Where does gas exchange primarily occur in the lungs?

The alveoli are the primary site for gas exchange in the lungs due to their unique structure and function. These small, air-filled sacs are located at the ends of the bronchioles and provide a large surface area for oxygen and carbon dioxide exchange to occur. The walls of the alveoli are extremely thin and are surrounded by a network of capillaries. This design facilitates the diffusion of gases; oxygen from the inhaled air moves into the blood in the capillaries, while carbon dioxide, which is a waste product of metabolism, moves from the blood into the alveoli to be exhaled. Because of this critical role in respiration, the alveoli are often referred to as the functional units of the lungs.

In contrast, the other options (atria, bronchi, and trachea) do not serve the same function in gas exchange. Atria are chambers of the heart and are not involved in lung function. The bronchi are air passages that lead air to the lungs but do not facilitate the actual exchange of gases. The trachea is the windpipe that conducts air from the throat to the bronchi but also lacks the structures necessary for gas exchange.