Role of perfusion and diffusion in 14CO2 exchange in the rabbit lung

The rate of transfer of H14CO-3 and 14CO2 from the alveoli to the capillaries was studied in rabbit lungs perfused without erythrocytes. Aliquots of 0.5 ml of buffered solutions containing these 14C indicators and 3H2) were injected into the distal airways, and the recoveries of 14C and 3H were compared in the left atrial outflow. It was assumed that 3H2O had equilibrated between the alveoli and fluid leaving the pulmonary capillaries, and a decline in the initial 14C recovery relative to that of 3H was attributed to incomplete equilibration of 14C between these compartments. No disequilibrium of 14C could be detected at pH 7.4 when excess carbonic anhydrase was present. When the pH was increased to 8.4, 14C equilibration was only 69% complete at 36 ml/min and 41% complete at 160 ml/min. Confirmatory evidence was obtained that carbonic anhydrase is associated with the endothelial side of the alveolar-capillary barrier but is absent on the epithelial surface. The data suggest that the barrier is at least 600 times more permeable to 14CO2 than to H14CO-3, and diffusion of 14CO2 would not limit exchange at normal pH unless pulmonary flow reached extremely high values.