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

The kidneys are not part of the respiratory system; they are vital organs in the urinary system responsible for filtering blood, producing urine, and regulating various bodily functions such as electrolyte balance and blood pressure.

In contrast, the nasal cavity, trachea, and bronchial tubes are essential components of the respiratory system. The nasal cavity serves as the entry point for air, filtering and humidifying it before it travels to the lungs. The trachea is the windpipe that connects the nasal cavity and larynx to the lungs, allowing the passage of air. The bronchial tubes branch off from the trachea and lead into the lungs, facilitating the distribution of air into the alveoli where gas exchange occurs. Thus, identifying the kidneys as outside the respiratory system underscores the distinct roles of these organ systems within the body.