## Surfactant Composition and Function **Key Point:** Dipalmitoylphosphatidylcholine (DPPC) is the primary surface-active phospholipid in pulmonary surfactant, accounting for ~50% of surfactant lipid mass and responsible for the majority of surface tension reduction. ### Structure and Mechanism DPPC is a saturated phospholipid with two 16-carbon palmitic acid chains. Its saturation allows tight molecular packing at the air-liquid interface, enabling maximal reduction in surface tension. This property is critical for: - Preventing alveolar collapse during expiration - Reducing the work of breathing - Maintaining functional residual capacity (FRC) ### Surfactant Composition Overview | Component | Percentage | Function | |-----------|-----------|----------| | Lipids (mainly DPPC) | ~90% | Surface tension reduction | | Proteins (SP-A, SP-B, SP-C, SP-D) | ~10% | Immune defense, spreading, recycling | | DPPC specifically | ~50% of lipids | Primary surface-active agent | **High-Yield:** DPPC is the target of exogenous surfactant replacement therapy in RDS. Natural surfactants (Survanta, Curosurf) contain high DPPC concentrations; synthetic surfactants attempt to replicate this composition. **Clinical Pearl:** The L/S ratio (lecithin-to-sphingomyelin ratio) on amniotic fluid analysis reflects fetal lung maturity; lecithin = DPPC, and a ratio >2:1 indicates adequate surfactant production. [cite:Nelson Textbook of Pediatrics 21e Ch 102] 
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