Which solid organ is part of the urinary system that controls fluid balance, filters waste, and controls pH?

• A. Kidneys
• B. Spleen
• C. Liver
• D. Ureter

Answer: (A)

Fluid balance, waste filtration, and pH control are the kidneys’ primary jobs. Blood is filtered through nephrons, where water and essential ions are reabsorbed to maintain proper volume and electrolyte levels, while wastes are excreted as urine. They also regulate acid-base balance by adjusting hydrogen ion excretion and bicarbonate reabsorption, keeping blood pH within a tight range. Hormones like ADH and aldosterone fine-tune water reabsorption and sodium handling to stabilize fluid volume and pressure. The other options don’t perform all these roles: a ureter is just a tube that carries urine to the bladder; the spleen is a immune-related organ that filters blood for immune purposes; the liver metabolizes nutrients and toxins but doesn’t directly regulate urine formation or acid-base balance.