TRANSLATIONAL PHYSIOLOGY Determinants of plasma water sodium concentration as reflected in the Edelman equation: role of osmotic and Gibbs-Donnan equilibrium

Nguyen, Minhtri K., and Ira Kurtz. Determinants of plasma water sodium concentration as reflected in the Edelman equation: role of osmotic and Gibbs-Donnan equilibrium. Am J Physiol Renal Physiol 286: F828–F837, 2004. First published January 20, 2004; 10.1152/ajprenal.00393.2003.—Edelman et al. have empirically shown that plasma water sodium concentration ([Na ]pw) is equal to 1.11(Nae Ke)/TBW 25.6 (Edelman IS, Leibman J, O’Meara MP, Birkenfeld LW. J Clin Invest 37: 1236–1256, 1958). However, the physiological significance of the slope and y-intercept in this equation has not been previously considered. Our analysis demonstrates that there are several clinically relevant parameters determining the magnitude of the y-intercept that independently alter [Na ]pw: 1) osmotically inactive exchangeable Na and K ; 2) plasma water K concentration; and 3) osmotically active non-Na and non-K osmoles. In addition, we demonstrate quantitatively the physiological significance of the slope in the Edelman equation and its role in modulating [Na ]pw. The slope of 1.11 in this equation which Edelman et al. determined empirically can be theoretically predicted by considering the combined effect of the osmotic coefficient of Na salts at physiological concentrations and Gibbs-Donnan equilibrium. In addition, our results demonstrate that the slope has an independent quantitative impact on the magnitude of the y-intercept in the Edelman equation. From a physiological standpoint, the components of both the slope and the y-intercept need to be addressed when considering the factors that modulate [Na ]pw.