Which structure is primarily responsible for gas exchange in the lungs?

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

Answer: (A)

Gas exchange happens primarily across the alveolar walls because they present a very thin barrier between air in the alveolar space and the blood in surrounding capillaries, and they provide a huge surface area for diffusion. The respiratory membrane, formed by the alveolar epithelium (mostly type I pneumocytes), its basement membrane, and the capillary endothelium, is only about a fraction of a micrometer thick, allowing O2 to diffuse into blood and CO2 to diffuse out efficiently. Surfactant from type II pneumocytes reduces surface tension, helping keep the surface area large and the alveoli stable for continuous gas exchange.

The other structures serve mainly to conduct air. The trachea and bronchi have thick walls reinforced with cartilage and mucus-secreting glands, which are ideal for airway passage and filtration but not for gas exchange. Bronchioles are smaller conducting pathways and, although some gas exchange can occur in very tiny regions, the vast majority of exchange occurs where alveoli are present.