Since the body fluids are nearly neutral and since they contain no appreciable quantities of organic bases, the concentration of cations in these fluids must be equal to the concentration of the inorganic cations found in them, [Na] + [K] +[Ca] + [Mg]. This, in turn, must be equal to the total concentration of anions. The distribution of water between cells and extracellular fluids in the body appears to be governed mainly by the concentration in the extracellular fluids of sodium, the principal one of those osmotically active components which are restrained from crossing the cellular membranes. In addition, sodium constitutes the chief cation of the extracellular fluids. In normal serum it makes up more than 90 per cent of the total concentration of cations. Although the concentrations of the other cations, profoundly influence physiological processes, they are so small and vary so little that their effects on osmotic and cation-anion equilibria are trifling. The concentration of sodium is, therefore, the best single criterion of the electrolyte osmotic pressure and the concentration of cations in the serum and extracellular fluids. The measurement of sodium has, however, been so laborious and time-consuming that it has been less useful for clinical than for investigative purposes.In lieu of direct measurement the concentration of sodium has usually been estimated from the combined concentrations of bicarbonate and chloride. In normal subjects these 2 ions together make up from 85 to 90 per cent of the anions of serum, falling only a little short of equaling sodium. The object of this paper is to examine the accuracy of such inferences. With the flame photometer it will become possible to measure the sodium of serum with sufficient speed and facility to make this measurement clinically available.
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