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 tens ions and pH were essentially identical to equilibrium values determined by in vitro tonometry.