Which structure is NOT included in the respiratory zone of the lung?

The respiratory zone of the lung is primarily involved in the gas exchange process and includes structures where this function occurs, such as respiratory bronchioles, alveolar ducts, and alveolar sacs.

The respiratory bronchioles are the transitional structures that connect the conducting zone to the gas exchange areas, and they are the first parts of the respiratory zone. Alveolar ducts lead into the alveolar sacs, which are clusters of alveoli where the exchange of oxygen and carbon dioxide takes place. These three structures are crucial for effective respiration, as they facilitate the movement of air into the regions where it can be absorbed into the bloodstream and where carbon dioxide is expelled.

In contrast, the trachea is part of the conducting zone of the respiratory system. It serves as the main airway that directs air into the lungs but is not directly involved in gas exchange. Instead, its primary role is to convey air to the bronchi and subsequently to smaller airway passages that lead to the respiratory zone. Thus, the trachea is excluded from the respiratory zone, marking it as the correct answer in this context.