Fig. 8. Relationship between alveolar–arterial P_O2 difference and the rate of oxygen uptake in Operation Everest II, a high-altitude chamber study designed to simulate an ascent by humans over 40 days to the summit of Mount Everest (8828m). These data were obtained from a multiple inert gas elimination technique that measures ventilation/perfusion relationships and show that the predicted alveolar–arterial P_O2 difference (mmHg) could be accounted for only in part by ventilation/perfusion inequality. The remaining difference was attributed to a diffusion limitation of oxygen from the air to the blood (from Wagner et al., 1987, with permission). 1mmHg=0.133kPa. Values are means + S.E.M.