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

Research output: Contribution to journalReview articlepeer-review


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 ([A(tot)]) 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 (HCO3), or nonvolatile buffer ions, indicates that plasma pH is determined by five independent variables: P(CO2), 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 [A(tot)] and K(a) of plasma. Application of the simplified strong ion model to CO2-tonometered horse plasma produced values for [A(tot)] (15.0 ± 3.1 meq/l) and K(a) (2.22 ± 0.32 x 10-7 eq/l) that were significantly different from the values commonly assumed for human plasma ([A(tot)] = 20.0 meq/l, K(a) = 3.0 x 10- 7 eq/l). Moreover, application of the experimentally determined values for [A(tot)] and K(a) to published data for the horse (known P(CO2), strong ion difference, and plasma protein concentration) predicted plasma pH more accurately than the values for [A(tot)] and K(a) commonly assumed for human plasma. Species-specific values for [A(tot)] and K(a) should be experimentally determined when the simplified strong ion model (or strong ion model) is used to describe acid-base equilibria.

Original languageEnglish (US)
Pages (from-to)297-311
Number of pages15
JournalJournal of Applied Physiology
Issue number1
StatePublished - Jul 1997


  • Acid-base balance
  • Acidosis
  • Alkalosis
  • Alphastat
  • Strong ion difference

ASJC Scopus subject areas

  • Physiology
  • Physiology (medical)


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