Exchange of gas between inhaled air and blood occurs in the:

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

Answer: (B)

Gas exchange between inhaled air and blood happens where the air meets blood across a very thin barrier in the lungs—the alveolar walls. The alveoli are tiny sacs with a single layer of epithelium and a dense network of capillaries, forming the respiratory membrane. Oxygen moves from the alveolar air, where its partial pressure is high, into the blood where it’s低, while carbon dioxide moves from blood, where its partial pressure is high, into the alveolar air. This diffusion is driven by partial pressure gradients and the enormous surface area provided by countless alveoli. The other structures are airways: bronchioles conduct air but lack the thin barrier and extensive capillary contact needed for diffusion, while the trachea and larynx mainly serve as passageways and for voice rather than gas exchange. So, the alveoli are the site where gas exchange occurs.