Which cells line the alveolar walls for gas exchange, and what other alveolar cell type is present?

• A. Type II pneumocytes line alveolar walls; Type I pneumocytes secrete surfactant.
• B. Type II pneumocytes line alveolar walls; Type I pneumocytes secrete surfactant.
• C. Alveolar walls are lined by club cells; Type I pneumocytes secrete mucus.
• D. Type I pneumocytes line alveolar walls; Clara cells secrete surfactant.

Answer: (C)

Gas exchange in the alveoli relies on very thin, flat cells that cover the alveolar walls—these are the Type I pneumocytes, forming the main diffusion barrier. The other alveolar cell type present is the Type II pneumocyte, which is cuboidal and produces surfactant to lower surface tension and prevent collapse; these cells also serve as progenitors that can regenerate Type I pneumocytes after injury.

Club (Clara) cells are located in the bronchioles, not the alveolar walls, and they are not responsible for surfactant production or gas exchange. They play a different protective/detoxifying role in the conducting airways. So the correct concept is: alveolar walls lined by Type I pneumocytes for gas exchange, with Type II pneumocytes present to secrete surfactant.