Which epithelium lines the air sacs of the lungs where gas exchange occurs?

Which epithelium lines the air sacs of the lungs where gas exchange occurs?

• A. Simple columnar epithelium
• B. Simple squamous epithelium
• C. Pseudostratified ciliated columnar epithelium
• D. Stratified squamous epithelium

Answer: (B)

Gas exchange relies on a very thin, smooth barrier between air and blood. The lining of the air sacs, the alveoli, is made up of simple squamous epithelium. This single layer of flat cells is short and flat, which minimizes the distance gases must diffuse, allowing oxygen to move into the blood and carbon dioxide to exit efficiently. While other epithelia have roles in protection, secretion, or lining larger airways, they are thicker or multi-layered and would impede diffusion. For example, simple columnar epithelium is thick and suited for absorption and secretion in the digestive tract, pseudostratified ciliated columnar epithelium lines the conducting airways and helps move mucus, and stratified squamous epithelium provides protection in areas of wear and tear. The alveolar lining, optimized for diffusion, is simple squamous.

Gas exchange relies on a very thin, smooth barrier between air and blood. The lining of the air sacs, the alveoli, is made up of simple squamous epithelium. This single layer of flat cells is short and flat, which minimizes the distance gases must diffuse, allowing oxygen to move into the blood and carbon dioxide to exit efficiently. While other epithelia have roles in protection, secretion, or lining larger airways, they are thicker or multi-layered and would impede diffusion. For example, simple columnar epithelium is thick and suited for absorption and secretion in the digestive tract, pseudostratified ciliated columnar epithelium lines the conducting airways and helps move mucus, and stratified squamous epithelium provides protection in areas of wear and tear. The alveolar lining, optimized for diffusion, is simple squamous.