In the respiratory system, the tiny air sacs are primarily responsible for which function?

• A. Filtration of air particles.
• B. Warming incoming air.
• C. Gas exchange.
• D. Vibration to produce sound.

Answer: (C)

Gas exchange. The tiny air sacs, called alveoli, provide a very thin, moist barrier surrounded by a dense network of capillaries. This setup lets oxygen move from the air into the blood and carbon dioxide move from the blood into the air to be exhaled, driven by differences in partial pressures. The large surface area and the thin walls optimize diffusion, and surfactant reduces surface tension to keep the sacs from collapsing and maintain efficient exchange. Filtration and warming of air happen earlier in the respiratory tract, and sound is produced by air vibrating the vocal cords, not by the alveoli. So the alveoli are where the actual exchange of gases occurs.