What type of epithelium lines the alveolar walls of the lungs, and which cells are involved in gas exchange?

• A. Type II pneumocytes; simple squamous epithelium; Type II produce surfactant
• B. Type II pneumocytes; simple squamous epithelium; Type I form gas exchange surface
• C. Type I pneumocytes; stratified squamous epithelium; macrophages assist gas exchange
• D. Type I pneumocytes; pseudostratified ciliated epithelium; Type II produce surfactant

Answer: (A)

The crucial idea is that the gas exchange surface in the lungs is built from very thin, flat epithelial cells known as Type I pneumocytes, which form a simple squamous lining of the alveolar walls. Their extreme thinness provides the rapid diffusion distance needed for oxygen and carbon dioxide to move between air and blood. Interspersed among them are Type II pneumocytes, which are cuboidal and specialize in making surfactant to reduce surface tension and in repairing the lining by differentiating into Type I cells when needed. So, the alveolar walls are lined by simple squamous epithelium formed by Type I pneumocytes, and these Type I cells are the ones that participate in gas exchange.