CO2 Diffusion Across Multiple Cell Membranes in Lungs

Steps:

• CO2 in blood plasma diffuses across the inner plasma membrane of the capillary endothelial cell (crossing 1).
• CO2 passes through the endothelial cell cytoplasm and crosses its outer plasma membrane into the pulmonary interstitium (crossing 2).
• CO2 diffuses across the inner plasma membrane of the type I alveolar epithelial cell (crossing 3).
• CO2 passes through the epithelial cell cytoplasm and crosses its outer plasma membrane into the alveolar space (crossing 4).

Why D is correct:

• The lung's blood-gas barrier consists of two cell layers (endothelium and epithelium), each requiring the gas to cross two plasma membranes to traverse the cell.

Why the others are wrong:

• A: Ignores the dual-membrane structure of each cell layer in the barrier.
• B: Counts only one membrane per cell, overlooking entry and exit crossings.
• C: Accounts for three crossings but misses the final exit from the alveolar epithelium.

Final answer: D