(See also Endocrine Glands.)
The endocrine system consists of a group of glands and organs that regulate and control various body functions by producing and secreting hormones. Hormones are chemical substances that affect the activity of another part of the body. In essence, hormones serve as messengers, controlling and coordinating activities throughout the body. To control endocrine functions, the secretion of each hormone must be regulated within precise limits. The body is normally able to sense whether more or less of a given hormone is needed.
Many endocrine glands are controlled by the interplay of hormonal signals between the hypothalamus, located in the brain, and the pituitary gland, which sits at the base of the brain. This interplay is referred to as the hypothalamic-pituitary axis. The hypothalamus secretes several hormones that control the pituitary gland.
The pituitary gland, sometimes called the master gland, in turn controls the functions of many other endocrine glands. The pituitary controls the rate at which it secretes hormones through a feedback loop in which the blood levels of other endocrine hormones signal the pituitary to slow down or speed up.
Many other factors can control endocrine function. For example, a baby sucking on its mother's nipple stimulates her pituitary gland to secrete prolactin and oxytocin, hormones that stimulate breast milk production and flow. Rising blood sugar levels stimulate the islet cells of the pancreas to produce insulin. Part of the nervous system stimulates the adrenal gland to produce epinephrine.