Carbon Dioxide Transport: From Tissues to Exhalation

CO₂ produced by cellular metabolism travels via blood to lungs for elimination. The pathway involves specific cardiovascular and respiratory structures.

Stepwise Pathway

1. Tissue Capillaries: CO₂ diffuses from cells → interstitial fluid → blood (plasma, RBCs)
2. Venous Return: Superior/inferior vena cava → Right Atrium
3. Right Ventricle: Tricuspid valve → RV contraction → Pulmonary Valve
4. Pulmonary Artery → Arterioles → Capillaries: Surrounding alveoli
5. Diffusion: CO₂ moves from blood (high PₐCO₂ ~45 mmHg) → alveoli (low PₐCO₂ ~40 mmHg) down gradient
6. Alveolar Ventilation: CO₂ exhaled via bronchi → trachea → larynx → mouth/nose

CO₂ Transport Forms in Blood

• Bicarbonate (HCO₃⁻): ~70% — carbonic anhydrase in RBCs: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (Cl⁻ shift / Hamburger phenomenon)
• Carbaminohemoglobin: ~23% — CO₂ bound to Hb amino groups
• Dissolved in plasma: ~7% — directly proportional to PₐCO₂

Regulation

• Central chemoreceptors (medulla) sense CSF pH (driven by PₐCO₂)
• Peripheral chemoreceptors (carotid/aortic bodies) sense PₐO₂, pH, PₐCO₂
• Ventilation adjusted to maintain PₐCO₂ ~40 mmHg

Clinical Relevance

• Hypercapnia: COPD, obesity hypoventilation, neuromuscular disease, sedation
• Hypocapnia: Hyperventilation (anxiety, sepsis, salicylate, stroke)
• ABG interpretation: pH, PₐCO₂, HCO₃⁻ distinguish respiratory vs. metabolic