Advantage or disadvantage of a decrease of blood oxygen affinity for tissue oxygen supply at hypoxia

Summary

A shift of the oxygen dissociation curve to the right is often interpreted as an adaptation to hypoxia favorable for tissue oxygen supply. However, animals native to high altitude tend to show a rather high oxygen affinity. In order to elucidate this apparent discrepancy we investigated by numerical computer studies 1. the effect of a shift of the dissociation curve to the right as reflected in the mixed venous oxygen pressure, and 2. the role of this displacement in pulmonary gas exchange with particular reference to the alveolar-arterial oxygen pressure difference and the pulmonary diffusing capacity for oxygen. A right shift had a favorable effect only in the range of moderate hypoxia (and of normoxia) whereas there was a detrimental effect with severe hypoxia. The most important criterion for this distinction was the direction of the change in steepness of the physiological dissociation curve (straight line between arterial and venous points). A favorable effect was associated with a steeper slope after the shift, an unfavorable effect with a less steep slope. There was only a minor influence of a right shift on the oxygen diffusion gradient in the lung. Comparison between man (higher affinity) and rat (lower affinity) suggests that animals of small size with high metabolic rate (high arteriovenous oxygen difference) living in normoxic or possibly exposed to moderately hypoxic condition only are better served by a relatively low oxygen affinity whereas animals native to high altitude are better adapted to severe hypoxia when having a high oxygen affinity.