Which statement best describes the function of pulmonary surfactant?

• A. Increase alveolar surface tension
• B. Decrease alveolar surface tension to prevent collapse
• C. Increase lung compliance by stiffening the lungs
• D. Neutralize reactive oxygen species

Answer: (B)

Pulmonary surfactant lowers surface tension at the air–liquid interface inside the alveoli. By reducing surface tension, it prevents small alveoli from collapsing at the end of expiration and makes inflation easier, which increases overall lung compliance and reduces the work of breathing. The effect is size-dependent: surfactant reduces tension more effectively in smaller alveoli, stabilizing them and preventing widespread collapse (atelectasis) as breathing cycles. It’s produced by type II pneumocytes and secreted into the alveolar lining when the lungs stretch. This is why the statement describing surfactant as decreasing alveolar surface tension to prevent collapse is the best choice. Increasing surface tension would promote collapse, stiffening the lungs would actually raise the work of breathing, and neutralizing reactive oxygen species is not a primary role of surfactant.