Which structure is the primary site of gas exchange in the lungs?

Which structure is the primary site of gas exchange in the lungs?

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

Answer: (C)

Gas exchange relies on diffusion across a very thin, moist barrier where oxygen moves into blood and carbon dioxide moves out. The alveoli provide the primary site because they offer an enormous surface area and an extremely thin barrier formed by the alveolar epithelium, the capillary endothelium, and their shared basement membranes. Surfactant keeps the alveoli stable, enabling efficient gas transfer by keeping the distance for diffusion minimal. In the alveoli, oxygen diffuses from the air into the pulmonary capillaries, while carbon dioxide diffuses from the blood into the alveolar air to be exhaled. The trachea and bronchi are conducting airways with thicker walls and cartilage, designed to move air rather than exchange gases. The pleura is a protective membrane around the lungs and does not participate in gas exchange.

Gas exchange relies on diffusion across a very thin, moist barrier where oxygen moves into blood and carbon dioxide moves out. The alveoli provide the primary site because they offer an enormous surface area and an extremely thin barrier formed by the alveolar epithelium, the capillary endothelium, and their shared basement membranes. Surfactant keeps the alveoli stable, enabling efficient gas transfer by keeping the distance for diffusion minimal.

In the alveoli, oxygen diffuses from the air into the pulmonary capillaries, while carbon dioxide diffuses from the blood into the alveolar air to be exhaled. The trachea and bronchi are conducting airways with thicker walls and cartilage, designed to move air rather than exchange gases. The pleura is a protective membrane around the lungs and does not participate in gas exchange.