Plasma bicarbonate and CO2 tension.

The relation between plasma bicarbonate concentration ([HCO3-]) and CO2 tension (PCO2) in normal subjects was studied by Madias et al. They calculated these values from measurements of plasma total CO2 content and blood pH using the Henderson-Hasselbalch equation. We calculated the relation between [HCO3-] and PCO2 using values obtained not from measurements, but from total CO2 and pH values chosen either randomly or uniformly within the range found by Madias et al. Both approaches yielded relations between [HCO3-] and PCO2 very similar to those found by Madias et al. Our findings indicate that the equation relating those variables depends mainly on the ranges of total CO2 and pH values, and how [HCO3-] and PCO2 are inherently related by the Henderson-Hasselbalch equation. Therefore, the mathematical relation between those variables should not be used to draw inferences about the physiological processes that relate them.