The oxygen equilibrium curves of human fetal and adult hemoglobins were reconstructed from the published Adair constants. The curves were then analyzed theoretically with respect to the amount of transferred oxygen, which is directly related to the saturation difference (▵S) of hemoglobin with oxygen in the artery and vein. In fetal blood, the oxygen affinity is optimized so as to provide the maximal ▵ S value in the fetus oxygen environment. In adult blood, on the other hand, the ▵ S value is far smaller than the theoretically obtained maximum, but it is most sensitive to the P50 changes around its physiological value of 27 torr. The present results imply that adult blood reserves an oxygen transport capacity for increased oxygen demands under resting conditions, and that the oxygen affinity is optimized so as to make the Bohr effect most effective to gaseous exchange.
