ROLE OF HEMATOCRIT IN THE RECRUITMENT OF PULMONARY DIFFUSING-CAPACITY- COMPARISON OF HUMAN AND DOG

In dogs, maximal O-2 uptake (VO2max) per kilogram of body weight is tw o- to threefold that in humans; the difference cannot be explained sol ely by differences in structural features between species. We compared the functional recruitment of pulmonary diffusing capacity (DL(CO)) d uring exercise in dogs with that in humans to determine whether pulmon ary gas exchange is matched to VO2max or the size of the lungs and to define the potential role of exercise-induced polycythemia in producin g the superior aerobic capacity of the dogs. We compared the relations hips of DL(CO), membrane diffusing capacity (DM(CO)), and pulmonary ca pillary blood volume (Vc) with respect to pulmonary blood flow (Qc) by a rebreathing method during steady-state exercise in adult male human subjects and in conditioned adult male foxhounds. The slopes and inte rcepts of the relationships of DL(CO) and DM(CO) to Q(c) are significa ntly greater in dogs than in humans; the slopes of the relationship of Vc to Qc are similar. In dogs diffusive pulmonary gas transport is ma tched to the higher VO2max. 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 h umans.