Carbon dioxide is transported in blood in three forms: dissolved in plasma, as bicarbonate, and coupled to proteins in the form of carbamino compounds.
Bicarbonate represents the largest fraction of the CO2 in blood (~88%).
On the venous side of systemic capillaries CO2 enters red blood cells (RBC) where it combines with water to form carbonic acid (H2CO3). This reaction is catalyzed by the enzyme carbonic anhydrase (CA) which is found in RBC’s. Carbonic acid then dissociates to form bicarbonate ions (HCO3-) and hydrogen ions (H+). This reaction also occurs outside the RBC’s, in the plasma, but it is much slower due to lack of CA.
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
In the pulmonary capillaries, the reverse occurs: bicarbonate ions enter the RBC and combine with H+ to form carbonic acid, which is broken down into CO2 and H2O, with CO2 diffusing out into alveoli.