## Pulmonary Surfactant Composition **Key Point:** Dipalmitoylphosphatidylcholine (DPPC) is the primary active component of pulmonary surfactant, accounting for ~50% of its lipid mass and responsible for reducing surface tension at the air-liquid interface. ### Surfactant Structure and Function Pulmonary surfactant is a complex mixture of lipids (~90%) and proteins (~10%): | Component | Percentage | Function | | --- | --- | --- | | DPPC | ~50% | Primary surface tension reducer | | Phosphatidylglycerol (PG) | ~10% | Stabilizes DPPC, enhances spreading | | Cholesterol | ~10% | Maintains membrane fluidity | | Proteins (SP-A, SP-B, SP-C, SP-D) | ~10% | Immune function, surfactant metabolism | ### RDS Pathophysiology **High-Yield:** In Respiratory Distress Syndrome (RDS), primarily affecting preterm infants <34 weeks, there is deficient production of surfactant due to immature type II pneumocytes. This leads to: 1. Increased alveolar surface tension 2. Alveolar collapse (atelectasis) at end-expiration 3. Increased work of breathing 4. Ventilation-perfusion mismatch 5. Respiratory failure ### Clinical Correlation **Clinical Pearl:** Antenatal corticosteroids (betamethasone, dexamethasone) accelerate fetal lung maturity by stimulating type II pneumocyte differentiation and surfactant synthesis. Exogenous surfactant replacement (containing DPPC) is the definitive postnatal treatment for RDS. **Mnemonic:** **DPPC** = **D**ipalmitoyl**P**hosphatidyl**C**holine — the "D" reminds you it's the primary ("Default") surfactant lipid. 
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