The prediction of human blood oxygen equilibrium curve under various conditions of pH, pCO2, and 2,3-DPG

We have analysed 56 continuous oxygen equilibrium curves of fresh human blood, each from 0 to 150 Torr PO2. The data were collected over a range of values of the [2,3-DPG]/[Hb] ratio of 0.2-2.7, of pH 7.0-7.8, and PCO2 7-70 Torr. The generalized Adair equation for the stepwise oxygenation of hemoglobin was fit to the data for each curve, so that the curve could be reproduced using the equation and the four constants a1, a2, a3 and a4. The behaviour of the constants in response to variations of pH, PCO2 and 2,3-DPG was characterized by empirical relationships, and a set of equations was developed which could estimate the Adair constants for the physiological range of these variables. Previous attempts to describe the effects of physiological conditions on the oxygen transport properties of red cells have dealt mainly with P50, the PO2 at one-half saturation, or have provided too little data for continuous simulations. Moreover, the mathematical descriptions of variations in the oxygen equilibrium curve have usually used the Hill equation which assumes that the shape of the curve is invariant. Recent evidence has shown this assumption to be incorrect, and the use of the Adair equation eliminates the need for such simplifying assumptions. This analysis provides a powerful tool to predict the shape of the oxygen equilibrium curve in various physiological and pathological states, as well as to study the affinity of hemoglobin for oxygen within the red cell. The equations we have developed can easily be incorporated into large programs for computations in acid-base and oxygen exchange properties of blood and for modelling gas exchange in man.