Ductless glands which secrete chemical substances called hormones into the bloodstream which control the internal environment not only of each cell and organ, but of the entire body.
The endocrine glands—the pineal, pituitary, thyroid, parathyroids, thymus, adrenals, pancreas and gonads (ovaries or testes)—comprise the endocrine system. The hypothalamus, the gland in the brain which serves as the command center, operates the endocrine system through the pituitary, a pea-sized gland located under it, which directs the work of all the other glands. The thyroid, a gland in the neck, regulates the body's metabolism. The parathyroids, which are attached to the thyroid, control the amount of calcium and phosphate in the bloodstream. The adrenal glands, located near the kidneys, produce adrenaline which arouses the body to respond to stress and emergencies and other hormones active in carbohydrate metabolism. The pancreas secretes insulin which regulates the level of sugar in the bloodstream. The gonads regulate sexual development, ovulation, and growth of sex organs.
The Endocrine System: Miraculous Messengers. New York: Torstar Books, 1985.