Pulmonary Surfactant: How It Keeps Alveoli Stable and Gas Exchange Efficient

Pulmonary surfactant is a lipoprotein complex—90% lipids (primarily dipalmitoylphosphatidylcholine) and 10% proteins (SP-A, B, C, D)—secreted by type II pneumocytes. It lines the alveolar air-liquid interface and is essential for viable respiration.

Key Functions

1. Reduces Surface Tension: By adsorbing at the air-liquid interface, surfactant lowers surface tension disproportionately at low lung volumes (LaPlace's law: P = 2T/r). This prevents alveolar collapse (atelectasis) at end-expiration.
2. Stabilizes Alveoli: Equalizes pressure across alveoli of different sizes, preventing small alveoli from emptying into larger ones.
3. Increases Compliance: Reduces the work of breathing by decreasing the elastic recoil pressure needed for inflation.
4. Enhances Gas Exchange: Maintains patent alveoli for optimal ventilation-perfusion matching and diffusion.
5. Immune Defense: SP-A and SP-D are collectins that opsonize pathogens and modulate inflammation.

Clinical Relevance

• Neonatal RDS: Surfactant deficiency in preterm infants causes atelectasis and hypoxia. Treated with exogenous surfactant plus CPAP or ventilation.
• ARDS: Surfactant inactivation and degradation contribute to alveolar collapse.
• Surfactant Therapy: Intratracheal instillation improves outcomes in RDS; investigational in ARDS.

Surfactant is essential for respiration at birth and throughout life.