Definitions from


The term is most often used in the sense of biological homeostasis.

homeo- similar or same.

stasis- standing or stopping.

Multicellular organisms require a homeostatic internal environment, in order to live; many environmentalists believe this principle also applies to the external environment.

Many ecological, biological, and social systems are homeostatic. They oppose change to maintain equilibrium. If the system does not succeed in reestablishing its balance, it may ultimately lead the system to stop functioning.

Complex systems, such as a human body, must have homeostasis to maintain stability and to survive. These systems do not only have to endure to survive; they must adapt themselves and evolve to modifications of the environment.

Properties of homeostasis Homeostatic systems show several properties:

They are ultrastable; Their whole organization, internal, structural, and functional, contributes to the maintenance of equilibrium They are unpredictable (the resulting effect of a precise action often has the opposite effect to what was expected). Main examples of homeostasis in mammals are as follows:

The regulation of the amounts of water and minerals in the body. This is known as osmoregulation. This happens in the kidneys. The removal of metabolic waste. This is known as excretion. This is done by the excretory organs such as the kidneys and lungs. The regulation of body temperature. This is mainly done by the skin. The regulation of blood glucose level. This is mainly done by the liver and the insulin secreted by the pancreas.

Mechanisms of homeostasis: feedback Main article: Feedback

When a change of variable occurs, there are two main types of feedback to which the system reacts:

Negative feedback is a reaction in which the system responds in such a way as to reverse the direction of change. Since this tends to keep things constant, it allows the maintenance of homeostasis. For instance, when the concentration of carbon dioxide in the human body increases, the lungs are signalled to increase their activity and expel more carbon dioxide. Thermoregulation is another example of negative feedback. When body temp rises (or falls), receptors in the skin and the hypothalamus sense a change, triggering a command from the brain, which in turn effects the correct response, this case being body temp decreases.

In positive feedback, the response is to amplify the change in the variable. This has a de-stabilizing effect, so does not result in homeostasis. Positive feedback is less common in naturally occurring systems than negative feedback, but it has its applications. For example, in nerves, a threshold electric potential triggers the generation of a much larger action potential. (See also leverage points.) Blood clotting and events in childbirth are other types of positive feedback.

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