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Homeostasis or homoeostasis (''homeo-'' + ''-stasis'') 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 French physiologist 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. ==Biological== 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 by sweating or panting, via the latent heat of evaporation. If it falls, this is counteracted by increased metabolic action, by shivering, and—in fur- or feather-coated creatures—by thickening of the coat. * Regulation of cell number and cell size to maintain organ size and function. * Regulation of the pH of the blood at 7.365 (a measure of alkalinity and acidity). * All animals also regulate their blood glucose concentration. Mammals regulate their blood glucose with insulin and glucagon. The human body maintains glucose levels constant most of the day, even after a 24-hour fast. Even during long periods of fasting, glucose levels are reduced only very slightly. Insulin, secreted by the beta cells of the pancreas, effectively transports glucose to the body's cells by instructing those cells to keep more of the glucose for their own use (see Dynamic equilibrium). If the glucose inside the cells is high, the cells will convert it to the insoluble glycogen to prevent the soluble glucose from interfering with cellular metabolism. Ultimately this lowers blood glucose levels, and insulin helps to prevent hyperglycemia. When insulin is deficient or cells become resistant to it, diabetes occurs. Glucagon, secreted by the alpha cells of the pancreas, encourages cells to break down stored glycogen or convert non-carbohydrate carbon sources to glucose via gluconeogenesis, thus preventing hypoglycemia. * The kidneys are used to remove excess water and ions from the blood. These are then expelled as urine. The kidneys perform a vital role in homeostatic regulation in mammals, removing excess water, salt, and urea from the blood. * If the water content of the blood and lymph fluid falls, it is restored in the first instance by extracting water from the cells. The throat and mouth become dry, so that the symptoms of thirst motivate the animal to drink. * If the oxygen content of the blood falls, or the carbon-dioxide concentration increases, blood flow is increased by more vigorous heart action and the speed and depth of breathing increases. * Sleep timing depends upon a balance between homeostatic sleep propensity, the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode, and circadian rhythms that determine the ideal timing of a correctly structured and restorative sleep episode. * Personality traits are often conceptualized as a person specific setpoint level around which mood states fluctuate in time. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Homeostasis」の詳細全文を読む スポンサード リンク
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