Which tissue type lines the air sacs (alveoli) of the lungs?

Which tissue type lines the air sacs (alveoli) of the lungs?

• A. Simple cuboidal epithelium
• B. Stratified squamous epithelium
• C. Simple squamous epithelium
• D. Pseudostratified

Answer: (C)

Gas exchange relies on a surface that is as thin as possible. The lining of the alveoli is a single layer of flat cells, which creates the shortest diffusion distance for oxygen to move into blood and for carbon dioxide to exit. This simple squamous epithelium forms the thin wall of the alveolar sacs together with a nearby capillary, making gas exchange efficient. A few cuboidal Type II cells are scattered among them to secrete surfactant and help keep the surface from collapsing, but the main lining remains simple squamous. The other options would not support efficient diffusion: simple cuboidal is thicker and typical of kidney tubules or glands, stratified squamous provides protection in areas of wear, and pseudostratified lines most of the conducting airways with cilia, not the delicate alveolar sacs.

Gas exchange relies on a surface that is as thin as possible. The lining of the alveoli is a single layer of flat cells, which creates the shortest diffusion distance for oxygen to move into blood and for carbon dioxide to exit. This simple squamous epithelium forms the thin wall of the alveolar sacs together with a nearby capillary, making gas exchange efficient. A few cuboidal Type II cells are scattered among them to secrete surfactant and help keep the surface from collapsing, but the main lining remains simple squamous. The other options would not support efficient diffusion: simple cuboidal is thicker and typical of kidney tubules or glands, stratified squamous provides protection in areas of wear, and pseudostratified lines most of the conducting airways with cilia, not the delicate alveolar sacs.