Fig. 6. Plots of the partial pressure of oxygen (mmHg) at sea level (A) and the summit of Mount Everest (B) versus the transit time of blood across the pulmonary capillary. These schematic representations demonstrate that, as one ascends, the driving pressure for oxygen from the air to the blood is lower, such that there is not time for equilibration for oxygen across the pulmonary capillary. This phenomen results in arterial oxygen desaturation, which is accentuated with higher levels of exercise (from West and Wagner, 1980, with permission). 1mmHg=0.133kPa.