The Mechanism of the Osmotic Adjustment of Body

Since permeability of cell membranes to water has been clearly demonstrated, there are several theoretically possible responses of cells to dilution of the extracellular fluid. Osmotic equilibrium might result from net movement of water into cells in response to extracellular dilution. On the other hand, net movement of water into cells might be averted by: 1) active transport of water out of cells preventing a change in intracellular osmotic activity; 2) reduction of intracellular osmotic activity by means of either osmotic inactivation of intracellular solutes or their extrusion from the cells. These possibilities were tested by determining the volume of distribution of a large water load which depends on movement of water through the body. The data are compared with the total body water content measured by the dilution of a tracer amount of deuterium oxide which is independent of net movement of water. In the dog the volume of distribution of a large water load was found to approximate closely the total body water content.