The respiratory membrane is the thin wall between the alveolar lumen and the ______.

• A. interstitial fluid
• B. capillary lumen
• C. pleural cavity
• D. bronchiolar lumen

Answer: (B)

Verified step by step guidance

1. Understand the structure of the respiratory membrane: The respiratory membrane is a thin barrier that facilitates gas exchange between the air in the alveoli and the blood in the capillaries.
2. Identify the components of the respiratory membrane: It consists of the alveolar epithelium, the capillary endothelium, and a shared basement membrane between them.
3. Recall the function of the respiratory membrane: Its primary role is to allow oxygen to diffuse from the alveolar air into the blood and carbon dioxide to diffuse from the blood into the alveolar air.
4. Eliminate incorrect options: The pleural cavity is the space between the lungs and the chest wall, which is not involved in gas exchange. The bronchiolar lumen is part of the airway leading to the alveoli but is not directly involved in the respiratory membrane. The interstitial fluid is present in tissues but does not form part of the respiratory membrane.
5. Conclude that the correct answer is the capillary lumen, as it is the space within the capillaries where blood flows and directly interfaces with the respiratory membrane for gas exchange.