Equilibrium of CO₂ reactions in the pulmonary capillary

Steady-state CO₂ excretion was measured in isolated blood-free rabbit lungs perfused with bicarbonate solutions. CO₂ in the expired ventilation was either present initially in the perfusate as dissolved CO₂ or produced from bicarbonate during pulmonary capillary transit. The two components were separated by measurement of simultaneous acetylene excretion. Bovine carbonic anhydrase and acetazolamide were sequentially added to the perfusate to determine the effects of maximal enzyme catalysis and inhibition of native lung carbonic anhydrase on CO₂ production. Control CO₂ production was significantly greater than that observed during inhibition of native lung carbonic anhydrase, confirming previous observations that bicarbonate has access to the tissue enzyme. Addition of excess carbonic anhydrase increased CO₂ production by a statistically, but not physiologically, significant amount. These data demonstrate that CO₂ reactions outside the erythrocyte attain 97% completion during pulmonary capillary transit. Under control and catalyzed conditions, alveolar and venous CO₂ tensions and pH were essentially identical to equilibrium values determined by in vitro tonometry.