The actual exchange of oxygen and carbon dioxide occurs in the

• A. pulmonary capillaries.
• B. apex of the lung.
• C. bronchioles.
• D. alveolar sacs.

Answer: (D)

Gas exchange between oxygen and carbon dioxide happens across the thin walls of the alveoli, inside the alveolar sacs. The alveoli are the tiny air sacs wrapped in a dense network of pulmonary capillaries, creating a huge surface area and a very thin barrier for diffusion. Oxygen moves from the alveolar air into the blood, while carbon dioxide moves from the blood into the alveolar air to be breathed out. The surrounding capillaries are essential for carrying blood to and from the lungs, but the actual exchange occurs across the alveolar membranes. Conducting airways like bronchioles don’t participate in diffusion, and the apex of the lung isn’t a specialized exchange site. So, the correct location is the alveolar sacs.