Which structure is primarily responsible for gas exchange between air and blood?

Which structure is primarily responsible for gas exchange between air and blood?

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

Answer: (C)

Gas exchange happens where air meets blood in the tiny air sacs of the lungs called alveoli. Alveoli are extremely small and numerous, with walls that are only one cell thick and surrounded by a dense capillary network. This thin respiratory membrane—composed of the alveolar epithelium, a shared basement membrane, and the capillary endothelium—creates a very short diffusion path and a large surface area, allowing oxygen to move into the blood and carbon dioxide to move out efficiently. Surfactant helps keep alveoli open so this surface area remains available for gas exchange. Structures like the bronchi are just air passages, the larynx is the voice box, and the pleura are membranes around the lungs, so they’re not the primary sites of gas exchange.