The adrenal glands: a) an adrenal gland found on top of each kidney b) each gland has two parts: a superficial adrenal cortex and an innermost adrenal medulla Adrenal cortex: --grayish yellow due to stored lipids(especially cholesterol and fatty acids --more than two dozen steroid hormones called corticosteroids are released ( if adrenal gland is ever destroyed or removed, the hormones must be administered or the individual will die (over or under production of these hormones is also dangerous THE CORTICOIDS Three Classes: 1) Glucocorticoids ----affect glucose metabolism--- ----cortisol,corticosterone, and cortisone are the most important ----secreted under ACTH stimulation--- ----affect glucose metabolism by... accelerating glucose synthesis, and glycogen formation in liver, adipose tissue releases fatty acids into the blood and fatty acids are broken down instead of glucose in tissues ----have an anti-inflammatory effect in that they suppress activities of WBCs (hydrocortisone is used for poison ivy) ---have a slow healing effect,so should never by applied to open wounds 2) Mineralcorticoids ----affect electrolyte composition of body fluids-- ---aldosterone is the major one, targets kidney cells, regulating ionic composition of urine. It causes retention of sodium ions and water reducing fluid losses by urine, reduces sodium and water losses at sweat glands, salivary glands and along the digestive tract ----sodium ions retained are exchanged for potassium ions so it lowers potassium concentrations in the blood ----aldosterone secretion occurs in response to a) stimulation by the hormone angiotensin II b) high extracellular potassium levels 3) Androgens (sex hormones released in small quantities - importance uncertain) Adrenal medulla ---reddish brown in color due to many blood vessels-- ---two populations of secretory cells a) epinephrine (E) (adrenalin) 75% -80% of secretions b) norepinephrine (NE) Noradrenalin ----accelerates cellular energy and mobilization of energy reserves (glycogen in the muscle) (accelerates the breakdown of glucose to form ATP) This results in increased muscular power and endurance. In liver glycogen is converted to glucose,fatty acids are released from fats,and the heart responds by increasing heart rate. ----The above changes peak 30 seconds after secretion and linger several minutes 7) The kidneys ---secrete three hormones: a) calcitrol ---a steroid hormone secreted in response to PTH-- ----synthesized from vitamin D (can be synthesized in skin or from diet, reaches the kidneys via the liver in the form of an intermediate product ----stimulates the absorption of calcium and phosphate ions in the digestive tract... b) erythropoietin (EPO) ---a peptide hormone secreted in response to low oxygen levels in kidney tissue ---stimulates the production of red blood cells by the bone marrow, results in an increase in blood volume and oxygen in the blood... c) renin ---released in response to a decline in blood volume and/or blood pressure ---functions as an enzyme eventually resulting in the formation of angiotension II which stimulates aldosterone production by the adrenal cortex 8) The heart ---endocrine cells are cardiac muscle cells in the walls of the right atrium ---releases ANP(atrial natriuretic peptide) when blood volume becomes too great in the atrium the walls are stretched, and ANP lowers blood volume and reduces stretching of the atrial walls 9) The pineal gland ----lies in the roof of the thalamus- ---secretes melatonin (lowest level during daylight; highest level during the night ----functions: a) slows the timing of sexual maturity (levels decline at puberty) (pineal tumors that eliminate melatonin production causes premature puberty) b) acts as an antioxidant in that it protects CNS neurons from free radicals (highly reactive atoms or molecules containing unpaired electrons in their outer shell, such as NO or H2O2 which are generated by active nerve tissue c) establishes day-night cycles of activity, or circadian rhythms (changes in physiological processes that follow regular day-night cycles) and increase in melatonin may cause seasonal affective disorder (SAD) : changes in mood, eating habits, sleeping patterns which occur at higher latitudes where there is little sunshine. 10) The gonads Consist of two types: a) The testis (in males) ---hormones called androgens (testosterone is the most important) determines secondary sex characteristics, production of functional sperm produced by the interstitial cells of the testis ----sustencular cells secrete a hormone called inhibin(stimulated by FSH) depresses the secretion of FSH, these two hormones interact to maintain normal sperm production levels as an adult. b) The ovaries (in females) ----Follicles in ovaries produce ova (egg cells) under the stimulation of FSH-- ---Follicle cells surrounding ova then produce hormones called estrogens, which stimulate secondary sex characteristics, egg production, maturation of the eggs and growth of the uterine lining. ---Under FSH stimulation (follicular cells release inhibin to suppress FSH in females as well as males--- --After ovulation, follicular cells form into the corpus luteum which release more hormones (estrogens and progestins), one called progesterone accelerates the movement of fertilized eggs and prepares the uterus for implantation. and also causes an enlargement of the mammary glands. ---The placenta functions also as an endocrine gland to help promote normal development of the fetus 11) The digestive system Endocrine hormones secreted locally: secretin,gastrin, CCK and GIP 12) The pancreas ----both an endocrine and an exocrine gland--- ----endocrine glands found in the Islands of Langerhans, contains two types of cells a) alpha cells (around the center) produce glucogon which stimulates glycogenolysis in the liver (the breakdown of glycogen into glucose) resulting in an increase of blood glucose b) beta cells (in center) produce insulin which stimulates glycogenesis (the formation of glycogen in the liver) and stimulates an increase in glucose transport through the cell membranes of our body cells. This results in a decrease in the sugar level of the blood. ****Inability of the beta cells to produce insulin or sufficient levels of insulin will increase sugar levels in the blood resulting in Diabetes Mellitus Two types: Type I (IDDM-insulin dependent diabetes mellitus) (Juvenile onset diabetes (least common 20% of all cases) Type II (NIDDM-non insulin dependent diabetes (used to occur to adults over the age of 40, but many teens are developing it, poor eating habits and lack of exercise
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