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Biology 55 Endocrine System ENDOCRINE SYSTEM – regulates body functions Endocrine Glands – ductless glands that secrete hormones hormones – control functions of other organs or tissues in the body usually diffuse into capillaries and circulate in blood target cells have receptors for hormones – when the hormone binds to the receptor, it changes the function of the cell in some way Chemical Classes of Hormones peptide hormones (modified amino acids or proteins) act as first messengers that bind to receptors on the cell membrane activates a second messenger inside the cell (eg. cAMP) second messenger turns on chains of enzymatic reactions that alter cell functions steroid hormones – (eg. sex hormones) enter cell and bind to receptor inside cell hormone-receptor complex binds to DNA turns genes on, resulting in synthesis of new proteins (enzymes) that alter cell functions Regulation of Hormone Secretion nervous control – signals from neurons control the secretion of hormones hormonal control – one hormone controls the secretion of another hormone negative feedback – secretion of most hormones is controlled by feedback of its product or a product of its action eg. high blood glucose causes release of insulin, which lowers blood glucose low blood glucose then inhibits release of insulin HYPOTHALAMUS AND PITUITARY GLAND – function together to regulate most aspects of growth, development, metabolism, and homeostasis in the body HYPOTHALAMUS – link between nervous and endocrine systems controls autonomic nervous system – emotions affect endocrine functions receives sensory information – water balance, blood pressure, concentration of various hormones in the blood, tactile sensations produces hormones hormones stored by posterior pituitary – released in response to a nerve signal from hypothalamus antidiuretic hormone (ADH), oxytocin regulatory hormones – regulate secretion of anterior pituitary hormones releasing hormones – stimulate anterior pituitary inhibiting hormones – inhibit anterior pituitary portal system – special blood network delivers regulatory hormones 1 PITUITARY GLAND – connected to the hypothalamus at base of brain anterior pituitary – "master gland" that controls secretion of other hormones posterior pituitary – stores oxytocin and ADH and secretes them in response to nerve signals from the hypothalamus Hormones of Posterior Pituitary – produced by hypothalamus oxytocin stimulates uterine contractions during childbirth stimulates milk let-down during nursing positive feedback – hypothalamus signals secretion in response to sensory stimuli (more stimulation = more hormone) stretch of the cervix causes secretion – stops after birth nursing of infant causes secretion antidiuretic hormone (ADH) – decreases volume of urine (conserves water) hypothalamus senses dehydration (blood concentrated) signals posterior pituitary to secrete ADH kidneys reabsorb more water, so less urine is formed negative feedback – as blood dilutes, less ADH is secreted Hormones of Anterior Pituitary secretion of most anterior hormones is controlled by releasing and inhibiting hormones from the hypothalamus thyroid-stimulating hormone (TSH) stimulates thyroid to secrete thyroid hormones controlled by negative feedback of thyroid hormones adrenocorticotropic hormone (ACTH) stimulates adrenal cortex to secrete its hormones controlled by negative feedback of adrenal cortex hormones gonadotropinic hormones (FSH & LH) female – stimulate ovaries to produce eggs and sex hormones male – stimulate testes to produce sperm and sex hormones controlled by negative feedback of sex hormones prolactin (PRL) stimulates mammary glands to produce milk many hormones are involved in mammary development and secretion secretion begins before childbirth and continues while child is nursing growth hormone (GH) stimulates cells in most body tissues to synthesize proteins, grow and divide, and use fat stores for energy controlled by negative feedback of GH giantism – too much GH during childhood pituitary dwarfism – too little GH during childhood acromegally – too much GH during adulthood 2 THYROID GLAND – attached to trachea, just below larynx thyroid hormones (T4 and T3) T4 (thyroxine) has 4 iodines (tetraiodothyronine) T3 has 3 iodines (triiodothyronine) iodine – required in diet for production of thyroid hormones