Carbon Dioxide Transport and Carbonic Anhydrase in Blood and Muscle

Abstract

CO2produced within skeletal muscle has to leave the body finally via ventilation by the lung. To get there, CO2 diffuses from the intracellular space into the convective transport medium blood with the two compartments, plasma and erythrocytes. Within the body, CO2 is transported in three different forms: physically dissolved, as HCO3 −, or as carbamate. The relative contribution of these three forms to overall transport is changing along this elimination pathway. Thus the kinetics of the interchange have to be considered. Carbonic anhydrase accelerates the hydration/dehydration reaction between CO2, HCO3 −, and H+. In skeletal muscle, various isozymes of carbonic anhydrase are localized within erythocytes but are also bound to the capillary wall, thus accessible to plasma; bound to the sarcolemma, thus producing catalytic activity within the interstitial space; and associated with the sarcoplasmic reticulum. In some fiber types, carbonic anhydrase is also present in the sarcoplasm. In exercising skeletal muscle, lactic acid contributes huge amounts of H+ and by these affects the relative contribution of the three forms of CO2. With a theoretical model, the complex interdependence of reactions and transport processes involved in CO2 exchange was analyzed.