In 1935 Darrow and Yannet1 demonstrated that water was freely diffusible between cells and extracellular fluid so that the solute concentrations of the 2 phases were always equal. More recently this concept has been confirmed by direct measurement of the osmolality of the 2 phases.2,3 Consequently an increase in the concentration of a solute of extracellular fluid that is not diffusible into cells, i.e., sodium chloride, will result in a movement of water out of the cells. The magnitude of water lost from cells can be calculated from the increase in sodium concentration. If the loss of water is not equal to that predicted by the change in sodium concentration, it must be assumed that solute content of the cells also increases.
Therefore, the state of hypernatremia, regardless of how it arises, should result in a loss of water from muscle cells that can be predicted from the