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Transcript
Chapter 8 THE ENDOCRINE SYSTEM Endocrine system 2nd great controlling system (behind nervous system) Not fast Uses chemical messages (hormones) to control major processes Reproduction, growth and development Body defenses, controlling electrolytes, water, nutrients in blood Regulates metabolism and energy balance Chemistry of hormones Hormones-chemical substances secreted by cells into extracellular fluid to regulate metabolic activity of other cells Classified as amino-acid based Proteins, peptides, amines Or prostaglandins (made from lipids) Or steroids (made from cholesterol) Sex hormones from gonads Hormones from adrenal cortex Mechanisms of hormone action Blood borne hormones circulate throughout the body Only affect cells that contain specific protein receptors (target cells) Hormones are used to alter cellular activity Mechanisms of hormone action After hormone binding to the protein receptor, one or more of the following occur: Changes in plasma membrane permeability or electrical state Synthesis of proteins or regulatory molecules (enzymes) Activation or inactivation of enzymes Stimulation of mitosis Mechanisms cont. Steroid hormones (lipid-soluble) Diffuse through the plasma membrane of target cell Inside the cell, steroid enters nucleus and binds a specific receptor Hormone-receptor complex binds specific sites on DNA to trigger transcription of mRNA mRNA then is translated into new proteins Mechanisms cont Nonsteroidal hormones (proteins and peptides) must bind receptors on the outside of the target cell Hormone binds membrane receptor Causes a series of reactions that activate enzymes Enzymes catalyze reactions to produce a second messenger molecule cAMP cAMP oversees the intracellular changes in response to the hormone Control of hormone release Negative feedback mechanisms control release of most hormones Hormone secretion is triggered by an internal or external stimulus Rising hormone levels inhibit further hormone release Stimuli that activate endocrine organs Hormonal Most common Endocrine organs are activated by other hormones Humoral Changing blood levels of certain ions and nutrients Neural Activation by nerve fibers Major endocrine organs Pituitary, thyroid, parathyroid, adrenal, pineal, thymus, pancreas, gonads, hypothalamus (also part of nervous system) Endocrine organs are duct-less, secreting hormones produced into blood or lymph Pituitary gland Hangs below hypothalamus Has two lobes Anterior pituitary (glandular tissue) Posterior pituitary (nervous tissue) Hormones of anterior pituitary Growth hormone and prolactin – exert effects on nonendocrine targets Thyrotropic hormone, adrenocorticotropic hormone, two gonadotropic hormones- are all tropic and stimulate target organs (endocrine glands) to secrete their own hormones All anterior pituitary hormones are Proteins, act through second messengers, regulated by hormonal stimuli (negative feedback) Growth hormone Effects directed to growth of skeletal muscles and long bones of the body Helps determine final body size Causes amino acids to be built into proteins Stimulate target cells to grow in size and divide Causes fats to be broken down for energy Maintains blood sugar homeostasis Homeostatic imbalance Hyposecretion of GH Pituitary dwarfism; body proportions are normal but maximum height is 4 feet Hypersecretion of GH In childhood leads to gigantism (8-9 feet tall) with normal body proportions After long bone growth has ended leads to acromegaly (thickening of soft tissues, malformation of facial features); can be treated with GH Prolactin PRL is a protein hormone Targets the breast to stimulate milk production after childbirth No known function in males Adrenocorticotropic hormone ACTH regulates endocrine activity of the adrenal cortex Thyroid-stimulating hormone TSH (or thyrotropic hormone – TH) Influences growth and activity of the thyroid gland Gonadotropic hormones Regulate hormonal activity of the gonads: ovaries and testes Follicle stimulating hormone FSH In females-stimulates follicle development in ovaries producing estrogen as they mature; prepares eggs for ovulation In males-stimulates sperm development in the testes Luteinizing hormone LH In females triggers ovulation of an egg from an ovary Causes ruptured follicle to become the corpus luteum which then