title: Determinants of plasma water sodium concentration as reflected in the Edelman equation: role of osmotic and Gibbs-Donnan equilibrium.
abstract: Edelman et al. have empirically shown that plasma water sodium concentration ([Na(+)](pw)) is equal to 1.11(Na(e) + K(e))/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).
authors.affiliation: Division of Nephrology, David Geffen School of Medicine at UCLA, Factor Bldg., Rm. 7-155, 10833 Le Conte Ave., Los Angeles, CA 90095-1689, USA. mtnguyen@mednet.ucla.edu
authors.forename: Minhtri K
authors.initials: MK
authors.lastname: Nguyen
authors.forename: Ira
authors.initials: I
authors.lastname: Kurtz
mesh_terms: D000818:Animals; D006801:Humans; D008954:Models, Biological; D012964:Sodium / Q000097:blood*; D014867:Water / Q000378:metabolism; D014882:Water-Electrolyte Balance / Q000502:physiology*
chemical_list: D014867:Water; D012964:Sodium