What tiny air sacs in the lungs enable the exchange of oxygen and carbon dioxide with surrounding capillaries?

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

Answer: (A)

Gas exchange occurs in the tiny air sacs called alveoli. These sacs are surrounded by a network of capillaries and have walls that are extremely thin, allowing oxygen from the alveolar air to diffuse into the blood and carbon dioxide from the blood to diffuse into the alveolar air to be exhaled. The large total surface area of countless alveoli and the thin respiratory membrane make this diffusion efficient. A layer of surfactant coats the alveolar surfaces to keep them open and prevent collapse between breaths. In contrast, the bronchi and trachea are airways that move air to and from the lungs, and the diaphragm is a muscle that helps change the chest cavity’s volume to draw in or push out air.