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College of Veterinary Medicine, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
Received 19 December 1994; accepted in final form 14 February 1997.
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
(Ka). A
simplified strong ion model, which was developed from the assumption
that plasma ions act as strong ions, volatile buffer ions
(HCO
3), or nonvolatile buffer ions,
indicates that plasma pH is determined by five independent variables:
PCO2, 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
Ka 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 Ka
(2.22 ± 0.32 × 107 eq/l) that were
significantly different from the values commonly assumed for human
plasma ([Atot] = 20.0 meq/l, Ka = 3.0 × 107 eq/l).
Moreover, application of the experimentally determined values for
[Atot] and
Ka to published
data for the horse (known PCO2,
strong ion difference, and plasma protein concentration) predicted
plasma pH more accurately than the values for
[Atot] and
Ka commonly
assumed for human plasma. Species-specific values for
[Atot] and
Ka should be
experimentally determined when the simplified strong ion model (or
strong ion model) is used to describe acid-base equilibria.
acid-base balance; acidosis; alkalosis; alphastat; strong ion difference
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