The primary function of the alveoli in the respiratory system is:

The primary function of the alveoli in the respiratory system is:

• A. Gas exchange
• B. Filtration of air
• C. Humidification
• D. Voice production

Answer: (D)

Gas exchange happens in the alveoli, the tiny air sacs at the end of the respiratory tree. Here, oxygen from the air within the alveolus diffuses across the very thin alveolar-capillary membrane into the surrounding capillaries and binds to hemoglobin. At the same time, carbon dioxide moves from the blood into the alveolar air to be exhaled. The alveolar walls are extremely thin and surrounded by a rich capillary network, and they provide a very large surface area to maximize diffusion. A thin layer of surfactant reduces surface tension, keeping the alveoli open between breaths so diffusion can continue efficiently. The other functions listed—filtering and humidifying air, or producing voice—occur in other parts of the respiratory or upper airway system, not in the alveoli.

Gas exchange happens in the alveoli, the tiny air sacs at the end of the respiratory tree. Here, oxygen from the air within the alveolus diffuses across the very thin alveolar-capillary membrane into the surrounding capillaries and binds to hemoglobin. At the same time, carbon dioxide moves from the blood into the alveolar air to be exhaled. The alveolar walls are extremely thin and surrounded by a rich capillary network, and they provide a very large surface area to maximize diffusion. A thin layer of surfactant reduces surface tension, keeping the alveoli open between breaths so diffusion can continue efficiently. The other functions listed—filtering and humidifying air, or producing voice—occur in other parts of the respiratory or upper airway system, not in the alveoli.