What are the sites of gas exchange in the lungs?

• A. Trachea
• B. Alveoli
• C. Bronchi
• D. Pleura

Answer: (B)

Gas exchange happens where air meets blood—in the alveoli, the tiny sacs at the ends of the airways. The walls of the alveoli are extremely thin and are surrounded by a dense network of capillaries, creating a large surface area for diffusion. Oxygen moves from the alveolar air into the blood, while carbon dioxide moves from the blood into the alveolar air to be exhaled. This diffusion is driven by the partial pressure differences of the gases across the respiratory membrane, which is formed by the alveolar epithelium, its basement membrane, and the capillary endothelium. The conducting airways, like the trachea and bronchi, have thicker walls and cartilage and are designed for moving air, not gas exchange. The pleura is a protective membrane around the lungs and does not participate in gas exchange.