What term describes the tiny sacs in the lungs where gas exchange occurs?

• A. Alveoli
• B. Bronchi
• C. Bronchioles
• D. Alveolar ducts

Answer: (A)

Gas exchange happens across the thin walls of tiny air sacs at the end of the airway tree. These sacs are alveoli. The walls of each alveolus are extremely thin and surrounded by a dense capillary network, so oxygen diffuses into the blood and carbon dioxide diffuses out into the alveolar air to be exhaled. The large total surface area of all alveoli, plus the presence of surfactant produced by type II cells to keep them from collapsing, makes this the primary site of gas exchange. The ducts that lead to alveoli and the larger airways like bronchi and bronchioles mainly conduct air and do not participate in gas exchange themselves, while alveolar ducts channel air to the alveoli. So the tiny sacs where gas exchange occurs are the alveoli.