Layers of the respiratory membrane barrier to hemoglobin

Steps:

• Oxygen in alveolar air crosses the thin plasma membrane of type I alveolar epithelial cells (layer 1).
• It passes the epithelial basement membrane (layer 2).
• It traverses the endothelial basement membrane (layer 3).
• It crosses the plasma membrane of capillary endothelial cells (layer 4).
• It enters the red blood cell by crossing the erythrocyte plasma membrane to reach hemoglobin (layer 5).

Why C is correct:

• The blood-air barrier includes four layers (alveolar epithelium, epithelial basement membrane, endothelial basement membrane, capillary endothelium) plus the RBC membrane, totaling five as defined in respiratory physiology.

Why the others are wrong:

• A. 3: Counts only cellular plasma membranes, ignoring the two basement membranes.
• B. 4: Reaches blood plasma but excludes the RBC membrane needed for hemoglobin access.
• D. 6: Overcounts by splitting the thin fused basement membrane or adding non-membrane elements like interstitial fluid.

Final answer: C