Homeostasis is defined as a stable internal environment. The endocrine system plays an important role in the maintenance of homeostasis because hormones regulate the activities of cells, tissues, and organs throughout the body. To maintain homeostasis, feedback mechanisms control hormone secretion. A feedback mechanism is one in which the last step in a series of events controls the first. Feedback mechanisms can be negative or positive. Most hormone systems use negative feedback.
In negative feedback, the final step in a series of events inhibits the initial signal in the series. An example of negative feedback in regulating the levels of thyroid hormones is shown in Figure 50-10. When the hypothalamus detects low levels of thyroid hormones, it secretes a hormone called thyrotropin releasing hormone (TRH) to the anterior pituitary. TRH stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH) into the bloodstream. TSH stimulates the thyroid gland to secrete thyroid hormones. When the thyroid hormone levels are high, two major negative feedback loops operate, as shown by the negative signs in Figure 50-10. In one loop, the thyroid hormones act on the hypothalamus to inhibit the release of TRH. In the second loop, the thyroid hormones act on the anterior pituitary to inhibit the release of TSH. The result is that the level of thyroid hormones in the blood decreases. In turn, this decrease causes the amount of negative feedback inhibition to decline. The interplay of these mechanisms helps to keep the concentration of thyroid hormones relatively stable.
www.scilinks.org Topic: Homeostasis Keyword: HM60753
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