hormone

A hormone is a chemical messenger a gland releases into the blood. Unlike a nerve impulse, which travels along a fiber to one target, a hormone travels wherever the blood goes, and only the cells built to answer it respond — which is why one gland can be the body's message for the whole body. Every hormone question has three answers: which gland makes it, what it acts on, and what it changes. Insulin moves glucose out of the blood and into the cells; glucagon makes the liver release glucose; calcitonin lowers the blood calcium level and parathyroid hormone raises it; ADH tells the kidneys to keep water. Most hormones are controlled by negative feedback: the response cancels the change that started it, so the gland slows or stops releasing the hormone once the level is back to normal.