Where does the oxygen and carbon dioxide exchange occur within the lungs?

Where does the oxygen and carbon dioxide exchange occur within the lungs?

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

Answer: (A)

Gas exchange happens where air in the lungs meets the blood—in the alveoli. These tiny sacs are surrounded by a dense network of capillaries, and their walls are extremely thin, with a moist surface that helps gases dissolve. Oxygen diffuses from the alveolar air, where its partial pressure is higher, across the alveolar and capillary walls into the blood. Carbon dioxide diffuses in the opposite direction, from blood (where its partial pressure is higher) into the alveolar air to be exhaled. The large total surface area of all alveoli and the thin diffusion barrier make this process efficient. Surfactant helps keep alveoli open so gas exchange can continue. By contrast, the trachea and bronchi are just air passages, not sites of gas exchange, and the pleura are membranes around the lungs.

Gas exchange happens where air in the lungs meets the blood—in the alveoli. These tiny sacs are surrounded by a dense network of capillaries, and their walls are extremely thin, with a moist surface that helps gases dissolve. Oxygen diffuses from the alveolar air, where its partial pressure is higher, across the alveolar and capillary walls into the blood. Carbon dioxide diffuses in the opposite direction, from blood (where its partial pressure is higher) into the alveolar air to be exhaled. The large total surface area of all alveoli and the thin diffusion barrier make this process efficient. Surfactant helps keep alveoli open so gas exchange can continue. By contrast, the trachea and bronchi are just air passages, not sites of gas exchange, and the pleura are membranes around the lungs.