CO2 transport: bicarbonate

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.