Gas exchange between inhaled air and blood occurs in which structure?

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

Answer: (B)

Gas exchange relies on a very thin, moist barrier with a large surface area that puts gases in contact with blood in close proximity. The alveoli provide exactly that. They are tiny air sacs surrounded by a dense capillary network, and their walls are extremely thin, forming the respiratory membrane (alveolar epithelium, fused basement membranes, and capillary endothelium). This setup lets oxygen diffuse from the air in the alveoli into the blood and carbon dioxide diffuse from the blood into the alveolar air, driven by their respective partial pressure differences.

Other airways, like the trachea and larger bronchioles, are designed for conducting air and have thicker walls with less surface area for diffusion, so they aren’t the sites where gas exchange happens. The larynx also functions mainly in air flow and sound, not diffusion of gases.