| Form | Fraction | Mechanism |
|---|
| Bicarbonate (HCO₃⁻) | ~70% | CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (catalyzed by carbonic anhydrase in RBC); Cl⁻ shift (Hamburger phenomenon) |
| Carbamino compounds | ~23% | CO₂ binds to N-termini of hemoglobin (and plasma proteins) |
| Dissolved CO₂ | ~7% | Physically dissolved in plasma (Henry's law) |
Haldane Effect
Deoxygenated Hb binds more CO₂ (as carbamino) and H⁺ (buffering) → facilitates CO₂ loading in tissues. Oxygenated Hb releases CO₂/H⁺ in lungs.
Pulmonary Exchange
- Venous blood → right heart (RA → RV) → pulmonary artery → pulmonary capillaries.
- High PAO₂, low PACO₂ → CO₂ diffuses out (Haldane + gradient).
- Carbonic anhydrase reverses reaction → CO₂ exhaled.
- Oxygenated blood → pulmonary veins → left heart → systemic arteries.
Heart's Role
- Right side: low-pressure pump for pulmonary circuit.
- Left side: high-pressure pump for systemic circuit.
- Septal separation prevents mixing.
Clinical Relevance
- ABG: PaCO₂ reflects alveolar ventilation (↑ PaCO₂ = hypoventilation).
- CO₂ retention (COPD, obesity hypoventilation, neuromuscular) → respiratory acidosis.
- Capnography (EtCO₂) monitors ventilation non-invasively.
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