Where does gas exchange occur in the lungs?

Gas exchange in the lungs specifically occurs across the blood-air barrier, which consists of the alveolar epithelium and the endothelium of the capillaries. This barrier is exceptionally thin, allowing for efficient diffusion of oxygen from the alveoli into the blood in the capillaries, while simultaneously enabling carbon dioxide to move from the blood into the alveoli to be exhaled. The design of the alveoli, with their large surface area and moist lining, further facilitates this crucial exchange process.

The other options involve parts of the respiratory system that are important for air conduction and protection but are not directly involved in the transfer of gases. For instance, the bronchi and bronchioles serve primarily as passageways for air rather than sites for gas exchange. The pleural cavity is the space between the two pleurae surrounding the lungs and helps with lung inflation but does not participate in gas exchange. The trachea functions as a major airway transporting air to and from the lungs but likewise does not engage in gas exchange. Thus, the blood-air barrier is indeed the correct location where gas exchange occurs in the lungs.