Which structure directly leads to alveolar sacs?

The structure that directly leads to alveolar sacs is the alveolar ducts. Alveolar ducts are the final passages in the respiratory system before air reaches the alveolar sacs, which are clusters of tiny air sacs where gas exchange occurs. The alveolar ducts branch from the respiratory bronchioles and are lined with alveoli along their walls, allowing for efficient gas exchange as air moves into the alveolar sacs.

Understanding the pathway of air through the respiratory system is crucial. The bronchial tree, for instance, includes the main bronchi, secondary and tertiary bronchi, and leads to smaller bronchi and bronchioles, but it does not connect directly to the alveolar sacs. The trachea serves as the primary airway into the lungs, branching into the bronchi but is not directly involved in the final passages where gas exchange occurs. Respiratory bronchioles also contribute to the pathway but lead into alveolar ducts rather than directly into the alveolar sacs themselves. Thus, alveolar ducts are specifically designed to connect to the alveolar sacs, making them the correct choice for this question.