Which of the following structures is NOT part of the upper respiratory tract?

The alveoli are tiny air sacs located in the lungs where the exchange of oxygen and carbon dioxide takes place, which is a function of the lower respiratory system. The lower respiratory tract includes structures such as the trachea, bronchi, and lungs, whereas the upper respiratory tract comprises structures that are involved in the initial phases of breathing and air filtration, including the nasal cavity, pharynx, and larynx.

In contrast, the frontal and sphenoidal sinuses, as well as the larynx, are integral components of the upper respiratory system. The frontal sinuses are located in the forehead region and help in humidifying and warming the air inhaled, while the sphenoidal sinuses are situated behind the nose and play a similar role. The larynx is crucial for voice production and acts as a passageway between the pharynx and the trachea, also contributing to the protection of the lower airways.

Understanding that the alveoli are fundamentally involved in gas exchange rather than the filtering and humidifying of inhaled air clarifies why they are categorized outside the upper respiratory system.