Fluids and Electrolytes: Physiology
Stanley Hellerstein MD1
1 Professor of Pediatrics, University of Missouri-Kansas City School of Medicine; Chief, Section of Pediatric Nephrology, The Children's Mercy Hospital, Kansas City, MO.
More than 100 years ago Claude Bernard (1813-1878) pointed out that higher animals "have really two environments: a milieu exterieur in which the organism is situated, and a milieu interieur in which tissue elements live. The living organism does not really exist in the milieu exterieur (the atmosphere it breathes, salt or fresh water if that is its element) but in the liquid milieu interieur formed by the circulating organic liquid which surrounds and bathes all the tissue elements: this is the lymph or plasma.....all the vital mechanisms, however varied they may be, have only one object, that of preserving constant the conditions of life in the internal environment." Although Bernard deduced the existence of the constancy of the internal environment from relatively few facts, this basic concept has been confirmed repeatedly. Walter B. Cannon extended Bernard's hypothesis by recognizing that "physiologic agencies" were excited to action by deviation from the constancy of the internal environment. Cannon designated the constancy of the internal environment as a homeostatic state that, when disturbed by environmental change, automatically elicits a corrective physiologic action. Although modern fluid and electrolyte physiology is based on recognition of these basic concepts, its application has been enormously enriched by detailed information on the specific mechanisms that foster the maintenance of the composition of individual cells and the regulatory processes that contribute to the overall body fluid homeostasis.
