Homeostasis is the ability of a living organism to control its internal environment despite fluctuations in the external environment. Claude Bernard proposed the concept of 'milieu interieur'. The hypothesis rests on the belief that cells need a precisely defined local environment in which to function optimally. The maintenance of the micro-environment is dependent on integration of the systems of the organism (body), including behaviour, to protect the cell(s) from external stressors. When the environment changes, the body needs to sense the alteration, then set in motion mechanisms which mitigate the change and restore the previous balance. If it fails to do this, the organism will ultimately die as systems fail.