What structure located in the lungs is the primary site of gas exchange?

What structure located in the lungs is the primary site of gas exchange?

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

Answer: (A)

Gas exchange happens when oxygen and carbon dioxide diffuse across the thin respiratory membrane between the air in the alveoli and the blood in the surrounding capillaries. Alveoli are tiny sacs at the ends of the airways and are present in huge numbers, giving the lungs a large surface area and very thin walls lined by Type I alveolar cells, with capillaries right beside them. This close contact and thin barrier allow rapid diffusion of gases, driven by differences in partial pressures. Surfactant from Type II cells helps keep the alveoli open so diffusion can occur efficiently. The other structures listed—trachea, bronchi, and larynx—are conducting airways that move air toward the alveoli but do not participate in gas exchange because their walls are thicker and lack the dense capillary network needed for diffusion.

Gas exchange happens when oxygen and carbon dioxide diffuse across the thin respiratory membrane between the air in the alveoli and the blood in the surrounding capillaries. Alveoli are tiny sacs at the ends of the airways and are present in huge numbers, giving the lungs a large surface area and very thin walls lined by Type I alveolar cells, with capillaries right beside them. This close contact and thin barrier allow rapid diffusion of gases, driven by differences in partial pressures. Surfactant from Type II cells helps keep the alveoli open so diffusion can occur efficiently. The other structures listed—trachea, bronchi, and larynx—are conducting airways that move air toward the alveoli but do not participate in gas exchange because their walls are thicker and lack the dense capillary network needed for diffusion.