Homeostasis is the self-regulating mechanisms whereby by biological systems attempt to maintain a stable internal conditions (e.g., blood pressure, body temperature, acid-base balance) in the face of changes in the external environment. It was the nineteenth-century French physiologist Claude Bernard who first realized that the internal environment of any free living organism was maintained constant within certain limits. Homeostasis is generally achieved through two types of regulating systems: on-off control and feedback control. Hormones often play a vital role in maintaining homeostatic stability.