Which of the following barriers must both \(\mathrm{O_2}\) and \(\mathrm{CO_2}\) cross to move between the air in the alveoli and the blood in pulmonary capillaries within the lungs?

• A. Only the alveolar epithelium
• B. Only the capillary endothelium
• C. Alveolar smooth muscle and capillary basement membrane
• D. Alveolar epithelium, fused basement membrane, and capillary endothelium

Answer: (D)

Verified step by step guidance

1. Understand the context: The question is asking about the barriers that oxygen (\(\mathrm{O_2}\)) and carbon dioxide (\(\mathrm{CO_2}\)) must cross during gas exchange in the lungs. This process occurs between the alveoli (air sacs in the lungs) and the pulmonary capillaries (tiny blood vessels).
2. Recall the anatomy of the respiratory membrane: The respiratory membrane is the structure that separates the air in the alveoli from the blood in the capillaries. It consists of three main layers: the alveolar epithelium, the fused basement membrane, and the capillary endothelium.
3. Break down the components: - The alveolar epithelium is a thin layer of epithelial cells lining the alveoli, allowing gases to diffuse. - The fused basement membrane is a shared structure between the alveolar epithelium and capillary endothelium, providing structural support and facilitating diffusion. - The capillary endothelium is the inner lining of the capillaries, where gases enter or leave the blood.
4. Explain the process of gas exchange: Oxygen (\(\mathrm{O_2}\)) diffuses from the alveoli into the blood, while carbon dioxide (\(\mathrm{CO_2}\)) diffuses from the blood into the alveoli. Both gases must cross all three layers of the respiratory membrane to complete this exchange.
5. Conclude with the correct answer: The barriers that \(\mathrm{O_2}\) and \(\mathrm{CO_2}\) must cross are the alveolar epithelium, the fused basement membrane, and the capillary endothelium. This ensures efficient gas exchange between the air and blood.