Lung Surface: Pulmonary Surfactant and the Mechanics of Gas Exchange

The alveolar surface is moist, lined by a thin film of pulmonary surfactant—a lipoprotein complex (90% lipids, 10% proteins) secreted by type II pneumocytes. This dynamic interface is essential for life.

Surfactant Functions

• Reduces surface tension: Prevents alveolar collapse at end-expiration (Laplace's law: P = 2T/r).
• Facilitates gas exchange: Thin fluid layer allows rapid O₂/CO₂ diffusion.
• Immune defense: SP-A, SP-D opsonize pathogens; modulate alveolar macrophages.
• Mechanical stability: Prevents edema, maintains lung compliance.

Clinical Relevance

• Neonatal RDS: Surfactant deficiency in preterm infants → respiratory failure. Treated with exogenous surfactant.
• ARDS: Surfactant inactivation/depletion contributes to stiff lungs.
• Pneumonia: Inflammatory proteins inhibit surfactant function.

The alveolar surface is neither smooth nor slimy—it is a dynamic, biologically active interface where physics and biology meet to sustain every breath.