RED-BLOOD-CELL FUNCTION IN HYPOXIA AT ALTITUDE AND EXERCISE

Oxygen transport by red blood cells is regulated by erythropoiesis and Hb-O-2-affinity. The O-2 carrying capacity is characterized by changes in hematocrit, red blood count or the mass of circulating red blood cells. Erythropoiesis is controlled by the hormone erythropoietin, which induces slow changes of the O-2-transport capacity. The Hb-O-2-affinity is modified mainly by pH and 2,3-DPG. Despite their apparently diverse effects e.g. in hypoxia at high altitude, a compromise seems to be adopted optimizing both arterial O-2-loading and peripheral O-2-unloading. In contrast to erythropoiesis, adjustments of the Hb-O-2-affinity occur fast and allow rapid adjustments of O-2-binding and release. In the intact organism the significance of changes in Hb-O-2-affinity for tissue oxygen supply relative to adjustments of cardiac output, microcirculation and O-2-transport capacity is not completely understood yet, but beneficial effects were demonstrated in isolated organs. It is, however, the least energy-demanding way of optimizing tissue O-2-supply, which might be of significance in extreme situations. In severe hypoxia adjustments of both, hematocrit and Hb-O-2-affinity, are insufficient to maintain tissue O-2-supply. Alterations of Hb-O-2-affinity are also insufficient to compensate for severe anemia.