At tissues, carbonic anhydrase facilitates the formation of carbonic acid from

RBCs contain a very high concentration of the enzyme, carbonic anhydrase and minute quantities of the same is present in the plasma too. This enzyme facilitates the following reaction in both directions.

CO₂ + H₂O $\stackrel{\mathrm{carbonic} \mathrm{anhydrase}}{\rightleftarrows }$ H₂CO₃ $\stackrel{\mathrm{carbonic} \mathrm{anhydrase}}{\rightleftarrows }$CO₂ + H₂O

At the tissue site where partial pressure of CO₂ is high due to catabolism, CO₂ diffuses into blood (RBCs and plasma) and forms HCO₃^– and H⁺. At the alveolar site where pCO₂ is low, the reaction proceeds in the opposite direction leading to the formation of CO₂ and H₂O. Thus, CO₂ trapped as bicarbonate at the tissue level and transported to the alveoli is released out as CO₂.