Where does gas exchange occur in the lungs?

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

Answer: (D)

Gas exchange occurs across the thin walls of the alveoli, the tiny air sacs at the ends of the airways. Alveoli provide a huge surface area and are wrapped in a dense capillary network, making the diffusion path extremely short. Oxygen moves from the air in the alveoli into the blood, while carbon dioxide moves from the blood into the alveolar air to be exhaled. This exchange is driven by the partial pressure gradients of the gases across the very thin respiratory membrane, which is formed by the alveolar epithelium, the capillary endothelium, and their basement membranes. Surfactant keeps the alveoli from collapsing and helps maintain efficient diffusion. The larynx and bronchi are air conduits, and the esophagus handles food; gas exchange itself happens mainly in the alveoli.