Question

Explain the transport of carbon dioxide.

Answer 1

Blood transports CO₂ from the tissue cells to the lungs in three ways:

1. Dissolved in plasma : About 7 – 10% of CO₂ is transported in a dissolved form in the plasma.

2. Bound to haemoglobin : About 20 – 25% of dissolved CO is bound and carried in the RBCs as carbaminohaemoglobin (Hb CO₂ ) CO₂ + Hb → Hb CO₂ .

3. As bicarbonate ions in plasma about 70% of CO₂ is transported as bicarbonate ions. This is influenced by pC0₂ and the degree of haemoglobin oxygenation. RBCs contain a high concentration of the enzyme, carbonic anhydrase, whereas small amounts of carbonic anhydrase is present in the plasma.

→ At the tissues the pCO₂ is high due to catabolism and diffuses into the blood to form HCO₂ and H ions. When CO₂ diffuses into the RBCs, it combines with water forming carbonic acid (H₂ CO₂ ) catalyzed by carbonic anhydrase. Carbonic acid is unstable and dissociates into hydrogen and bicarbonate ions.

Carbonic anhydrase facilitates the reaction in both directions.

The HCO₃^- moves quickly from the RBCs into the plasma, where it is carried to the lungs. At the alveolar site where pCO₂ is low, the reaction is reversed leading to the formation of CO₂ and water. Thus CO₂ trapped as HCO₃^- at the tissue level is transported to the alveoli and released out as CO₂ . Every 100 mL of deoxygenated blood delivers 4 mL of CO₂ to the alveoli for elimination.