Endocrine System Help
The endocrine system consists of endocrine glands that secrete specific chemicals called hormones into the blood or surrounding interstitial fluid. The endocrine system functions closely with the nervous system in regulating and integrating body processes. More specifically, hormones cause changes in the metabolic activities in specific cells, and nerve impulses cause muscles to contract or glands to secrete. In general, the action of hormones is relatively slow and the effects are prolonged, whereas the action of nerve impulses is fast and the effects are of short duration.
A hormone is a chemical messenger secreted by an endocrine gland. Its chemical composition is such that it has its effect on specific receptor sites on target cells causing a series of biochemical events that leads to a specific response. Hormones are classified according to chemical structure and the location of the cell membrane receptors on their target cells.
Hormones are divided into two groups, based on the location of the receptors on the target cells. Group I hormones bind to intracellular receptors and are lipophilic. These include the steroid hormones. Group II hormones bind to cell surface receptors and are hydrophilic. These hormones include polypeptide, glycoprotein, and catecholamine hormones.
Negative and Positive Feedback
Negative feedback involves a chain of biochemical or physiological events. Generally, an increased amount of end product inhibits the production, mechanism, or action of a starting substance to prohibit further synthesis of the end product. For example: A > B > C > D. As A progresses through B and C to D the amount of D increases. Substance D, however, is an inhibitor of substance A. As levels of substance D increase, substance A receives "negative feedback" to prevent the process from continuing to produce more D. An example of this mechanism is the production of cortisol and its regulation along the hypothalamic-pituitary-adrenal axis.
In the case of positive feedback, D, in the example above, would stimulate A to further produce increased amounts of B, and so on to D. This is less common in the body. One example is the secretion of oxytocin stimulating contraction of uterine muscles during labor. As labor progresses pressure on the cervix continues to stimulate the release of oxytocin, which continues to stimulate uterine muscles to contract.
Homeostatsis is maintained by the continual adjustments of endocrine function in response to changes in our environment. Negative feedback inhibits the factors that produced a product, so as to maintain a normal range of values. Positive feedback increases the deviation from normal values, and thus is not homeostatic.
Endocrine Glands and Their Secretions
Endocrine organs are widely scattered throughout the body with no anatomical continuity. In addition to the discrete endocrine organs, several other organs, referred to as mixed organs, have an endocrine function. These include the thymus, stomach, duodenum, placenta and the heart.
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