Which of the following organs is NOT involved in the respiratory system?

The kidneys are not involved in the respiratory system as they are primarily part of the urinary system, responsible for filtering blood and excreting waste products through urine. In contrast, the diaphragm, alveoli, and trachea play essential roles in the process of respiration. The diaphragm is a muscle that aids in inhalation and exhalation by contracting and relaxing to change the volume of the thoracic cavity. The alveoli are tiny air sacs in the lungs where gas exchange occurs; they allow oxygen to enter the bloodstream and carbon dioxide to be expelled. The trachea, or windpipe, serves as the primary airway, conducting air in and out of the lungs. Understanding the distinct functions of these organs helps clarify why the kidneys do not belong to the respiratory system.