Which epithelial tissue type is most common in the alveolar sacs of the lungs?

Which epithelial tissue type is most common in the alveolar sacs of the lungs?

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

Answer: (D)

Gas exchange in the lungs relies on a very thin barrier between the air in the alveoli and the blood in surrounding capillaries. That barrier is formed by a single layer of flat, thin epithelial cells—the simple squamous epithelium. In the alveolar sacs, this thin lining (type I pneumocytes) is ideal for rapid diffusion of oxygen and carbon dioxide. A small population of cuboidal cells (type II pneumocytes) sits among them to produce surfactant and help with repair, but they do not form the main lining. The other epithelium types wouldn’t support diffusion as effectively: simple cuboidal is thicker and found in glands and kidney tubules; pseudostratified is characteristic of most conducting airways but not the delicate gas-exchange surface; stratified squamous is protective and suited to areas subject to abrasion, not the delicate alveolar walls. So the alveolar sacs are lined predominantly by simple squamous epithelium to maximize gas exchange.

Gas exchange in the lungs relies on a very thin barrier between the air in the alveoli and the blood in surrounding capillaries. That barrier is formed by a single layer of flat, thin epithelial cells—the simple squamous epithelium. In the alveolar sacs, this thin lining (type I pneumocytes) is ideal for rapid diffusion of oxygen and carbon dioxide. A small population of cuboidal cells (type II pneumocytes) sits among them to produce surfactant and help with repair, but they do not form the main lining. The other epithelium types wouldn’t support diffusion as effectively: simple cuboidal is thicker and found in glands and kidney tubules; pseudostratified is characteristic of most conducting airways but not the delicate gas-exchange surface; stratified squamous is protective and suited to areas subject to abrasion, not the delicate alveolar walls. So the alveolar sacs are lined predominantly by simple squamous epithelium to maximize gas exchange.