Chapter 33 THE ENDOCRINE SYSTEM

The endocrine system: ] interacts with the nervous system to maintain homeostasis. ] consists of endocrine glands & isolated cells that secrete hormones.

A. Hormones Chemicals synthesized in one part of an organism & transported to another, where they affect target cells. Target cells have receptors on or in them that bind only to certain hormones. Hormones Target cells

1. Classes of Hormones ] Peptide hormones F water-soluble F bind to receptors on target cell’s surface ] Steroid hormones F lipid-soluble F bind to receptors in target cell’s cytoplasm or nucleus

2. Hormone Regulation ] Positive feedback loop Accumulation of a certain biochemical stimulates an increase in its production. Ex. release of oxytocin during childbirth ] Negative feedback loop Accumulation of a certain biochemical stimulates a decrease in its production. Ex. release of most hormones

B. Diversity of Endocrine Systems All animals produce hormones. 1. Invertebrate Hormones Most are secreted by neurosecretory cells. Invertebrate hormones regulate: ] regeneration (Hydra, flatworms, annelids) ] color change (crustaceans) ] gonad maturation (flatworms, mollusks, arthropods) ] molting & metamorphosis (insects)

2. Vertebrate Hormones Secreted by neurosecretory cells & endocrine glands. All vertebrates produce the same basic hormones, but may have different functions in different species. Ex. Thyroxine ] sets pace of metabolism in humans ] controls metamorphosis in frogs ] controls heat production & oxygen consumption in birds.

All vertebrate endocrine systems are regulated by the hypothalamus and the pituitary gland.

C. The Human Endocrine System Consists of neurosecretory cells, endocrine glands & scattered hormone-secreting cells.

1. Pituitary Gland Anterior Pituitary (regulated by releasing & inhibiting hormones of hypothalamus)

] Growth hormone (GH) F targets all cells  promotes growth & development Hyposecretion in children causes pituitary dwarfism. Hypersecretion in children causes pituitary gigantism. Hypersecretion in adults causes acromegaly. ] Prolactin F targets mammary glands  stimulates milk production

] Follicle-stimulating hormone (FSH) F targets ovaries  initiates development of ovarian follicles, oocyte maturation, and release of estrogen F targets testes  promotes development of testes & production of sperm ] Luteinizing hormone (LH) F targets ovaries  stimulates ovulation F targets testes  stimulates production of testosterone

] Thyroid-stimulating hormone (TSH) F targets thyroid gland  stimulates release of thyroxine & triiodothyronine ] Adrenocorticotropic hormone (ACTH) F targets adrenal cortex  stimulates release of glucocorticoids FSH, LH, TSH & ACTH are tropic hormones.

Intermediate lobe of pituitary (regulated by releasing & inhibiting hormones of hypothalamus) ] Melanocyte-stimulating hormone (MSH) F targets melanocytes in skin of many vertebrates  causes color changes Intermediate lobe is poorly developed in humans.

Posterior Pituitary (stores & releases hormones produced by hypothalamus) ] Antidiuretic hormone (ADH) F targets kidneys  stimulates reabsorption of water F targets blood vessels  regulates blood pressure ] Oxytocin F targets uterus  stimulates contraction F targets mammary glands  stimulates release of milk when baby nurses

2. Thyroid Gland ] Thyroxine (T 4 ) & Triiodothyronine (T 3 ) F target all cells of body  increase metabolic rate Dietary iodine is required to produce these two hormones. How does the body respond to a lack of iodine in diet? Why is iodine-deficient goiter rarely encountered in the US today?

TRH = thyroid releasing hormone

Hyposecretion in infants causes cretinism. - physical & mental retardation Hyposecretion in adults causes myxedema. - low metabolic rate, weight gain, lethargy, puffy face Hypersecretion causes hyperthyroidism. - high metabolic rate, weight loss, irritability, elevated blood pressure ] Calcitonin F targets bones, kidneys & intestines  reduces level of calcium in blood

3. Parathyroid Glands ] Parathyroid hormone (PTH) F targets bones, kidneys & intestines  increases level of calcium in blood Calcitonin & PTH are antagonistic - interact to maintain normal blood calcium levels. Regulation of blood calcium level

4. Adrenal Glands Adrenal Medulla ] Epinephrine & Norepinephrine F target heart, blood vessels, muscles, liver  increase heart rate, breathing rate, blood glucose  redirect blood flow toward brain & muscles Epinephrine & norepinephrine help body cope with emergencies (“fight-or-flight” response).

Adrenal Cortex ] Mineralocorticoids (aldosterone) F targets kidneys  retains sodium ions & water (increases blood volume/pressure) ] Glucocorticoids (cortisol) F targets all cells  increases glucose synthesis & suppresses immune system Mineralocorticoids & Glucocorticoids help body cope with long term stress.

Hyposecretion of adrenal cortex hormones causes Addison’s disease. - dehydration, low blood pressure, mental fatigue, weakness, impaired resistance to stress, darkening of skin Hypersecretion of adrenal cortex hormones causes Cushing’s disease. - redistribution of body fat, depresses immune system

5. Pancreas Classified as exocrine & endocrine gland. ] Endocrine portion (islets of Langerhans) secretes hormones. ] Exocrine portion secretes digestive enzymes.

] Insulin (secreted by beta cells of islets) F targets all cells  decreases blood glucose levels [stimulates conversion of glucose to glycogen] ] Glucagon (secreted by alpha cells of islets) F targets all cells  increases blood glucose levels [stimulates breakdown of glycogen to glucose] Insulin & Glucagon are antagonistic - interact to maintain normal blood glucose levels.

Diabetes mellitus Blood glucose level is too HIGH. Type I (insulin-dependent) F autoimmune disease - immune system destroys beta cells F usually develops during childhood F treated with daily injections of insulin Type II (insulin-independent) F defective insulin receptors F usually develops in adulthood F treated with diet & exercise

Gestational diabetes F temporary - develops during pregnancy F placental hormones interfere with insulin action Hypoglycemia Blood glucose level is too LOW. F excess insulin production F treated by following diet of frequent, small meals low in carbohydrates & high in protein

6. Gonads Ovaries ] Estrogen F targets most cells  develops & maintains sexual characteristics in females; stimulates growth of uterine lining ] Progesterone F targets uterus  stimulates growth of uterine lining; maintains pregnancy

Testes ] Testosterone F targets most cells  develops & maintains sexual characteristics in males; promotes sperm formation ] Inhibin F targets anterior pituitary  inhibits FSH release (decreases sperm formation)