Hormones are chemical substances produced by the body that affect the activity of another part of the body (target site). In essence, hormones serve as messengers, controlling and coordinating activities throughout the body. Hormones are secreted by a number of different glands, including the hypothalamus, the pituitary gland, the thyroid gland, the parathyroid glands, the islet cells of the pancreas, the adrenal glands, the testes in men, and the ovaries in women. These glands make up the endocrine system.
Not all organs that secrete hormones or hormone-like substances are considered part of the endocrine system. For example, the kidneys produce the hormone renin to help control blood pressure and the hormone erythropoietin to stimulate the bone marrow to produce red blood cells. In addition, the digestive tract produces a variety of hormones that control digestion, affect insulin secretion from the pancreas, and alter behaviors, such as those associated with hunger. Fat (adipose) tissue also produces hormones that regulate metabolism and appetite. Additionally, the term "gland" does not mean that the organ is part of the endocrine system. For example, sweat glands, glands in mucus membranes, and mammary glands secrete substances other than hormones.