CO2 diffusion across membranes in pulmonary gas exchange

Steps:

• CO2 inside red blood cell diffuses across RBC plasma membrane into blood plasma (crossing 1).
• CO2 diffuses through plasma and across capillary endothelial cell membrane into interstitial fluid (crossing 2).
• CO2 diffuses through interstitial fluid and across alveolar type I epithelial cell membrane (crossing 3).
• CO2 enters alveolar air space.

Why B is correct:

• Gas exchange requires crossing three distinct cell surface membranes: RBC, endothelium, and alveolar epithelium, as defined in respiratory physiology.

Why the others are wrong:

• A: Underestimates by omitting one cell layer in the blood-air barrier.
• C: Overcounts by treating each epithelial layer as two separate crossings.
• D: Excessively overcounts additional hypothetical barriers not present.

Final answer: B