Hormones regulate various biological activities including growth, development, reproduction, energy use and storage, and water and electrolyte balance. They are molecules that act as chemical messengers in the body's endocrine system. Hormones are produced by certain organs and glands and are secreted into the blood or other bodily fluids. Most hormones are carried by the circulatory system to different areas, where they influence specific cells and organs.
Hormones that are circulated in the blood come in contact with a number of cells. However, they influence only target cells, which have receptors for each specific hormone. Target cell receptors can be located on the surface of the cell membrane or inside of the cell. When a hormone binds to a receptor, it causes changes within the cell that influence cellular function. This type of hormone signaling is described as endocrine signaling because the hormones influence target cells over a long distance from where they are secreted. For example, the pituitary gland near the brain secretes growth hormones affecting widespread areas of the body. Not only can hormones affect distant cells, but they can also influence neighboring cells.
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