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The exchange of gases in the lungs occurs in the alveoli, which are tiny, sac-like structures that are part of the lungs. The alveoli are surrounded by a network of capillaries, allowing oxygen and carbon dioxide to diffuse through their walls. This diffusion happens because of the concentration gradient; oxygen moves from the high concentration in the alveoli to the lower concentration in the blood, while carbon dioxide moves in the opposite direction from the blood (where its concentration is higher) to the alveoli (where its concentration is lower) to be exhaled.

The other structures listed play different roles in the respiratory system. The bronchi are larger air passages that lead from the trachea into the lungs but do not participate in gas exchange. Similarly, the bronchioles are smaller branches of the bronchi that direct air into the alveoli but are not the site of gas exchange themselves. The trachea serves as the main airway path that carries air to the bronchi but, like the bronchi and bronchioles, it also does not exchange gases. Hence, it is the alveoli that are specifically designed for the critical process of gas exchange, making them the correct answer.