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physiology week1 chapter2-bookHOMEOSTASIS THE INTERNAL ENVIRONMENT More than a century ago a great French physiologist, Claude Bernard (1813–1878), made a remarkable observation. He noted that body cells survived in a healthy condition only when the tem-perature, pressure, and chemical composition of their fluid environ-100 Increase 90 Normalment remained relatively constant. He called the environment of cells the internal environment, or milieu intérieur. Bernard realized that although many elements of the external environment in which we live are in a constant state of change, important elements of the internal environment, such as body tem-perature, remain remarkably stable. For example, Bernard’s neigh-bor, who travels from his Paris home heated with a fireplace to the snowy slopes of the Alps in January, is exposed to dramatic changes in air temperature within a few hours. Fortunately, in a healthy individual, body temperature will re-main at or very near normal regardless of temperature changes that may occur in the external environment. Just as the external environ-ment surrounding the body as a whole is subject to change, so too is the fluid environment surroundingeach body cell. The remarkable fluid that bathes each cell contains literally dozens of different sub-stances. Good health, indeed life itself, depends on the correct and constant amount of each substance in the blood and other body fluids.The precise and constant chemical composition of the internal environment must be maintained within very narrow limits (“normal ranges”), or sickness and death will result.RELATIVE STABILITYIn 1932 a famous American physiologist, Walter B. Cannon (1871-1945), suggested the name homeostasis for the relatively con-stant states maintained by the body. Homeostasis is a key
word in modern physiology. It comes from two Greek words (homoios, “the same,” and stasis, “standing”). “Standing or staying the same,” then, is the literal meaning of homeostasis. In his classic publication titled The Wisdom of the Body, Cannon advanced one of the most unifying and important themes of physiol-ogy. He suggested that every regulatory mechanism of the bodyexists to maintain homeostasis, or constancy, of the body’s internal fluid environment. However, as Cannon emphasized, homeostasis does not mean something set and immobile that stays exactly the same all the time. In his words, homeostasis “means a condition that may vary, but which is relatively constant.”It is the maintenance of relatively constant internal conditions despite changes in either the internal or the external environment that characterizes homeostasis. For example, even if externaltem-peratures vary, homeostasis of body temperature means that it re-mains relatively constant atabout 37° C (98.6° F), although it may vary slightly above or below that point and still be “normal.” The fasting concentration of blood glucose, an important nutrient, can also vary somewhat and still remain within normal limits (Figure 2-1).

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