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

The kidneys are a crucial part of the urinary system, responsible for filtering blood to remove waste products and excess substances, which are then excreted as urine. They play a vital role in maintaining fluid and electrolyte balance in the body, regulating blood pressure, and contributing to the body's overall homeostasis.

In the context of the options provided, the lungs, heart, and liver do not belong to the urinary system. The lungs are part of the respiratory system, facilitating gas exchange. The heart is part of the circulatory system, pumping blood throughout the body. The liver, while essential for various metabolic processes, including detoxification and bile production, is part of the digestive system. Thus, the kidneys distinctly align with the functions and components of the urinary system, making them the correct answer.