Name the two main alveolar cell types and their primary functions.

• A. Type II pneumocytes (surfactant production) and Type III pneumocytes (repair)
• B. Type I pneumocytes (gas exchange) and Type III pneumocytes (immune)
• C. Alveolar macrophages and Type I pneumocytes
• D. Type I pneumocytes (gas exchange) and Type II pneumocytes (surfactant production and repair)

Answer: (D)

The main idea is that the alveolar lining relies on two specialized epithelial cell types with distinct jobs: one type forms the thin gas-exchange barrier, and the other handles surfactant production and repair. Type I pneumocytes are very thin, squamous cells that cover most of the alveolar surface, creating an almost unfettered path for oxygen and carbon dioxide to diffuse between air in the alveoli and blood in the capillaries. Type II pneumocytes are cuboidal and fewer in number, but they are crucial because they produce pulmonary surfactant, a substance that lowers surface tension to keep the alveoli from collapsing with each breath; they also serve as progenitor cells that can proliferate and differentiate into Type I cells to aid repair after injury. Alveolar macrophages (a separate cell type) reside in the air spaces to remove debris and pathogens, but they are not the primary epithelial cells responsible for gas exchange or surfactant production. Therefore, pairing gas exchange with surfactant production and repair best captures the two main alveolar cell roles.