Which of the following is NOT a part of the upper respiratory system?

The alveoli are not part of the upper respiratory system; they are located in the lower respiratory system. Specifically, alveoli are tiny air sacs within the lungs where gas exchange occurs—oxygen is absorbed into the blood, and carbon dioxide is expelled. The respiratory system is often divided into the upper and lower portions, with the upper respiratory system consisting of structures such as the nose, nasal cavity, paranasal sinuses, pharynx (including both the nasopharynx and oropharynx), and the larynx. Understanding this distinction between upper and lower respiratory components is essential for recognizing where various functions, such as gas exchange, occur within the respiratory system.

The other structures listed, including the nasopharynx, oropharynx, and larynx, are all integral parts of the upper respiratory system, as they are involved in the passage of air from the outside environment to the lower respiratory structures.