The actual exchange of oxygen and carbon dioxide occurs in which of the following structures?

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

Answer: (B)

Gas exchange happens by diffusion across the alveolar–capillary membrane. The place where this occurs is the alveolus, tiny air sacs in the lungs surrounded by a dense network of capillaries. Oxygen moves from the air inside the alveolus into the blood because the partial pressure of oxygen is higher in the alveolar air than in the blood, while carbon dioxide moves from the blood into the alveolar air because its pressure is higher in the blood. The alveoli provide the enormous surface area and the extremely thin barrier needed for rapid diffusion. The trachea and bronchi just conduct air and don’t participate in gas exchange, and while capillaries are involved in carrying blood, the actual transfer of gases happens across the alveolar–capillary membrane.