Which cells secrete pulmonary surfactant, and what is the importance of surfactant?

• A. Type I pneumocytes; surfactant reduces surface tension to prevent alveolar collapse.
• B. Type II pneumocytes; surfactant reduces surface tension to prevent alveolar collapse.
• C. Club cells; surfactant reduces surface tension to prevent alveolar collapse.
• D. Type II pneumocytes; surfactant increases surface tension to promote collapse.

Answer: (B)

Pulmonary surfactant is produced by alveolar type II pneumocytes. Its role is to lower surface tension at the air–liquid interface inside the alveoli, which prevents alveolar collapse during expiration and reduces the work of breathing. Type II cells store surfactant in lamellar bodies and can proliferate to replenish the epithelium, while type I pneumocytes form the thin gas-exchange surface and do not produce surfactant. Club cells reside in the bronchioles and are not responsible for alveolar surfactant. Surfactant increasing surface tension would promote collapse, which is opposite of its function. In premature infants, lack of surfactant leads to respiratory distress syndrome due to alveolar collapse.