Eliminates nitrogenous wastes and regulates water, electrolyte and acid-base balance.

• A. Urinary system
• B. Digestive system
• C. Circulatory system
• D. Lymphatic system

Answer: (A)

The kidneys are responsible for filtering blood to remove nitrogenous wastes and for finely tuning body fluids, electrolytes, and pH. They convert waste products from protein metabolism into urine (nitrogenous wastes like urea and creatinine) that is eliminated from the body. At the same time, they regulate water balance by adjusting how much water is reabsorbed into the bloodstream versus excreted, which controls urine concentration and overall fluid volume. They manage electrolyte levels by reabsorbing or secreting ions such as sodium, potassium, and bicarbonate, helping maintain osmolarity and nerve/muscle function. For acid-base balance, the kidneys reclaim bicarbonate and excrete hydrogen ions, keeping blood pH within a narrow range. While other systems interact with waste and fluids, the urinary system is specialized for eliminating nitrogenous wastes and maintaining water, electrolyte, and acid-base balance.