Transport of Carbon-dioxide

Blood carries CO2 from the tissues to the lungs in three ways. A small fraction (about 7-10%) simply dissolves in the plasma. A further 20-25% binds directly to haemoglobin (at a site different from where O2 binds) to form carbaminohaemoglobin (HbCO2): CO2 + Hb is in equilibrium with HbCO2. The majority, about 70%, is carried as bicarbonate ions in the plasma.

The bicarbonate route depends on the enzyme carbonic anhydrase, which is present in large amounts inside red blood cells (and in much smaller amounts in plasma). At the tissues, where pCO2 is high, CO2 diffuses into the red blood cells and combines with water to form carbonic acid (CO2 + H2O -> H2CO3), catalysed by carbonic anhydrase. Carbonic acid is unstable and immediately dissociates into a hydrogen ion (H+) and a bicarbonate ion (HCO3-). The HCO3- then moves out of the red blood cell into the plasma, in exchange for an incoming chloride ion (the chloride shift), and is carried in the plasma to the lungs. At the alveoli, where pCO2 is low, the whole reaction runs in reverse: HCO3- re-enters the red blood cell, recombines with H+ to reform carbonic acid, which carbonic anhydrase then splits back into CO2 and water, allowing the CO2 to diffuse out into the alveolar air and be exhaled. Every 100 mL of deoxygenated blood delivers about 4 mL of CO2 to the alveoli by this combined route. …