The only place in the respiratory system where oxygen and carbon dioxide are exchanged

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

Answer: (B)

Gas exchange happens where a very thin barrier separates air from blood, which is the alveoli. These tiny sacs provide a huge surface area and are surrounded by a dense network of capillaries, with a moist, 1-cell-thick wall that allows diffusion. Oxygen moves from the alveolar air into the blood, while carbon dioxide moves from the blood into the alveolar air to be exhaled. The conducting airways—the trachea, bronchi, and bronchioles—are built to move air and protect the lungs, not to exchange gases; their walls are thicker and they lack the close contact with blood vessels required for diffusion.