Which substance reduces alveolar surface tension and helps prevent alveolar collapse?

• A. Oxygen concentration.
• B. Surfactant.
• C. Mucus layer.
• D. Partial pressure of carbon dioxide.

Answer: (B)

Surfactant lowers surface tension at the air–liquid interface inside the alveoli, which prevents small alveoli from collapsing at the end of expiration and makes breathing easier. It is produced by type II pneumocytes and consists of phospholipids and specific proteins that spread along the alveolar surface. Because it reduces the energy needed to keep alveoli open, more of the lung remains inflated, improving compliance and decreasing work of breathing. In premature infants, insufficient surfactant leads to widespread alveolar collapse (respiratory distress syndrome), highlighting its crucial role in maintaining alveolar stability. Oxygen concentration changes oxygen delivery, not surface tension. The mucus layer protects and humidifies airways but doesn’t prevent alveolar collapse. Partial pressure of carbon dioxide influences breathing drive and acid-base balance, not the surface tension of alveoli.