Oxygen transport in blood at high altitude: role of the hemoglobin-oxygen affinity and impact of the phenomena related to hemoglobin allosterism and red cell function

Altitude hypoxia is a major challenge to the blood O-2 transport system, and adjustments of the blood-O-2 affinity might contribute significantly to hypoxia adaptation. In principle, lowering the blood-O-2 affinity is advantageous because it lowers the circulatory load required to assure adequate tissue oxygenation up to a threshold corresponding to about 5,000 m altitude, whereas at higher altitudes an increased blood-O-2 affinity appears more advantageous. However, the rather contradictory experimental evidence raises the question whether other factors superimpose on the apparent changes of the blood-O-2 affinity. The most important of these are as follows: (1) absolute temperature and temperature gradients within the body; (2) the intracapillary Bohr effect; (3) the red cell population heterogeneity in terms of O-2 affinity; (4) control of altitude alkalosis; (5) the possible role of hemoglobin as a carrier of the vasodilator nitric oxide; (6) the effect of varied red cell transit times through the capillaries.