Pulmonary Gas Exchange: Mechanism of Oxygen and Carbon Dioxide Transfer

Blood is not "absorbed" in the lungs. Instead, gas exchange occurs across the alveolar-capillary membrane.

Process

1. Ventilation: Air reaches alveoli (tiny air sacs) during inspiration.
2. Diffusion: Down partial pressure gradients:
   • O₂ moves from alveolar air (PO₂ ~100 mmHg) → capillary blood (PO₂ ~40 mmHg).
   • CO₂ moves from capillary blood (PCO₂ ~45 mmHg) → alveolar air (PCO₂ ~40 mmHg).
3. Transport: O₂ binds to hemoglobin in red blood cells (98.5%) and dissolves in plasma (1.5%). CO₂ is transported as bicarbonate (70%), carbaminohemoglobin (23%), and dissolved (7%).
4. Circulation: Oxygenated blood travels via pulmonary veins → left heart → systemic arteries → tissues. Deoxygenated blood returns via systemic veins → right heart → pulmonary arteries.

Key Structural Features Optimizing Exchange

• Massive surface area (~70 m²) from ~300 million alveoli.
• Minimal diffusion distance (<0.5 μm) due to thin type I pneumocytes and capillary endothelium sharing a basement membrane.
• Ventilation-perfusion matching.

This efficient system ensures continuous oxygen delivery for cellular respiration and CO₂ removal.