Where does gas exchange occur in the lungs?

• A. At the trachea
• B. In the bronchi
• C. Across the alveolar-capillary membrane
• D. Through the pleural cavity

Answer: (C)

Gas exchange occurs across the alveolar-capillary membrane, which is specifically designed for this function. The alveoli are tiny air sacs located at the ends of the bronchial tubes, and they are surrounded by capillaries, which are small blood vessels. This unique structural arrangement allows oxygen from the inhaled air to diffuse through the thin walls of the alveoli into the blood in the capillaries, while carbon dioxide in the blood diffuses in the opposite direction to be exhaled.

The efficiency of gas exchange is facilitated by several factors, including the large surface area of the alveoli, the thinness of the alveolar-capillary membrane, and the concentration gradients of the gases. Oxygen levels are higher in the alveoli compared to the blood, prompting oxygen to enter the bloodstream, whereas carbon dioxide levels are higher in the blood than in the alveoli, driving carbon dioxide out of the blood.

The other options play different roles in respiration and ventilation but are not where gas exchange occurs. For instance, the trachea and bronchi are primarily involved in conducting air to the lungs without participating in gas exchange. The pleural cavity is a space that surrounds the lungs and helps with lung inflation but does not facilitate gas exchange itself.