Where in the respiratory system does gas exchange take place?

• A. The Alveoli
• B. The Trachea
• C. The Bronchi
• D. The Larynx

Answer: (A)

Gas exchange happens where the air sacs, the alveoli, meet the blood in the surrounding capillaries. Oxygen in the alveolar air diffuses through the very thin alveolar and capillary walls into the blood, while carbon dioxide moves from the blood into the alveolar air to be breathed out. This occurs across the respiratory membrane, a tiny barrier formed by the alveolar epithelium, its basement membrane, and the capillary endothelium, which together create a short diffusion path and a huge surface area for efficient exchange. Surfactant keeps the alveoli open and stable, supporting consistent gas exchange. The other airway parts—the trachea, bronchi, and larynx—are conducting passages; they transport, warm, and humidify air but do not participate in gas exchange due to thicker walls and lack of the close, extensive blood vessel network that alveoli have.