Most gas exchange occurs in the?

• A. Respiratory bronchioles
• B. Alveoli
• C. Alveolar ducts
• D. Bronchioles

Answer: (B)

Gas exchange happens mainly in the alveoli because they provide a vast, thin surface where gases can diffuse rapidly. The walls of the alveoli are extremely thin and closely surrounded by a dense network of capillaries, so the distance for diffusion is minimal—about 0.5 micrometers. This tiny barrier plus the enormous surface area (roughly 70–100 square meters in an adult) makes diffusion of oxygen into blood and carbon dioxide out of blood very efficient. Other parts of the airway, like the bronchioles and ducts, are primarily conducting airways with thicker walls and less surface area for diffusion, so they don’t contribute to gas exchange to the same extent. A small amount of exchange can occur in respiratory bronchioles, but the bulk of gas exchange takes place in the alveoli.