The endocrine system, made up of all the body's different hormones, regulates all biological processes in the body from conception through adulthood and into old age, including the development of the brain and nervous system, the growth and function of the reproductive system, as well as the metabolism and blood sugar. Endocrine hormones travel throughout the body in the blood. However, each endocrine hormone affects only certain cells, called target cells. A target cell is affected by a given hormone because it has proteins on its surface to which the hormone can bind. When the hormone... Show more

The endocrine system, made up of all the body's different hormones, regulates all biological processes in the body from conception through adulthood and into old age, including the development of the brain and nervous system, the growth and function of the reproductive system, as well as the metabolism and blood sugar.
Endocrine hormones travel throughout the body in the blood. However, each endocrine hormone affects only certain cells, called target cells. A target cell is affected by a given hormone because it has proteins on its surface to which the hormone can bind. When the hormone binds to a target cell protein, it causes changes inside the cell. For example, binding of the hormone might cause the release of enzymes inside the cell. The enzymes then influence cell processes.    
Endocrine hormones control many cell activities, so they are very important for homeostasis. But what controls the hormones? Most endocrine hormones are controlled by feedback loops. In a feedback loop, the hormone produced by a gland feeds back to control its own production by the gland. A feedback loop can be negative or positive. Most endocrine hormones are controlled by negative feedback loops. Negative feedback occurs when rising levels of a hormone feed back to decrease secretion of the hormone or when falling levels of the hormone feed back to increase its secretion.    
An example of a negative feedback loop is the one that controls the thyroid gland. This loop involves the hypothalamus and pituitary gland as well as the thyroid. Low levels of thyroid hormones in the blood cause the release of hormones by the hypothalamus and pituitary gland. These hormones stimulate the thyroid gland to secrete more hormones. The opposite happens with high levels of thyroid hormones in the blood. The hypothalamus and pituitary gland stop releasing hormones that stimulate the thyroid, so the thyroid stops secreting its hormones.

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