Review of the endocrine system
Endocrinology is the study of mechanisms of communication and control at work within a living organism by means of internally synthesized chemical messengers. The endocrine system regulates the functions of various organs, tissues, and cells in order to maintain homeostasis or elicit some other necessary function. Endocrine tissue ceils secrete hormones, which travel through the bloodstream to other target cells where the hormones regulate the function of those cells. Metabolism, growth, and sexual development are all regulated by these hormones.
A hormone is any substance, of endogenous or exogenous origin, produced by the cell to play a role in the control of the function of another cell. Hormones are classified by their chemical structure and generally fall into the following general categories:
Some hormones, like steroid hormones, bind to receptors within the cell’s cytoplasm, altering gene activity. Others, such as peptide hormones, bind to receptors embedded in the cell’s membrane. Many hormones require second messengers to effect their molecular message. Multiple second messengers enable varied control of each hormonal system.
Various types of hormonal communication exist. Paracrine hormones affect neighboring ceils by diffusing through interstitial fluid and acting on cellular receptors. Autocrine hormones act on the cells that produce them, and neuroendocrine hormones have components of endocrine and nervous system function.
The nervous system and the endocrine system are related intimately, and both provide a communication system for the body. For example, recovery from hypoglycemia involves the concerted effort of the autonomic nervous system and the endocrine system.
Hormone levels are regulated finely by various types of feedback mechanisms, both positive and negative, and display intrinsic rhythms that vary by the minute, daily, or over longer periods (for example, circadian, ultradian, menstrual, or seasonal).
Concentration of hormone in the blood depends on both the rate of hormone secretion and clearance. The liver and kidneys are responsible primarily for removing and metabolizing hormones.
The major organs comprising the endocrine system are:
The hypothalamus lies at the base of the brain in the diencephalon, and controls many endocrine functions. The hypothalamic-pituitary-target gland is an intricate system that regulates the function of the thyroid gland, adrenal cortex, ovaries, and testes. The hypothalamus produces regulatory hormones, which travel through short.
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