Transport of carbon dioxide

Roughly 70 per cent of CO₂ travels as bicarbonate, about 20–25 per cent is bound to haemoglobin as carbamino-haemoglobin, and about 7 per cent is dissolved in plasma.

CO₂ binding to haemoglobin depends on pCO₂, with pO₂ also a major influence. In the tissues, high pCO₂ and low pO₂ favour more CO₂ binding; in the alveoli, low pCO₂ and high pO₂ make CO₂ dissociate from carbamino-haemoglobin.

The enzyme carbonic anhydrase is present at very high concentration in RBCs, and in minute amounts in plasma as well.

Carbonic anhydrase speeds up both directions of the reaction CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺.

In the tissues, catabolism raises pCO₂, so CO₂ diffuses into the blood (both RBCs and plasma), where HCO₃⁻ and H⁺ are formed.

At the alveoli, where pCO₂ is low, the reaction runs the other way, giving back CO₂ and H₂O. CO₂ trapped as bicarbonate at the tissues is thus released at the alveoli.

Every 100 mL of deoxygenated blood delivers roughly 4 mL of CO₂ to the alveoli.