The alveolar barrier consists mainly of which two cell types?

• A. Type II pneumocytes and goblet cells
• B. Type I pneumocytes and capillary endothelium
• C. Basal cells and lamina propria
• D. Alveolar macrophages and Type II pneumocytes

Answer: (B)

The barrier for gas exchange in the alveoli is formed mainly by the air-blood interface: the thin, flat Type I pneumocytes lining the alveolar walls and the adjacent capillary endothelium, with their shared basement membranes. This pairing creates the very short diffusion distance that allows oxygen and carbon dioxide to move between air and blood efficiently. Type I pneumocytes cover most of the alveolar surface and are specialized for gas exchange, while Type II pneumocytes produce surfactant and help repair the epithelium, but they’re not the primary components of the diffusion barrier. The other cell types mentioned are found in conducting airways (goblet cells, basal cells) or are immune cells on the surface (alveolar macrophages) and do not constitute the main barrier.