[Affinity of hemoglobin for oxygen and tolerance to hypoxia]

The transfer of O2 in the system between the lung captation and the delivery to tissues is insured by a whole of factors that are components of Fick's equation. The two most important in this O2 transfer are heart output and blood concentration in hemoglobin (transfer capacity of O2). The affinity of blood for O2, evaluated by the determination of P50 (PO2 of half saturation of Hb) interfers also, but we have till presently no precision for what concerns its relative importance. Two demonstrative situations of chronic hypoxia are taken in consideration: adaptation to high altitude hypoxia and the reaction of the system to hypoxia in chronic respiratory deficiency. The influence of displacements of the dissociation curve of HbO2 (Bohr effect, variation in the erythrocytary DPG) is object of discussion. Authors insist particularly on the fact that the fall in the affinity of Hb for O2 (increase in P50) is not always a beneficial mechanism for the delivery of O2 to tissues, particularly in the case of severe hypoxia.