Gas Exchange in Alveoli

Steps:

• Gas exchange occurs via diffusion across the thin respiratory membrane between alveolar air and blood.
• Oxygen moves from alveoli (high concentration) into blood (low), while CO2 moves from blood (high) to alveoli (low).
• The membrane is extremely thin, consisting of alveolar epithelial cells and capillary endothelial cells, effectively one cell layer.
• Evaluate choices: A reverses directions; C reverses directions and misattributes cause; D misstates pressure role; B accurately describes the barrier.

Why the correct option is correct:

• B aligns with the definition of the respiratory membrane as a single, thin cell layer (alveolar epithelium fused closely with endothelium), enabling rapid diffusion per Fick's law of diffusion.

Why the others are wrong:

• A: Reverses diffusion gradients; oxygen enters blood, CO2 exits.
• C: States oxygen diffuses into alveoli (opposite of gradient) and credits surface area incorrectly for direction.
• D: Blood pressure is low in pulmonary capillaries for slow flow, but not "lowest" overall, and it doesn't make cells "remain."

Final answer: B