Pulmonary Surfactant: The Molecule That Keeps Alveoli Open

Pulmonary surfactant is a lipoprotein complex secreted by Type II pneumocytes (alveolar cells) that lines the alveoli and reduces surface tension at the air-liquid interface.

Composition

• ~90% Lipids: 80% phospholipids (mostly dipalmitoylphosphatidylcholine / DPPC — the surface-active component), 10% neutral lipids/cholesterol.
• ~10% Proteins: SP-A, SP-D (innate immunity, host defense), SP-B, SP-C (critical for spreading, adsorption, film stability).

Mechanism: Laplace's Law

\[ P = \frac{2T}{r} \] Pressure (P) to keep alveolus open ∝ Surface Tension (T) / Radius (r).

• Without surfactant: High T → small alveoli collapse into larger ones (atelectasis) → ↑ work of breathing, V/Q mismatch, hypoxemia.
• With surfactant: DPPC + SP-B/C → dramatically lowers T, especially at low lung volumes (small r) → stabilizes all alveoli, prevents collapse at end-expiration.

Clinical Relevance

• Neonatal RDS (Hyaline Membrane Disease): Preterm <34 wks → surfactant deficiency. Exogenous surfactant (beractant, poractant alfa, calfactant) via endotracheal tube is lifesaving.
• ARDS: Surfactant inactivation/degradation by inflammatory proteins, edema. Exogenous trials mixed; not standard.
• Surfactant Protein Mutations (SP-B, SP-C, ABCA3): Rare genetic causes of interstitial lung disease in infants/children.

Surfactant is the linchpin of alveolar stability. Without it, breathing is unsustainable.