Which organ is primarily involved in excretion of waste and water balance as part of the urinary system?

• A. Heart
• B. Kidneys
• C. Lungs
• D. Liver

Answer: (B)

Understanding how the urinary system excretes waste and maintains water balance is the main idea here. The kidneys are the key players because they filter the blood to remove waste products like urea and creatinine, and they adjust how much water and electrolytes are kept or lost. In the kidneys, blood is filtered through tiny units called nephrons; the filtrate becomes urine, and along the tubules, essential substances are reabsorbed while wastes are secreted into the filtrate. This process concentrates waste into urine and keeps body fluids, electrolytes, and pH in balance, which is why the kidneys are the primary organ responsible for both waste excretion and water regulation in the urinary system. Other organs listed have different roles: the heart pumps blood, the lungs remove gases and some water vapor, and the liver handles metabolism and detoxification, but they do not perform the main excretion and water-balance functions of the urinary system.