Where does gas exchange occur in the lungs?

• A. Trachea and bronchi
• B. Pleura and mediastinum
• C. Alveoli and capillaries
• D. Diaphragm and intercostal muscles

Answer: (C)

Gas exchange happens at the alveolar-capillary interface, where the thin walls of the alveoli are surrounded by a dense network of capillaries. This thin respiratory membrane allows oxygen to diffuse from the air in the alveoli into the blood and carbon dioxide to diffuse from the blood into the alveolar air. The driving force is the difference in partial pressures: oxygen moves from higher to lower PO2, and carbon dioxide moves from higher to lower PCO2.

Airways like the trachea and bronchi simply conduct air and don’t participate in gas exchange. The pleura and mediastinum are membranes and spaces around the lungs, not sites of gas exchange. The diaphragm and intercostal muscles are involved in moving air in and out of the lungs rather than exchanging gases themselves.