The Distribution of Sodium and Chloride and the Extracellular Fluid Volume in the Rat1

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Abstract
Measurement of extracellular volume in the rat was made by correcting total body Na and total body Cl, determined by carcass analysis, for the amounts of these ions present outside the extracellular space. Corrections for body Na included the amounts present in bone salt, in the lumen of the gut and that located intracellularly in muscle and in erythro-cytes. Corrections for Cl included the "excess" Cl present in connective tissue, Cl in the lumen of the gut and that located intracellularly in liver, in viscera of the respiratory and gastrointestinal tract and in erythrocytes. The values for these corrections were either obtained experimentally or from data in the literature. Dividing the values for total body Na and Cl, corrected as described above, by the serum ultra-filtrate concentrations of Na and Cl, respectively, gave volumes of distribution which agreed within 1%. In the 220 gram rat, the extracellular volume amounted to 21.8% of body weight or 32% of body water. The Cl and Na present in extracellular fluid amounted to 87.3%, and 68.6% respectively, of the total amounts of these ions in the body.