SHORT COMMUNICATION DETERMINATION OF THE CONSTANTS OF THE HENDERSON–HASSELBALCH EQUATION, aCO2 AND pKa, IN SEA TURTLE PLASMA

Abstract

Acid–base physiologists traditionally simplify the reaction by omitting the H2CO3 term and lumping all ionic CO2 species into the HCO32 term. The simplified reaction forms the basis for the familiar Henderson–Hasselbalch equation of the CO2–HCO32 buffer system: pH = pKa+log([HCO32]/aCO2PCO), (2) where aCO2 is the solubility coefficient relating [CO2] and PCO (Henry’s Law). The apparent pK (pKa) in this equation lacks a rigorous thermodynamic definition. Instead, it is an empirical factor relating pH, the product of aCO2 and PCO, and the apparent [HCO32] (i.e. the sum of all ionic CO2species). aCO2 and pKa are sensitive to the temperature, pH and/or the ionic strength of the reaction medium. aCO2 and pKa of normal mammalian blood plasma have been well defined over a range of temperatures and pH values (e.g. Severinghaus, 1965; Siggaard-Andersen, 1974; Reeves, 1976). These mammalian values are commonly used in analyses of the acid–base status of non-mammalian species, despite evidence that such practices can produce misleading results (Nicol et al.1983). As an alternative, Heisler (1984; erratum in Heisler, 1986) developed complex equations for aCO2 (mmoll21mmHg1) (1mmHg=133.22Pa) and pKa that are purported to be generally applicable to aqueous solutions (including body fluids) between 0 and 40˚C and incorporate the molarity of dissolved species (Md), solution pH, temperature (T, ˚C), sodium concentration ([Na+], mol21), ionic strength of nonprotein ions (I, mol21) and protein concentration ([Pr], gl21): aCO2 = 0.1008 2 2.98031022Md+ (1.21831023Md2 3.63931023)