Pulmonary Surfactant Prevents Alveolar Collapse

Steps:

• Identify the problem: Surface tension pulls alveolar walls inward, risking collapse per Laplace's law (pressure = 2 × surface tension / radius).
• Recall key mechanism: Pulmonary surfactant, produced by type II alveolar cells, reduces surface tension in alveoli.
• Evaluate statements: Assume 1 involves lung elastic recoil (opposes expansion, not collapse prevention); 2 involves positive pressure (not primary); 3 is surfactant (directly lowers tension).
• Select option: Only 3 addresses surface tension directly, so D.

Why D is correct:

• Surfactant decreases surface tension, stabilizing small alveoli via Laplace's law, preventing collapse.

Why the others are wrong:

• A, B, C include 1 or 2, which do not reduce surface tension (elastic recoil aids expiration; pressure gradients maintain ventilation but not tension).

Final answer: D