What are the two major alveolar cell types and their functions?

• A. Alveolar macrophages for gas exchange; Endothelial cells secrete surfactant.
• B. Type I pneumocytes for gas exchange; Type II pneumocytes secrete surfactant and can proliferate.
• C. Type II pneumocytes for gas exchange; Type I pneumocytes secrete surfactant.
• D. Type I pneumocytes transport nutrients; Type II pneumocytes regulate fluid balance.

Answer: (B)

The main alveolar cell types are Type I pneumocytes and Type II pneumocytes. Type I pneumocytes are extremely thin, covering most of the alveolar surface, and form the thin barrier through which gases diffuse between air and blood. Type II pneumocytes are cuboidal and produce surfactant, a substance that lowers surface tension to keep alveoli from collapsing; they also act as progenitors, able to proliferate and differentiate into Type I cells to repair the epithelium after injury. This combination—gas exchange by Type I cells and surfactant production plus regenerative capacity by Type II cells—perfectly matches the described roles. Alveolar macrophages are important for immune defense but are not the primary cells responsible for gas exchange or surfactant production.