Diffusion distance across respiratory membrane

Steps:

• Identify the respiratory membrane as the barrier between alveolar air and capillary blood, consisting of alveolar epithelium, interstitial fluid, and capillary endothelium.
• Recall that this membrane's total thickness is approximately 0.2–1 μm to enable rapid oxygen diffusion.
• Note that oxygen must cross this full barrier to reach the red blood cell interior.
• Conclude the minimum diffusion distance is 1 μm, as the upper limit of membrane thickness.

Why D is correct:

• The respiratory membrane thickness is defined as ~1 μm in standard biology, matching the path from alveolar air to RBC.

Why the others are wrong:

• A: 0.1 mm is 100 μm, far exceeding the microscopic scale of lung tissue.
• B: 1 mm is macroscopic, irrelevant to cellular diffusion.
• C: 0.1 μm underestimates the full membrane thickness, ignoring interstitial and endothelial layers.

Final answer: D