Which structure is NOT a site of gas exchange?

• A. Trachea
• B. Alveoli
• C. Capillaries
• D. Lungs

Answer: (A)

Gas exchange happens where air meets blood across a very thin barrier with a large surface area. The alveoli, tiny air sacs wrapped in a dense network of capillaries, provide this ideal interface, so oxygen diffuses into the blood and carbon dioxide diffuses out into the air in the alveoli.

The trachea is a conducting airway. Its walls are thick and reinforced with cartilage, and it lacks alveoli and a thin diffusion barrier. It mainly serves to carry and condition the air we breathe, not to exchange gases. So, although the lungs contain the sites of gas exchange, the trachea itself does not participate in the process.