Updated February 06, 2008.

Definition: A specific physiologic process that allows organs to determine their own blood supply.

Examples: Exercising muscles require more blood flow. When muscles are stressed, as in exercise, they release chemicals that cause nearby blood vessels to dilate. This thus increases the amount of blood flowing through the blood vessels and, ultimately, to the muscle. When the muscles are no longer stressed, they stop releasing these compounds and the blood vessels once again constrict to normal size.