In the respiratory system, where does the actual site of gas exchange between air and blood occur?

Gas exchange between air and blood occurs at the alveoli, the tiny air sacs clustered at the terminal ends of the bronchiolar tree. The alveolar walls are extremely thin and surrounded by a dense network of pulmonary capillaries, creating the ideal structure for oxygen to diffuse from inhaled air into the bloodstream and for carbon dioxide to move in the opposite direction out of the blood. This interface is called the alveolar-capillary membrane, and its enormous collective surface area — estimated at around 70 square meters in an adult — makes efficient gas exchange possible. The pleura is the double-layered membrane surrounding the lungs; it provides mechanical protection and reduces friction during breathing but plays no direct role in gas exchange. The bronchi are large conducting airways that direct airflow toward the lungs but are lined with mucus-producing cells and cilia rather than a gas-exchange epithelium. Bronchioles are smaller conducting passages that continue to branch before terminating in alveolar ducts and sacs; while terminal bronchioles lead to alveoli, the bronchioles themselves do not perform gas exchange. Only the alveoli possess the ultra-thin type I pneumocyte lining and the proximity to capillaries needed for the diffusion of respiratory gases.