[The critical pressure of oxygen]

A review of the metabolic response (M) to hypoxia measured in different mammals shows that the critical ambient O2 pressure (Pc), is a linear function of the normoxic metabolic demand. Values for Pc at different metabolic loads may range from about 10 torr to close to 150 torr. However, the ratio M/Pc provides a useful index that seems to be independent of the metabolic demand. Critical ambient pO2 for reducing O2 uptake at comparable loads has been found to be lower in highland than in lowland species. Below the Pc the metabolism changes sharply or more gradually with further reduction in pO2. This reduction may be better expressed by a single hypoxic coefficient that is independent of the metabolic demand, representing the fractional reduction of M per torr. Regression analysis of Pc on body weight shows independence of the critical pO2 on body size within the range 8-480 g.