Adaptive variation in the mammalian respiratory system in relation to energetic demand: VII. Flow of oxygen across the pulmonary gas exchanger

This paper addresses the question of whether the lung's gas exchange capacity limits O₂ uptake in exercising animals. Physiological data measured whiel animals exercised at V̇_O2max are used together with morphometric data on the pulmonary diffusing capacity (DL_O2) to calculate the change in O₂ concentration and P_O2 of the blood as it transits the lung. The 2.5-fold difference in V̇_O2max between the athletic dog and pony and the less athletic goat and calf was used to test for a match between structure and function at the level of O₂ transfer in the lung. Since the 2.5-fold difference in V̇_O2max is not matched by a 2.5-fold difference in diffusing capacity of the lung for O₂, we find that the less athletic species utilize only 1/3 of their capacity at V̇_O2max while the athletic species utilize 3/4. The excess diffusing capacity can be thought of as a 'safety factor' allowing animals to tolerate a decrease in PA_O2 or DL_O2 that may occur normally at altitude or during disease, but it is not clear why this factor should be smaller in the more athletic species.