Which structure does NOT belong to the major structures of the respiratory system?

The diaphragm is primarily a muscular structure involved in the mechanics of breathing but is not classified as a major structure of the respiratory system itself. The diaphragm facilitates the movement of air into and out of the lungs by contracting and relaxing, which changes the volume of the thoracic cavity. However, it does not comprise the air passage or the gas exchange structures.

In contrast, the nasal cavities, trachea, and alveoli are integral components of the respiratory system. The nasal cavities filter, warm, and humidify the air we breathe, while the trachea serves as the main airway leading to the bronchi and lungs. The alveoli are tiny sacs where gas exchange occurs, allowing oxygen to enter the bloodstream and carbon dioxide to be expelled. Thus, the diaphragm's role, while essential for respiration, is supportive rather than structural in the context of the respiratory system's major components.