Homeostasis is a phenomenon initially observed by the French physiologist Claude Bernard (1813-1878) and later coined by Walter Cannon (1871-1945), an American physiologist. Homeostasis is the tendency of the living body to maintain fairly stable chemical and physical internal fluid conditions.

The body cells are the functional unit of life. They function only when bathed by fluid in which the physical and chemical composition are relatively stable. This internal stability of the body is best described as a dynamic equilibrium or a balanced change in which there is a certain set point or an average value for a given variable, and conditions fluctuate slightly around this point.

The functions of each body system aid in maintaining homeostasis or internal stability. Much of physiology therefore pertains to the regulatory mechanisms that maintain homeostasis. These regulatory mechanisms are frequently negative feedback systems, where an alteration is detected and the body responds with various responses that reverse the initial change.