Rds Surfactant Physiology

Pulmonary surfactant is a phospholipid-protein mixture made by type II alveolar cells. Its dominant phospholipid, dipalmitoylphosphatidylcholine, and proteins such as SP-B and SP-C spread across the fluid lining of the alveoli and lower surface tension. Without surfactant, the wet inner surfaces of alveoli pull inward at end-expiration. Small alveoli are especially vulnerable to collapse. Surfactant acts like a stabilizing film: it reduces the pressure needed to keep alveoli open and prevents repeated opening and closing with every breath. This effect preserves functional residual capacity—the air remaining in the lungs after a normal expiration. Functional residual capacity matters because it provides a continuing oxygen reservoir between breaths. When it is lost, the infant must repeatedly recruit collapsed alveoli, breathing becomes inefficient, and compliance falls. Surfactant production begins at approximately 24 weeks’ gestation, but quantities sufficient for independent air breathing are usually not present until about 34 to 36 weeks. This explains the central pattern of neonatal respiratory distress syndrome (RDS): the earlier the gestational age, the greater the risk and usual severity. The sequence is predictable: 1. Insufficient surfactant allows progressive...