Which structure in the respiratory system leads to the alveoli?

The structure in the respiratory system that leads to the alveoli is the alveolar ducts. These ducts are small passageways that branch from the terminal bronchioles and lead directly to the alveolar sacs, where gas exchange occurs. The alveolar ducts are lined with alveoli and are critical in facilitating the movement of air into the alveolar sacs, enhancing the surface area available for oxygen and carbon dioxide exchange.

The other structures listed have different roles in the respiratory process. Bronchodilators are medications used to relax the muscles around the airways, allowing for easier airflow, but they do not directly lead to the alveoli. Alveolar sacs are the clusters of alveoli but are destination points rather than conduits. The trachea, while essential for conducting air to the lungs, branches into the bronchi and does not connect directly to the alveoli. Thus, the alveolar ducts serve the specific function of transporting air directly to the sites of gas exchange within the lungs.