Pulmonary Gas Exchange: Oxygen In, Carbon Dioxide Out

Ventilation & Perfusion

• Inspiration: Diaphragm contracts → thoracic volume ↑ → pressure ↓ → air enters.
• Alveolar Gas: PₐO₂ ~100 mmHg, PₐCO₂ ~40 mmHg.
• Mixed Venous Blood: PᵥO₂ ~40 mmHg, PᵥCO₂ ~45 mmHg.

Diffusion

• O₂: Alveolus → capillary (down 60 mmHg gradient) → binds Hb.
• CO₂: Capillary → alveolus (down 5 mmHg gradient) → exhaled. Higher solubility = faster diffusion.

Transport

• O₂: 98% on Hb (cooperative binding), 2% dissolved.
• CO₂: 70% as HCO₃⁻ (carbonic anhydrase), 23% on Hb (carbamino), 7% dissolved.

Expiration

Passive elastic recoil (quiet breathing); active (forced) uses internal intercostals, abdominals.

Regulation

Central chemoreceptors (CSF H⁺) → primary drive; peripheral (PaO₂ <60, PaCO₂ ↑, pH ↓) → secondary. V/Q matching optimizes exchange.