Which structure is the site of gas exchange in the respiratory system?

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

Answer: (A)

Gas exchange happens in the tiny air sacs called alveoli. Their walls are extremely thin and they sit right next to a dense network of capillaries, forming a thin respiratory membrane where oxygen can move from the air into the blood and carbon dioxide can move from the blood into the air to be exhaled. The vast number of alveoli provides a large surface area and a moist environment that helps gases dissolve and diffuse efficiently, with diffusion driven by differences in partial pressures. Surfactant in the alveoli also keeps them from collapsing, ensuring these sites stay open for continuous gas exchange. The other structures—trachea, bronchi, and larynx—are mainly air passages and airways for protection and conduction; they don’t have the thin walls and close capillary contact needed for diffusion, so they aren’t the sites where gas exchange occurs.