Which structure is the site of gas exchange located within the lungs?

Which structure is the site of gas exchange located within the lungs?

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

Answer: (A)

Gas exchange happens where oxygen moves from air into the blood and carbon dioxide moves from blood into the air. That occurs in the alveoli, tiny air sacs at the ends of the airways. Their walls are extremely thin and there is a dense network of tiny capillaries surrounding them, creating a very large surface area and a short diffusion distance. This setup allows efficient diffusion of gases between air and blood. The other structures—airways like the trachea, bronchi, and larynx—are designed to conduct air to the lungs. Their walls are thicker and they lack the close contact with blood vessels and the thin barrier needed for gas diffusion, so they’re not sites of gas exchange.

Gas exchange happens where oxygen moves from air into the blood and carbon dioxide moves from blood into the air. That occurs in the alveoli, tiny air sacs at the ends of the airways. Their walls are extremely thin and there is a dense network of tiny capillaries surrounding them, creating a very large surface area and a short diffusion distance. This setup allows efficient diffusion of gases between air and blood.

The other structures—airways like the trachea, bronchi, and larynx—are designed to conduct air to the lungs. Their walls are thicker and they lack the close contact with blood vessels and the thin barrier needed for gas diffusion, so they’re not sites of gas exchange.