Pulmonary Surfactant: Composition, Function, and Clinical Significance

Pulmonary surfactant is a lipoprotein complex secreted by type II pneumocytes that lines the alveolar air-liquid interface.

Composition

• Phospholipids (80–85%): Dipalmitoylphosphatidylcholine (DPPC) – main surface-active component.
• Neutral lipids (5–10%): Cholesterol – modulates fluidity.
• Proteins (8–10%): SP-A, SP-D (innate immunity); SP-B, SP-C (hydrophobic, critical for adsorption and film stability).

Function

• Reduces surface tension: DPPC monolayer compresses on expiration → near-zero tension → prevents alveolar collapse (atelectasis) per Laplace's law (P = 2T/r).
• Increases compliance: Less work of breathing.
• Immune defense: SP-A/D opsonize pathogens, modulate macrophages.

Clinical Relevance

• Neonatal RDS: Surfactant deficiency in preterm infants (<34–36 wks). Exogenous surfactant (poractant alfa, beractant, calfactant) given intratracheally → ↓ mortality, pneumothorax.
• ARDS: Surfactant inactivation/degradation. Trials of exogenous surfactant mixed results.
• Genetic disorders: SP-B, SP-C, ABCA3 mutations → fatal surfactant deficiency.

Regulation

Stretch, glucocorticoids, thyroid hormone ↑ production. Hypoxia, inflammation ↓ production.