produces progesterone and estrogen In males (interstitial cell-simulating hormone) stimulates testosterone production by interstitial cells of testes Homeostatic imbalance Hyposecretion of FSH or LH leads to sterility in both males and females Hypersecretion does not seem to cause problems but multiple births are common Pituitary/hypothalamus relationship Pituitary release of hormones is controlled by releasing or inhibiting hormones from the hypothalamus Hypothalamus sends those hormones into the portal circulation (connects hypothalamus to pituitary) Hypothalamus also produces oxytocin and antidiuretic hormone (which travel down the neurosecretory cells) to posterior pituitary for storage Later released into blood after nerve impulses from hypothalamus Hormones of posterior pituitary Hormones are made by hypothalamic neurons Oxytocin – released only during childbirth and nursing women; stimulates contractions of the uterus during labor, sex, and breastfeeding; causes let-down of milk Antidiuretic hormone (ADH) prevents urine production and causes kidneys to reabsorb more water; blood volume increases as does blood pressure Homeostatic imbalance ADH is inhibited by alcohol consumption resulting in output of large amounts of water (urine); dry mouth and intense thirst are often part of the “hangover” Diuretics antagonize ADH and result in more urine output; these are used to manage edema and congestive heart failure Hyposecretion of ADH leads to excessive urine output or diabetes insipidus resulting in the consumption of large amounts of water Thyroid gland Located at the base of the throat, inferior to adam’s apple Has 2 lobes joined by the isthmus Produces 2 hormones Thyroid hormone calcitonin Thyroid gland Thyroid hormone is 2 active iodine- containing hormones: thyroxine and triiodothyronine (T4 and T3 respectively) T4 has 4 iodine atoms, T3 has 3 iodine atoms Controls the rate at which glucose is burned and converted to heat / energy Needed for normal tissue growth and development, especially reproductive and nervous Homeostatic imbalance Lack of iodine causes goiter, enlarged thyroid, which fails to inhibit FSH release Cretinism is hyposecretion in childhood which does not stimulate TH; results in dwarfism, mental retardation In adults, hypothyroidism results in myxedema, physical and mental sluggishness but no retardation, poor muscle tone, low body temperature, obesity, dry skin Homeostatic imbalance Hyperthyroidism results from a tumor in the thyroid gland producing high metabolic rate, intolerance of heat, rapid heartbeat, weight loss, nervous behavior Grave’s disease is hyperthyroidism with enlarged thyroid and bulging eyes Thyroid gland Calcitonin (thyrocalcitonin) decreases blood calcium levels by depositing calcium into bones Secretion of calcitonin declines in adults with age resulting in progressive decalcification of bones Parathyroid glands Tiny masses of glandular tissue on the posterior surface of the thyroid gland Secrete parathyroid hormone (PTH) which helps reglulate Ca in the blood Ca levels decline, PTH stimulates destruction of osteoclasts which release Ca to the blood PTH also stimulates kidneys and intestine to absorb more Ca Adrenal gland Bean-shaped glands that curve over the top of both kidneys Has glandular cortex and neural tissue (medulla) Medulla is enclosed by the cortex Hormones of adrenal cortex 3 groups of corticosteroids: mineralcorticoids, glucocorticoids, and sex hormones Mineralcorticoids Aldosterone – from outer adrenal cortex Regulate mineral (salt) content of blood Help regulate water and electrolyte balance Hormones of adrenal cortex Glucocorticoids (cortisone, cortisol) Promote normal cell metabolism and resist stressors by increasing blood glucose levels Can also be used to reduce inflammation paincausing molecules called prostaglandins Glucocorticoids are released in response to high blood levels of ACTH Hormones of adrenal cortex Sex hormones – produced in adrenal cortex for both sexes in small amounts Androgens are the majority (male hormones) but small amounts of estrogen are also produced Homeostatic imbalance Addison’s disease – hyposecretion of adrenal cortex hormones Bronze color of skin Problems with water/electrolyte balance Weak muscles, hypoglycemia Cushing’s disease-tumor in middle cortex Edema, hypertension, hyperglycemia, weak bones Hypersecretion – masculinization (masculine patter of body hair, more pronounced in females Hormones of adrenal medulla Stimulated by sympathetic nervous system Releases catecholamines for “fight-or-flight” response to stressors epinephrine (adrenaline) into the blood stream Norepinephrine (noradrenaline) Increase heart rate, blood pressure, blood glucose levels, dilates passages in lungs Pancreatic islets Called islets of Langerhans Endocrine organ located near the stomach Over a million islets Produce insulin and glucagon Insulin is released from beta cells, increases cells’ abilities to transport glucose across the plasma membrane; removes glucose from blood Glucagon is released from alpha cells; targets the liver; stimulates breakdown of glycogen to glucose to increase blood glucose levels Homeostatic imbalance Diabetes mellitus caused by sluggish pancreatic islets that don’t produce enough insulin 3 signs of diabetes 1. polyuria – excessive urination to flush out sugars and ketones 2. polydipsia – excessive thirst due to water loss 3. polyphagia – hunger due to inability to use glucose; loss of fat and proteins Pineal gland Found on roof of third ventricle in brain Releases melatonin, levels rise and fall with cycles of day and night Peak levels occur at night, lowest levels at noon Melatonin is a “sleep trigger” Melatonin regulates mating behavior in some animals; in humans it coordinates hormones of fertility and prevents sexual maturity during childhood Thymus Found in upper thorax, posterior to sternum Large in infants and children; decreases in size with adulthood Produces thymosin During childhood incubates white blood cells called T lymphocytes used in the immune response Hormones of ovaries Paired, almond-sized organs in the pelvic cavity Produce ova (eggs) and two hormones Estrogens (estrone and estradiol) Stimulate secondary sex characteristics (maturation of reproductive organs and hair in pubic and axillary regions) Prepares uterus to receive the zygote (begins menses) Help maintain pregnancy (but estrogens come from placenta at this time) Prepare breasts for lactation Hormones of ovaries Progesterone (acts with estrogens) to begin menses Quiets uterine muscles in pregnancy to prevent abortion Prepares breasts for lactation Can also be secreted by corpus luteum Hormones of testes Paired testes suspended in the scrotum, outside the pelvic cavity Produces sperm Produces male hormones called androgens Testosterone from interstitial cells Development of male secondary sex characteristics Maturation of reproductive organs; stimulates sex drive Needed for continuous production of sperm in adulthood Other hormone-producing tissues/organs Hormone producing cells can be found in Small intestine Stomach Kidneys Heart Placenta Some tumors (lung and pancreatic cancers) See Table 9.2 on page 301 for hormones produced by these structures Placenta Temporary organ produced in the uterus during pregnancy Has roles as respiratory, excretory, and nutrition delivery systems for the fetus Also maintains pregnancy and prepares for birth Produces human chorionic gonadotropin (hCG) Stimulates corpus luteum to continue making the estrogens and progesterone Prevents sloughing of uterine lining (no periods) Eventually placenta takes over producing hormones To prepare for birth To prepare for lactation Placenta Human placental lactogen (hPL) works with estrogens and progesterone to prepare breasts for lactation Relaxin causes mother’s pelvic ligaments and pubic symphysis to relax and become flexible for passage of infant through birth canal Developmental aspects Embryonic development of endocrine glands varies by gland (some are neural, some are eptithelial) For the most part, endocrine glands work smoothly until old age (barring any diseases) Menopause occurs in women during late middle age Ovaries begin to atrophy and are not efficient Child bearing years are over Other problems begin with decreased estrogens: arteriosclerosis, osteoporosis, decreased skin elasticity, “hot flashes”, fatigue, anxiety, and depression are common Developmental aspects Men typically continue to produce testosterone in adequate amounts throughout life For both sexes Decline in overall endocrine function Many elderly become hypothyroid and have decreased ability to resist stress and infection Decreased immune function is due to decreased melatonin production