Oxygen's diffusion path across five cell membranes in lungs

Steps:

• Oxygen crosses the apical membrane of the alveolar epithelial cell from air space (1st membrane).
• It exits via the basolateral membrane of the epithelial cell into interstitium (2nd membrane).
• Oxygen enters the apical membrane of the capillary endothelial cell (3rd membrane).
• It exits via the basolateral membrane of the endothelial cell into plasma (4th membrane).
• Finally, oxygen crosses the red blood cell membrane to reach hemoglobin (5th membrane).

Why C is correct:

• Oxygen diffusion requires passing through plasma membranes of epithelial cells (2), endothelial cells (2), and erythrocytes (1), as defined by the alveolar-capillary barrier structure.

Why the others are wrong:

• A: Counts only major barriers, ignoring individual plasma membranes per cell.
• B: Omits the erythrocyte membrane entry for hemoglobin binding.
• D: Overcounts by including non-cellular structures like basement membranes.

Final answer: C