A simplified strong ion model for acid-base equilibria: application to horse plasma

Abstract

Constable, Peter D. A simplified strong ion model for acid-base equilibria: application to horse plasma. J. Appl. Physiol. 83(1): 297–311, 1997.—The Henderson-Hasselbalch equation and Stewart’s strong ion model are currently used to describe mammalian acid-base equilibria. Anomalies exist when the Henderson-Hasselbalch equation is applied to plasma, whereas the strong ion model does not provide a practical method for determining the total plasma concentration of nonvolatile weak acids ([Atot]) and the effective dissociation constant for plasma weak acids ( K a). A simplified strong ion model, which was developed from the assumption that plasma ions act as strong ions, volatile buffer ions ([Formula: see text]), or nonvolatile buffer ions, indicates that plasma pH is determined by five independent variables:[Formula: see text], strong ion difference, concentration of individual nonvolatile plasma buffers (albumin, globulin, and phosphate), ionic strength, and temperature. The simplified strong ion model conveys on a fundamental level the mechanism for change in acid-base status, explains many of the anomalies when the Henderson-Hasselbalch equation is applied to plasma, is conceptually and algebraically simpler than Stewart’s strong ion model, and provides a practical in vitro method for determining [Atot] and K a of plasma. Application of the simplified strong ion model to CO2-tonometered horse plasma produced values for [Atot] (15.0 ± 3.1 meq/l) and K a(2.22 ± 0.32 × 10−7 eq/l) that were significantly different from the values commonly assumed for human plasma ([Atot] = 20.0 meq/l, K a = 3.0 × 10−7 eq/l). Moreover, application of the experimentally determined values for [Atot] and K a to published data for the horse (known [Formula: see text], strong ion difference, and plasma protein concentration) predicted plasma pH more accurately than the values for [Atot] and K a commonly assumed for human plasma. Species-specific values for [Atot] and K a should be experimentally determined when the simplified strong ion model (or strong ion model) is used to describe acid-base equilibria.