The Endocrine System
33.1 Hormones Are Chemical Messengers and Regulators
1. The endocrine system includes several glands and scattered cells and the hormones they secrete into the bloodstream.
2. A hormone exerts a specific physiological effect on target cells, which have receptors for it.
3. The nervous and endocrine systems interact to maintain homeostasis. The nervous system acts faster and more locally than the endocrine system. Neurosecretory cells, which are neurons that secrete hormones, are a physical link between the two systems.
4. Diverse species have hormonelike regulation, and many use the signaling molecule cAMP.
5. Local-acting hormones include those that regulate digestion, and atrial natriuretic hormone.
33.2 How Hormones Exert Their Effects
6. Peptide hormones are water-soluble and bind to the surface receptors of target cells, stimulating conversion of ATP to cAMP on the inner face of the cell membrane. The cAMP then triggers a specific metabolic effect.
7. The lipid-soluble steroid hormones cross target cell membranes, bind to receptors in the cytoplasm or nucleus, and activate genes to direct synthesis of proteins that provide the cell’s response.
8. In a negative feedback loop, excess of a hormone or the product of a hormone-induced response suppresses further synthesis or release of that hormone until levels return to normal.
9. In a positive feedback loop, the hormone causes an event that increases its production.
10. Ions or nutrient levels near the endocrine cells, input from the nervous system, and other hormones control feedback loops.
33.3 Diversity of Endocrine Systems
11. Simpler invertebrates have neurosecretory cells. More complex invertebrates have interacting hormones.
12. Changing levels of juvenile hormone and molting hormone control insect metamorphosis.
13. The same hormones may function differently in different species.
14. The hypothalamus produces releasing hormones, which travel in neurosecretory cells to the anterior lobe of the pituitary gland.
33.4 A Closer Look at the Endocrine System in Humans
15. Anterior pituitary hormones include growth hormone, which stimulates cell division, protein synthesis, and growth in all cells; thyroid-stimulating hormone, which prompts the thyroid gland to release thyroxine and triiodothyronine, which regulate metabolism; adrenocorticotropic hormone, which stimulates the adrenal cortex to release hormones that enable the body to cope with a serious threat; prolactin, which stimulates milk production; and the sex hormones, which control sex cell development.
16. The hypothalamus manufactures two hormones that are stored in and released from the posterior lobe of the pituitary gland—antidiuretic hormone, which regulates body fluid composition, and oxytocin, which contracts the uterus and milk ducts.
17. In many vertebrate species, the region between the anterior and posterior lobes of the pituitary secretes melanocyte-stimulating hormone, which affects skin coloring.
18. The parathyroid glands secrete parathyroid hormone, which increases blood calcium level by releasing calcium from bone and increasing its absorption in the gastrointestinal tract and kidneys. Calcitonin, which the thyroid gland secretes, lowers the level of calcium in the blood.
19. The adrenal cortex secretes mineralocorticoids and glucocorticoids, which mobilize energy reserves during stress and maintain blood volume and blood composition. The adrenal medulla secretes epinephrine and norepinephrine, which ready the body to cope with an emergency.
20. The endocrine portion of the pancreas secretes insulin, which stimulates cells to take up glucose; glucagon, which increases blood glucose levels; and somatostatin, which controls the rate at which the blood absorbs nutrients.
21. The pineal gland may regulate the responses of other glands to light-dark cycles through its hormone, melatonin.
22. The ovaries secrete estrogen and progesterone, hormones that stimulate development of female sexual characteristics and with GnRH, FSH, and LH control the menstrual cycle. The testes secrete testosterone, which stimulates sperm cell production, the development of secondary sexual characteristics, and controls prostate gland function.
23. Prostaglandins are lipids that form enzymatically when disturbed cell membranes release fatty acids. They function at the site where they are released, and are diverse.