Role of hematocrit in the recruitment of pulmonary diffusing capacity: Comparison of human and dog

In dogs, maximal O₂ uptake (V̇O(2max)) per kilogram of body weight is two- to threefold that in humans; the difference cannot be explained solely by differences in structural features between species. We compared the functional recruitment of pulmonary diffusing capacity (DL(CO)) during exercise in dogs with that in humans to determine whether pulmonary gas exchange is matched to V̇O(2max) or the size of the lungs and to define the potential role of exercise-induced polycythemia in producing the superior aerobic capacity of the dogs. We compared the relationships of DL(CO), membrane diffusing capacity (DM(CO)), and pulmonary capillary blood volume (Vc) with respect to pulmonary blood flow (Q̇c) by a rebreathing method during steady-state exercise in adult male human subjects and in conditioned adult male foxhounds. The slopes and intercepts of the relationships of DL(CO) and DM(CO) to Q̇c are significantly greater in dogs than in humans; the slopes of the relationship of Vc to Q̇c are similar. In dogs diffusive pulmonary gas transport is matched to the higher V̇O(2max). The enhanced recruitment of DL(CO) and DM(CO) in dogs during exercise could potentially be explained entirely by the exercise-induced polycythemia, which is seen in dogs but not in humans.