Alveoli Lined for Gas Exchange

Steps:

• Identify alveoli as tiny lung sacs where oxygen and CO2 exchange occurs.
• Recall that alveoli walls are simple squamous epithelium for thin diffusion barrier.
• Note the moist surface reduces surface tension and enables gas solubility.
• Match to option describing moisture film, eliminating structural mismatches.

Why B is correct:

• Alveoli walls are coated with a thin surfactant-containing moisture film, per respiratory physiology, to prevent collapse and dissolve gases for diffusion.

Why the others are wrong:

• A: Cilia line upper airways to trap particles, not alveoli.
• C: Capillaries surround alveoli for exchange, but do not line walls.
• D: Alveoli lack muscle; walls are elastic epithelium only.

Final answer: B