Homeostasis

Homeostasis^[1] is self-regulation, a basic property of all self-organising systems. In biology, it is the keeping of a stable internal environment.^[2][3][4]

Homeostasis is life's ability to stay balanced, when the environment changes. Animals keep their body in a stable condition. They do so by regulating their inner equilibrium. For example, they adjust their pH, temperature, amount of oxygen or carbon dioxide in the blood and so on. In living things, the study of how they keep in a stable condition is called physiology. Mostly, our physiology works unconsciously throughout life. We have many negative feedback systems which adjust our physiology so that we stay alive.^[5][6] These systems are self-organising and do not need to be learnt. They are inherited.

The concept was described by Claude Bernard,^[7] and the term was later coined by Walter Cannon in 1926,^[8] 1929,^[9] and 1932.^[2][10] Walter Cannon thought these were the features of homeostasis:^[2]

1. Constancy in an open system, such as our bodies, is done by mechanisms which maintain this constancy. Cannon based this on insights into the ways by which steady states such as glucose concentrations, body temperature and acid-base balance were regulated.
2. To keep a steady-state condition, any change automatically meets with factors that resist change. An increase in blood salt results in thirst as the body attempts to dilute the concentration of salt in the extracellular fluid.
3. The regulating system doing homeostasis has a number of cooperating mechanisms which act simultaneously or successively. Blood sugar is regulated by insulin, glucagons, and other hormones that control its release from the liver or its uptake by the tissues.

Examples of homeostasis:

• The operation of a thermostat
• The regulation of water and minerals in the body
• The regulation of body temperature: mammals and birds have complicated systems which keep their body temperature within close limits.

In mammals, the main organs involved with homeostasis are:

• The hypothalamus and pituitary gland
• the lungs
• the skin
• the muscles
• the kidneys
• the liver and pancreas

The brain is also central to homeostasis. It controls behaviour, and the basic function of behaviour is to support life by taking action.

1. ↑ homeo meaning "similar" and stasis meaning "standing, condition"
2. ↑ ^{2.0 2.1 2.2} Cannon, Walter B. 1932. The wisdom of the body. New York:Norton.
3. ↑ Bradfield, Phil; Potter, Steve (2009). Edexcel IGCSE Biology: Student Book. Pearson Education. p. 83. ISBN 9780435966881.
4. ↑ More formal: homeostasis keeps essential variables within physiological limits. Ashby, Ross 1960. Design for a brain: the origin of adaptive behaviour. 2nd ed, Chapman & Hall, London. p58
5. ↑ Widmaier E.P; Raff H. & Strang K.T. 2009. Vander's human physiology. 11th ed, McGraw-Hill.
6. ↑ Schmidt-Nielsen K. 1997. Animal physiology: adaptation and environment. Cambridge: Cambridge University Press.
7. ↑ Bernard C. 1974. Lectures on the phenomena common to animals and plants. Trans Hoff HE, Guillemin R, Guillemin L, Springfield (IL): Charles C Thomas. ISBN 978-0-398-02857-2
8. ↑ Cannon W.B 1926. Physiological regulation of normal states: some tentative postulates concerning biological homeostatics. In: Pettit A. (ed) À Charles Richet: ses amis, ses collègues, ses élèves. Paris: Éditions Médicales, p. 91.
9. ↑ Cannon W.B. 1929. Organization for physiological homeostasis. Physiol Rev. 9: 399-431.
10. ↑ Karl Ludwig von Bertalanffy. 1970. ... aber vom Menschen wissen wir nichts, English title Robots, Men and Minds, translated by Dr. Hans-Joachim Flechtner. Econ Verlag GmbH, Düsseldorf, Wien, p115.