Surfactant: How It Keeps Your Lungs Inflated and Healthy

Pulmonary Surfactant

A complex lipoprotein mixture secreted by type II alveolar epithelial cells (pneumocytes). Composed of ~90% lipids (mostly dipalmitoylphosphatidylcholine — DPPC) and 10% proteins (SP-A, SP-B, SP-C, SP-D).

Mechanism of Action

• Reduces surface tension at the air-liquid interface of alveoli.
• By Laplace's law (P = 2T/r), lower surface tension (T) prevents small alveoli from collapsing into larger ones (atelectasis) during expiration.
• DPPC forms a monolayer that compresses during expiration, drastically lowering surface tension to near zero.
• SP-B and SP-C are essential for rapid adsorption and respreading of the monolayer.

Physiological Importance

• Maintains alveolar stability and compliance (ease of inflation).
• Reduces work of breathing.
• Prevents pulmonary edema by lowering transudative forces.
• SP-A and SP-D contribute to innate immunity (opsonization, pathogen recognition).

Clinical Disorders

• Neonatal Respiratory Distress Syndrome (RDS): Surfactant deficiency in preterm infants (<37 wks). Treated with exogenous surfactant (beractant, poractant alfa, calfactant) + CPAP/ventilation.
• Acute Respiratory Distress Syndrome (ARDS): Surfactant inactivation/degradation by inflammatory proteins, edema, meconium. Exogenous surfactant trials show mixed results.
• Genetic surfactant disorders (SP-B, SP-C, ABCA3 mutations): Rare, severe interstitial lung disease in infants/children.

Summary

Surfactant is the lung's "anti-collapse" system. Without it, breathing would require prohibitive pressure; with it, gas exchange is efficient and effortless.