Krebs and his associates1 first showed that the water content of tissues in vitro was dependent on tissue metabolism. With inhibition of metabolism tissues swell and with restoration of metabolism, if not too delayed, the water content returns to normal. It has been shown that the changes affect the intracellular fluids rather than the interstitial fluid of the tissue and the swelling and shrinking of the cells is secondary to their changes in content of solutes. 2, 3, 4

The concentration of water is the same inside and outside of cells. 5 6 7 Cell membranes are generally so permeable to water that a uniform chemical potential of water or tonicity exists throughout intra-and extracellular fluids. This means that the volume of cells is determined by the quantity of solutes which they contain. The major osmotically active solute within cells is potassium ions. Sodium which is high in the plasma and extracellular fluids is present at low concentrations within most cells, eg, at about one-tenth its extracellular fluid concentration within muscle cells. But cell mem-branes are permeable to sodium, and positive