Homeostasis, also spelled homoeostasis (from Greek: ὅμοιος, "hómoios", "similar"), is the property of a system in which variables are regulated so that internal conditions remain stable and relatively constant. Examples of homeostasis include the regulation of temperature and the balance between acidity and alkalinity (pH). It is a process that maintains the stability of the human body's internal environment in response to changes in external conditions.
The concept was described by Claude Bernard in 1865 and the word was coined by Walter Bradford Cannon in 1926. Although the term was originally used to refer to processes within living organisms, it is frequently applied to automatic control systems such as thermostats. Homeostasis requires a sensor to detect changes in the condition to be regulated, an effector mechanism that can vary that condition; and a negative feedback connection between the two. All living organisms depend on maintaining a complex set of interacting metabolic chemical reactions. From the simplest unicellular organisms to the most complex plants and animals, internal processes operate to keep the conditions within tight limits to allow these reactions to proceed. Homeostatic processes act at the level of the cell, the tissue, and the organ, as well as for the organism as a whole.
Principal Homeostatic processes include the following:
"Warm-blooded" (endothermic) animals (mammals and birds) maintain a constant body temperature, whereas ectothermic animals (almost all other animals) exhibit wide body temperature variation. An advantage of temperature regulation is that it allows an organism to function effectively in a broad range of environmental conditions. For example, ectotherms tend to become sluggish at low temperatures, whereas a co-located endotherm may be fully active. That thermal stability comes at a price, since an automatic regulation system requires additional energy. If the temperature rises, the body loses heat...
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