Shape of the haemoglobin-oxygen equilibrium curve and oxygen transport efficiency

The sigmoid shape of the oxygen equilibrium curve (OEC) of haemoglobin permits large changes in oxygen saturation (S) upon narrow variations of the partial pressure of oxygen (P). The slope of the Hill plot (n) calculated from the Adair constants for human adult haemoglobin is usually maximised at S values greater than 0.5, implying that co-operative interactions of haemoglobin subunits are maximised above half-saturation. Our analysis of OECs based on the Adair equation shows that the slope of the OEC, S', which is proportional to the capacitance coefficient beta (Piiper et al., Respir. Physiol. 13: 292-304, 1971) and is a direct measure for oxygen-transport efficiency of haemoglobin, is maximised at S values smaller than 0.5, usually at S = 0.38. The analysis also gives the relations: Pdmax < P50 < Pnmax and Sdmax < 0.5 < Snmax where Pdmax, P50 and Pnmax are P at which S' is maximised, P at half-saturation and P at which n is maximised, respectively, and Sdmax and Snmax are S at Pdmax and S at Pnmax, respectively. Thus the most efficient point of OEC does not coincide with the point of maximal cooperativity nor with the oxygen saturation level of mixed venous blood in resting conditions (S approximately = 0.75). The steep portion of the OEC around S = 0.38 is exploited for large oxygen demands under conditions of exercise. The most efficient oxygen unloading region around S = 0.38 could be revealed by the Adair analysis but not by analysis using the conventional Hill equation.