For example, norepinephrine raises the blood pressure when the adrenal gland secretes it into the blood, but when norepinephrine is released within the nervous system, it stimulates the activity of only nearby nerve cells without affecting the blood pressure. Hormones are substances released into the bloodstream from a gland or organ that affect activity in cells at another site. Most hormones are proteins composed of amino acid chains of varying length. Others are steroids, fatty substances derived from cholesterol. Very small amounts of hormones can trigger very large responses in the body.
Hormones bind to receptors on a cell's surface or inside a cell.
The binding of a hormone to a receptor speeds up, slows down, or in some other way alters the cell's function. Ultimately, hormones control the function of entire organs. They control growth and development, reproduction, and sexual characteristics. They influence the way the body uses and stores energy. Hormones also control the volume of fluid and the levels of salt and sugar in the blood.