The Endocrine System

Chapter 9 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 • Bloodborne 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 breast-feeding; 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 pain-causing 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

The Endocrine System