stimulates most body tissues to increase metabolic rate controlled by TSH and negative feedback of T3 goiter – enlarged thyroid calcitonin lowers blood calcium (increases bone density) inhibits osteoclasts – decreases resorption of bone controlled by negative feedback of blood calcium PARATHYROID GLANDS – 4 small glands attached to thyroid gland parathyroid hormone (PTH) – major regulator of calcium level in blood increases blood calcium (decreases bone density) stimulates osteoclasts – increases resorption of bone controlled by negative feedback of blood calcium hypocalcemia – low blood calcium; may be caused by low PTH causes nerve depolarization and muscle spasms (tetany) ADRENAL GLANDS – found on top of each kidney Adrenal Medulla – central portion of adrenal gland epinephrine and norepinephrine controlled by sympathetic nervous system fight-or-flight responses – secreted during stress Adrenal Cortex – outer portion of adrenal gland; produces 3 types of steroid hormones mineralocorticoids – regulate mineral(Na+ and K+) and water homeostasis aldosterone – main mineralocorticoid produced stimulates kidneys to conserve Na+ and water and excretion of K+ (increases blood pressure and blood Na+, decreases blood K+ ) controlled by renin-angiotensin-aldosterone (RAA) pathway 1) low blood volume and pressure 2) kidneys secrete renin (enzyme) into blood 3) renin converts angiotensinogen into angiotensin I 4) lungs convert angiotensin I to angiotensin II 5) angiotensin II stimulates aldosterone secretion 6) blood volume and pressure increase also controlled by negative feedback of blood Na+ and K+ 3 glucocorticoids – increases blood glucose and helps resist stress cortisol (hydrocortisone) – main glucocorticoid produced increases blood glucose available for brain (main energy source for brain) stimulates break down of fats and proteins(muscle) stimulates most cells use of these other nutrients for energy stimulates liver to convert these other nutrients to glucose reduces pain anti-inflammatory suppresses immune system secreted in response to ACTH and prolonged stress controlled by negative feedback of glucocorticoids sex hormones amount is low compared to that produced in gonads PANCREATIC ISLETS – clusters of endocrine cells in pancreas pancreas – between stomach and sm. intestine 99% exocrine glands producing digestive enzymes 1% pancreatic islets (islets of Langerhans) – produce hormones glucagon – increases blood glucose (between meals) stimulates break down of fat to supply energy for cells stimulates liver to convert other nutrients to glucose controlled by negative feedback of blood glucose insulin – decreases blood glucose (after eating) stimulates most body cells to take in glucose (carrier proteins) and use it cells use glucose for ATP synthesis glucose stored as glycogen in liver and muscles glucose converted to fat by adipose tissue controlled by negative feedback of glucose diabetes mellitus – inability to produce or respond to insulin cells can’t take in or use glucose Type I (insulin-dependent) – no insulin produced Type II (insulin-resistant) – cells don't respond to insulin hyperglycemia – high blood glucose frequent urination – glucose gets in urine, water follows(osmosis) hunger and weight loss (can't use carbohydrates for energy) insulin shock – low blood glucose due to insulin overdose or not eating brain can't function without glucose PINEAL GLAND – found in diencephalon of brain regulated by light and darkness melatonin – sets circadian rhythm (sleep and awakening cycles) controlled by light and darkness THYMUS – found over base of heart and distal trachea produces hormones that regulate development of immune cells 4 GONADS – testes and ovaries Testes – found in scrotum of male androgens – male sex hormones (mainly testosterone) stimulates development of male reproductive organs in fetus and their growth at puberty causes development of male secondary sexual characteristics beard, body hair, deepening of voice anabolic – increases growth of tissues, especially bones and muscles Ovaries – found in pelvic cavity of female estrogens and progesterone stimulates development of female reproductive organs in fetus and their growth at maturity causes development of female secondary sexual characteristics breasts, body hair and fat, wider pelvis cause menstrual cycles anabolic – though less so than testosterone 5