ABSTRACT
Alterations of the oxygen affinity of hemoglobin can alter the oxygen dissociation curve at high altitude and therefore affect oxygen transport by the blood. Many animals that live in oxygen deprived environments have high oxygen affinities of their hemoglobin. This is the case in the human fetus. It is interesting that climbers at extreme altitude increase their oxygen affinity by extreme hyperventilation which causes a marked respiratory alkalosis. The effect of the alkalosis overwhelms the small decrease in oxygen affinity caused by the increased concentration of 2,3-diphosphoglycerate in the red blood cells. The P50 of high altitude natives is essentially the same as the sea level value according to most studies. However, lowlanders living at high altitude for weeks tend to have a reduced P50, indicating an increased oxygen affinity of hemoglobin. An increased oxygen affinity is advantageous at high altitude because it assists in the loading of oxygen by the pulmonary capillaries. The acid-base status of high altitude natives is a little controversial but many studies have found a normal arterial pH, indicating a fully compensated respiratory alkalosis. However, acclimatized lowlanders usually have a slightly alkaline pH, indicating that metabolic compensation is not complete. There is evidence that at extreme altitude, metabolic compensation for the respiratory alkalosis is slow, possibly because of chronic volume depletion caused
Physiological changes in the blood play an important role in acclimatization and adaptation to high altitude. In this chapter, the main topics considered are the changes in oxygen affinity of hemoglobin, and the alterations of the acid-base status of the blood. The increase in red cell concentration of the blood was discussed in Chapter 8, where the regulation of erythropoiesis was described. Some of the consequences of an altered oxygen affinity of hemoglobin are alluded to in other chapters, especially Chapter 6 on diffusion of oxygen across the bloodgas barrier, and Chapter 12 on limiting factors at extreme altitude.
