The tiny air sacs where gas exchange occurs are called what?

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

Answer: (D)

Gas exchange takes place in the alveoli, the tiny sacs at the ends of the airways. Their walls are extremely thin and wrapped in a network of capillaries, creating a huge surface area for 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 lining is moist and maintained by surfactant from type II cells, which lowers surface tension and prevents collapse. In contrast, the bronchioles are air passages that conduct air but do not participate in gas exchange; the trachea is the main windpipe; and the pleura are membranes surrounding the lungs. So the site of gas exchange is the alveoli.