Which of the following organs is part of the respiratory system?

The trachea is a crucial part of the respiratory system because it serves as the main airway that directs air into and out of the lungs. It connects the larynx (voice box) to the bronchi, which lead to each lung. The trachea is lined with ciliated mucous membrane that helps filter and humidify the air before it reaches the delicate tissues of the lungs. Its structure, a series of cartilaginous rings, provides both flexibility and stability, allowing it to maintain an open passage for air flow during breathing. This function is essential for gas exchange, which is the primary purpose of the respiratory system.

In contrast, the stomach, liver, and kidneys belong to different systems: the stomach is part of the digestive system, the liver serves various roles including detoxification and metabolism, and the kidneys are key components of the urinary system, responsible for filtering blood and producing urine. Each of these organs performs vital functions unrelated to respiration, highlighting the specificity of the trachea's role in breathing and air passage.