Carbonic acid dissociates into carbon dioxide and water in the

CO₂ + H₂O $\stackrel{\mathrm{Carbonic} \mathrm{anhydrase}}{\to }$ H₂CO₃ $\stackrel{\mathrm{Carbonic} \mathrm{anhydrase}}{\to }$ HCO₃^- + H⁺

HCO₃^-  + H⁺ $\stackrel{\mathrm{Carbonic} \mathrm{anhydrase}}{\to }$H₂CO₃ $\stackrel{\mathrm{Carbonic} \mathrm{anhydrase}}{\to }$ 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 H₂CO₃ which dissociates into HCO₃^– and H⁺. At the alveolar site where pCO₂ is low, the reaction proceeds in the opposite direction leading to the dissociation of H₂CO₃ into CO₂ and H₂O.