Figure 3. CO2 is transported by red blood cells (RBCs) as bicarbonate ions, the formation of which is accelerated by the enzyme, carbonic anhydrase. In the tissues (high PPCO2), carbon dioxide diffuses into RBCs where it combines with water to form carbonic acid, H2CO3, which then dissociates into H+ and bicarbonate ion, HCO3-. In the lungs (low PPCO2), the process is reversed and carbon dioxide diffuses out of RBCs. The key to all of this are the reversible reactions
CO2 + H2O X H2CO3 X H+ + HCO3-
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