The direct exchange of oxygen and carbon dioxide in the lungs is a function of which of the following structures?

The alveoli are small, balloon-like structures located at the end of the bronchioles in the lungs and are crucial for the process of gas exchange. Each alveolus is surrounded by a network of capillaries, where the exchange of oxygen and carbon dioxide occurs. Oxygen from the air within the alveoli diffuses through the alveolar walls and into the bloodstream, while carbon dioxide from the blood diffuses into the alveoli to be exhaled. This efficient exchange is facilitated by the thin walls of the alveoli, which allow for the rapid transfer of gases.

In contrast, the bronchi serve primarily as air passages that direct air into the lungs but do not play a direct role in gas exchange. The trachea acts as the main airway, conducting air to the bronchi but also without involvement in gas exchange. The pleurae are membranes that envelop the lungs and facilitate movement during respiration, but they do not participate in the exchange of gases themselves.

Thus, the alveoli are essential for the direct exchange of oxygen and carbon dioxide in the lungs, making them the correct answer to the question.