ABSTRACT
Homeostasis is concerted action of various systems within an organisms that act to maintain the internal environment, in particular the physico-chemical properties of the fluids that bathe tissues, cells, and organelles. Virtually every component of an organismal system thus both benefits from, and contributes to, the maintenance of this milieu intérieur, as opposed to the milieu extérieur, the external (ambient) environment which is at best partially under control of the organism's actions, and which therefore must be regarded as exerting perturbations that are to some extent unpredictable in nature. In this chapter we introduce the key notions of the feedback loop and the set-point, as well as the distinction between ‘physiological’ and ‘actuator’ state variables.
