Oxygen diffusion across the respiratory membrane

Steps:

• Alveolus is lined by type I alveolar cells, which are squamous epithelial cells.
• Oxygen diffuses from alveolar air through the squamous epithelium into interstitial fluid.
• It then crosses the basement membrane and enters the capillary wall.
• Capillary wall consists of squamous endothelial cells, allowing oxygen into blood plasma.

Why C is correct:

• The respiratory membrane is defined as the thin barrier of alveolar squamous epithelium and capillary endothelium, enabling simple diffusion of gases per Fick's law of diffusion.

Why the others are wrong:

• A: Repeats epithelium, ignoring the distinct endothelial layer of capillaries.
• B: Reverses order, starting with endothelium, which is not adjacent to alveolar air.
• D: Repeats endothelium, omitting the initial epithelial barrier of the alveolus.

Final answer: C