Form of CO₂ Transport (70%) in Blood

70% of CO₂ transport in the blood takes place in which form?

Correct Answer: Bicarbonate ion

The majority (~70%) of CO₂ in blood is transported as bicarbonate ions (HCO₃⁻), formed inside RBCs via the enzyme carbonic anhydrase.

CO₂ Transport Mechanism

• CO₂ enters RBC → reacts with water: CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻ (catalyzed by carbonic anhydrase — 5,000× faster than spontaneous reaction).
• HCO₃⁻ exits RBC into plasma via chloride shift (Hamburger phenomenon) — Cl⁻ moves from plasma into RBC in exchange for HCO₃⁻.
• H⁺ ions are buffered by hemoglobin (which acts as a base).

Summary of CO₂ Transport

• Bicarbonate (HCO₃⁻): 70% (most important)
• Carbaminohemoglobin (Hb-CO₂): 23% — CO₂ binds to amino groups of globin chains
• Dissolved in plasma: 7%

Note

• Carboxyhemoglobin is formed by CO (carbon monoxide) binding to Hb — it is a toxic compound, not a normal CO₂ transport form.