Which structure is the site of gas exchange with the bloodstream?

Which structure is the site of gas exchange with the bloodstream?

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

Answer: (A)

Gas exchange happens where air is in very close contact with blood, across walls that are extremely thin and offer a large surface area for diffusion. That role is fulfilled by the alveoli, tiny sacs at the ends of the respiratory tree. Their walls are only about one cell thick and are surrounded by a dense network of capillaries, so oxygen in the alveolar air diffuses into the bloodstream and carbon dioxide diffuses from the blood into the alveoli to be exhaled. The surface is kept moist and coated with surfactant to optimize diffusion and prevent collapse, which further enhances exchange efficiency. In contrast, the bronchi and bronchioles mainly serve to conduct air to the lungs, and the trachea is the main airway, not the site of gas exchange.

Gas exchange happens where air is in very close contact with blood, across walls that are extremely thin and offer a large surface area for diffusion. That role is fulfilled by the alveoli, tiny sacs at the ends of the respiratory tree. Their walls are only about one cell thick and are surrounded by a dense network of capillaries, so oxygen in the alveolar air diffuses into the bloodstream and carbon dioxide diffuses from the blood into the alveoli to be exhaled. The surface is kept moist and coated with surfactant to optimize diffusion and prevent collapse, which further enhances exchange efficiency. In contrast, the bronchi and bronchioles mainly serve to conduct air to the lungs, and the trachea is the main airway, not the site of gas exchange.