Lung carbonate dehydratase (carbonic anhydrase), CO2 stores and CO2 transport

A study of CO2 storage in excised, exsanguinated lungs revealed that CO2 stores include a compartment which reaches equilibration very rapidly (less than 3s) and a slower compartment which equilibrates with a half-time of approximately 15 s. Inhibition of carbonate dehydratase (carbonic anhydrase, EC 4.2.1.1) does not change the slope of the total CO2 dissociation curve of the lung but does increase the slow compartment at the expense of the fast. CO2 diffusion across the pleura is approximately 20 times faster than that of O2, a relationship that is not affected by inhibition of carbonate dehydratase. The role of tissue CO2 stores in limiting respiratory fluctuations of PCO2 or pH in arterial blood is only minor and may be of significance only in rapid, deep inspiration. CO2 uptake or release by the stores is out of phase with blood CO2 exchange. As a consequence, the time course of CO2 exchange at the mouth during expiration cannot be used to predict alveolar or capillary CO2 exchange.