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=__**ENDOCRINOLOGY**__=

//How is it that humans and animals maintain quite constant blood concentrations of glucose throughout their lives despite wildly varying frequencies of meals?// If your blood glucose concentration drops much below 1 mg per ml, your neurons will begin to misbehave, leading ultimately to coma and death. Yet skipping breakfast is rarely life-threatening. The answer is that a battery of chemical messengers - //hormones// - are secreted into blood in response to rises and falls in blood glucose concentration and stimulate metabolic pathways that pull glucose concentrations back into the normal range. Two systems control all physiologic processes: As will be repeatedly demonstrated, the nervous and endocrine systems often act together to regulate physiology. Indeed, some neurons function as endocrine cells. Endocrinology is the study of hormones, their receptors and the intracellular signalling pathways they invoke. Distinct endocrine organs are scattered throughout the body. These are organs that are largely or at least famously devoted to secretion of hormones, and no introduction to endocrinology would be complete without some kind of endocrine organ "map" such as that below: Your browser is not Java-enabled: image will not be visible ||
 * [[image:http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/basics/anim_endo.gif]] ||
 * **The nervous system** exerts point-to-point control through nerves, similar to sending messages by conventional telephone. Nervous control is electrical in nature and fast.
 * **The endocrine system** broadcasts its hormonal messages to essentially all cells by secretion into blood and extracellular fluid. Like a radio broadcast, it requires a receiver to get the message - in the case of endocrine messages, cells must bear a //receptor// for the hormone being broadcast in order to respond.
 * [[image:http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/basics/anim_nerve.gif]] ||

**Hormones coordinate cell, tissue, and organ activities on a sustained basis. They circulate in the extracellular fluid and bind to specific receptors on or in target cells. They then modify cellular activities by altering membrane permeability, activating or inactivating key enzymes, or changing genetic activity.

The hypothalamus produces regulatory factors that adjust the activities of the anterior pituitary gland, which produces seven hormones. Most of these hormones control other endocrine organs, including the thyroid gland, adrenal gland, and gonads. It also produces growth hormone, which stimulates cell growth and protein synthesis. The posterior pituitary gland releases two hormones produced in the hypothalamus. ADH restricts water loss and promotes thirst, and oxytocin stimulates smooth muscle contractions in the mammary glands and uterus (in females) and the prostate gland (in males).

The thyroid gland produces (1) hormones that adjust tissue metabolic rates and (2) a hormone that usually plays a minor role in calcium ion homeostasis by opposing the action of parathyroid hormone.

The adrenal glands produce hormones that adjust metabolic activities at specific sites, affecting either the pattern of nutrient utilization, mineral ion balance, or the rate of energy comsumption by active tissues.

The pancreatic islets release insulin and glucagon. Insulin is released when blood glucose levels rise, and it stimulates glucose transport and utilization by peripheral tissues. Glucagon is relased when blood glucose levels decline, and it stimulates glycogen breakdown, glucose synthesis, and fatty acid release.**