Lung Surfactant: DPPC and Its Critical Role in Respiratory Function

What Is Pulmonary Surfactant?

A lipoprotein complex (90% lipids, 10% proteins) secreted by type II alveolar epithelial cells that lines the alveoli.

DPPC: The Primary Surface-Active Component

• Dipalmitoylphosphatidylcholine (DPPC) = ~50% of surfactant lipids.
• Saturated phospholipid → tightly packs at air-liquid interface → maximally reduces surface tension (to near 0 mN/m at end-expiration).
• Prevents alveolar collapse (atelectasis) by Laplace's law: P = 2γ/r. Low γ → low pressure needed to keep alveoli open.

Surfactant Proteins (SP-A, SP-B, SP-C, SP-D)

• SP-B, SP-C: Hydrophobic; essential for DPPC adsorption, film respreading.
• SP-A, SP-D: Collectins; innate immune defense (opsonization, pathogen clearance).

Clinical Significance

• Neonatal RDS: Surfactant deficiency in preterm infants → exogenous surfactant replacement therapy.
• ARDS: Surfactant inactivation/depletion → adjunctive surfactant trials.
• Genetic mutations (SP-B, SP-C, ABCA3): Rare interstitial lung diseases.