Neonatal surfactant deficiency (respiratory distress syndrome) results in reduced lung compliance because:

• A Without surfactant, surface tension at the air-liquid interface is high and varies with alveolar radius, causing alveolar collapse at end-expiration ✓
• B Surfactant increases alveolar surface tension, requiring more pressure to inflate the lungs
• C Surfactant deficiency increases pulmonary blood flow, flooding alveoli with oedema fluid
• D Absence of surfactant inhibits type II pneumocyte differentiation, reducing gas exchange surface area

Explanation

Surfactant (dipalmitoylphosphatidylcholine, DPPC) reduces surface tension at the alveolar air-liquid interface and, crucially, reduces tension more in smaller alveoli (where molecules are compressed), stabilising alveolar size and preventing collapse on expiration. Without surfactant, Laplace's law (P = 2T/r) dictates that small alveoli generate higher inward pressure, causing them to collapse (atelectasis) at end-expiration. This dramatically reduces the available surface area and compliance. The high work of breathing required to re-expand collapsed alveoli with each breath leads to respiratory failure in preterm neonates.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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