Alveoli: Structure, Function, and Role in Gas Exchange

Alveoli are microscopic, thin-walled air sacs at the terminal ends of the bronchial tree. They are the primary site of gas exchange in the lungs, providing an enormous surface area (~70–100 m² in adults) for oxygen and carbon dioxide diffusion between air and blood.

Structure

• Number: ~300–500 million per adult lung.
• Size: 200–300 μm diameter.
• Wall: Single layer of epithelium (type I and II pneumocytes) + capillary endothelium + fused basement membranes = blood-air barrier (0.5–2 μm thick).
• Type I pneumocytes: Cover 95% of surface; optimized for gas diffusion.
• Type II pneumocytes: Secrete pulmonary surfactant (dipalmitoylphosphatidylcholine) reducing surface tension, preventing collapse (atelectasis).
• Alveolar macrophages: Phagocytose debris, pathogens.

Gas Exchange Physiology

Oxygen diffuses from alveolar air (PO₂ ~100 mmHg) into capillary blood (PO₂ ~40 mmHg). CO₂ moves in reverse (PCO₂ ~45 mmHg → ~40 mmHg). Diffusion is perfusion-limited under normal conditions—blood flow, not membrane thickness, is rate-limiting.

Developmental Note

Alveolarization begins ~36 weeks gestation and continues until ~8 years of age. Premature infants often lack sufficient alveoli and surfactant, causing respiratory distress syndrome (RDS).

Clinical Relevance

• Emphysema: Alveolar wall destruction → reduced surface area, airflow limitation.
• ARDS: Barrier disruption → pulmonary edema, hypoxemia.
• Pneumonia: Alveolar filling with inflammatory exudate.
• Pulmonary fibrosis: Thickened barrier → diffusion impairment.