In the blood, approximately what fraction of total CO₂ is transported as bicarbonate ion (HCO₃⁻)?

Explanation

## CO₂ Transport in Blood **Key Point:** The majority of CO₂ produced by tissues is transported as bicarbonate ion (HCO₃⁻), which accounts for 70–80% of total CO₂ transport. ### Three Routes of CO₂ Transport | Transport Form | Percentage | Location | Mechanism | | --- | --- | --- | --- | | Bicarbonate (HCO₃⁻) | 70–80% | Plasma | CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (carbonic anhydrase) | | Dissolved CO₂ | 5–10% | Plasma | Physical dissolution (Henry's Law) | | Carbaminohemoglobin | 5–10% | RBC | CO₂ binds directly to globin chains (not heme) | **High-Yield:** The bicarbonate buffer system is the primary mechanism for CO₂ transport and is essential for maintaining blood pH. The enzyme **carbonic anhydrase** (present in RBCs) catalyzes the rapid conversion of CO₂ + H₂O to H₂CO₃, which then dissociates to HCO₃⁻ and H⁺. **Clinical Pearl:** In the lungs, this reaction reverses: HCO₃⁻ + H⁺ → H₂CO₃ → CO₂ + H₂O, allowing CO₂ to be exhaled. Impaired lung function leads to CO₂ retention and respiratory acidosis. **Mnemonic:** **"BIG 3"** — **B**icarbonate (70–80%), dissolved (5–10%), carb**A**minohemoglobin (5–10%) — remember bicarbonate dominates.