Gas exchange of oxygen and carbon dioxide occurs primarily in which structure of the lungs?

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

Answer: (C)

Gas exchange happens at the alveoli. These tiny air sacs have walls only one cell thick and are surrounded by a dense network of capillaries, forming the respiratory membrane where gases diffuse. Oxygen moves from the alveolar air into the blood, while carbon dioxide moves from the blood into the alveoli to be exhaled. The large surface area of all the alveoli and the very thin barrier between air and blood make diffusion efficient. Surfactant helps keep alveoli open so this surface area is maintained. The other structures are airways or membranes that don’t participate in gas exchange: conducting passages like the trachea and bronchioles transport air, while the pleura lines the lungs but isn’t involved in diffusion.