In the respiratory system, where does diffusion of respiratory gases primarily occur?

• A. In the bronchial tubes
• B. In the alveoli
• C. In the diaphragm
• D. In the trachea

Answer: (B)

Gas diffusion occurs where there is a large, very thin surface area in close contact with blood, allowing gases to move down their partial pressure gradients. In the lungs, that happens in the alveoli, tiny air sacs surrounded by a dense network of capillaries. The walls of the alveoli and the capillary endothelium form a respiratory membrane only about a cell thick, so the distance for diffusion is minimal. Oxygen in the alveolar air has a higher partial pressure than in the blood, so it diffuses into the blood, while carbon dioxide diffuses from the blood into the alveolar air where it is exhaled. The alveoli provide a huge surface area—crucial for efficient gas exchange. The diaphragm and other muscles control airflow, not diffusion, and the trachea and bronchial tubes are conducting passages with no gas exchange occurring across their walls.