Osmotic Volumes of Distribution

Abstract

Osmotic volumes of distribution of sodium, chloride, sulfate, bicarbonate and sucrose, and volumes of distribution of urea and osmotic pressure were determined in anesthetized, nephrectomized dogs and cats. Total body water was measured in cats by oven drying the entire comminuted carcass. Total body water is closely measured by urea (chemically and osmotically determined) volumes of distribution and by the osmotic volume of distribution of sodium (chemically determined). The volume of distribution of osmotic pressure when NaCl, Na2SO4 or sucrose is given is approximately 11% less than total body water reflecting, it is believed, an idiogenic increase of osmotic pressure in proportion (0.15:1) to applied osmotic stresses tending to dehydrate cells. The possible idiogenic decrease of osmotic pressure under osmotic stresses tending to hydrate cells is considered. In experiments where NaCl is loaded, the equilibrium concentration of chloride in extracellular water may exceed that of sodium or appraoch it closely enough to be taken as evidence of ionic exchanges across cell membranes. It is shown how chemical and osmotic analyses of a single substance such as sucrose may be applied to measure total body water as well as extracellular and intracellular fluid volumes. Evidence is presented that the body does not behave as a "perfect" osmometer insofar as the quantity of osmotically active material in the body may vary under stated conditions. With this limitation, which is described quantitatively, it remains useful under the conditions of these experiments to regard the cells of the body as a whole to behave as an osmometer. Accordingly, theory, application, and critique of osmometric analysis are outlined.