Explain How Carbon Dioxide Is Carried as Bicarbonate in Blood

Nearly 70% of the total carbon dioxide produced by body tissues is transported in the form of bicarbonate ions (HCO₃⁻) dissolved in the plasma. This process occurs through a reversible chemical reaction catalyzed by the enzyme carbonic anhydrase present in red blood cells.

Stepwise process of CO₂ transport as bicarbonate:

1. At tissue level: CO₂ diffuses from body cells into RBCs.
2. Inside RBCs: CO₂ + H₂O ⇌ H₂CO₃ (carbonic acid) — catalyzed by carbonic anhydrase.
3. H₂CO₃ quickly dissociates: H₂CO₃ ⇌ H⁺ + HCO₃⁻
4. Bicarbonate ions (HCO₃⁻) diffuse out into plasma, while chloride ions move into RBCs to maintain ionic balance — known as the chloride shift.

At the lungs:

• The process reverses — HCO₃⁻ reenters RBCs, recombines with H⁺ to form H₂CO₃, which splits into CO₂ and H₂O.
• The CO₂ diffuses into alveoli and is exhaled out.

This bicarbonate buffering system also helps in maintaining the acid-base balance of blood (pH ≈ 7.4).