3.   How do hormones act?

Hormones are substances produced by glands and released into the bloodstream that affect activity in cells at other locations of 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.

Hormones can be either a protein or a steroid. Protein hormones bind with receptors on the surface of cells. Steroid hormones are small molecules that are fat-soluble and thus easily diffuse through cell walls to bind with receptors inside of cells.

For more detailed information on hormones go to the "endocrine primer" section.