Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 33

The endocrine system includes several glands and scattered cells and the hormones they secrete into the bloodstream. A hormone exerts a specific physiological effect on target cells, which have receptors for it. Exocrine glands deliver substances into ducts, which open onto body surfaces.
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 and evolutionary link between the two systems.
Diverse species have hormonelike regulation, and many use the signaling molecule cAMP.
The gastrointestinal tract secretes several hormones that regulate digestion. In mammals, the atrium of the heart secretes atrial natriuretic hormone, which regulates blood pressure, blood volume, and the excretion of sodium ions, potassium ions, and water. Pheromones are hormonelike biochemical signals between individuals.
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. The lipid-soluble steroid hormones cross target cell membranes, bind receptors in the cytoplasm or nucleus, and activate genes to direct synthesis of proteins that provide the cell's response.
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. In a positive feedback loop, the hormone causes an event that increases its production. Ions or nutrient levels near the endocrine cells, input from the nervous system, and other hormones control feedback loops.
Simpler invertebrates may have only neurosecretory cells. More complex invertebrates have interacting hormones. Changing levels of juvenile and molting hormone control insect metamorphosis. The same hormones may function differently in different vertebrate species. Reproductive cycles are hormonally controlled and are diverse.
The endocrine system functions in a hierarchy. The hypothalamus produces releasing hormones, which travel in neurosecretory cells to the anterior lobe of the pituitary gland. There they stimulate release of anterior pituitary hormones: 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. The hypothalamus also 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. In many vertebrate species, the region between the anterior and posterior lobes of the pituitary secretes melanocyte-stimulating hormone, which colors skin.
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. Another hormone that affects calcium metabolism is calcitonin, which the thyroid gland secretes. Calcitonin lowers the level of calcium in the blood by increasing the rate at which it enters bone.
The adrenal cortex secretes mineralocorticoids and glucocorticoids, which mobilize energy reserves during times of stress and maintain blood volume and blood composition. The adrenal medulla secretes epinephrine and norepinephrine, which ready the body to cope with an emergency.
The endocrine portion of the pancreas secretes insulin, which stimulates cells to take up glucose; glucagon, which breaks down glycogen to glucose; and somatostatin, which regulates insulin and glucagon production. Lack of insulin or the inability to use it causes diabetes mellitus.
The female gonads, the ovaries, secrete estrogen and progesterone, hormones that stimulate development of female sexual characteristics and together with GnRH, FSH, and LH control the menstrual cycle. The male gonads, the testes, secrete testosterone, which stimulates the development of secondary sexual characteristics and controls prostate gland function.
The pineal gland may regulate the responses of other glands to light-dark cycles through its hormone, melatonin.
Prostaglandins are lipids that form enzymatically when disturbed cell membranes release fatty acids. Prostaglandins function at the site where they are released, and the different types exert different effects on the body.