Alveolar Wall Composition for Gas Exchange

Steps:

• Alveoli are tiny air sacs where oxygen and carbon dioxide diffuse across thin walls.
• Walls must be extremely thin and flexible to maximize diffusion efficiency.
• Cartilage stiffens larger airways but would hinder alveolar thinness, so it's absent.
• Cilia line bronchioles to clear debris but aren't needed in sterile alveoli, so absent.
• Elastic fibers enable lung expansion and recoil during breathing, so present.

Why B is correct:

• Alveolar walls consist of simple squamous epithelium with elastic fibers for recoil, per respiratory histology definitions, excluding rigid cartilage and motile cilia.

Why the others are wrong:

• A: Cartilage and cilia are in bronchi, not alveoli, thickening walls unnecessarily.
• C: Includes cartilage and cilia, which disrupt alveolar thinness and gas exchange.
• D: Omits elastic fibers, essential for lung elasticity and ventilation.

Final answer: B