Which organ is primarily responsible for filtering blood to form urine?

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

Answer: (B)

The kidneys are responsible for filtering blood to form urine. They remove waste products like urea and creatinine and regulate the body's fluid and electrolyte balance, which helps maintain overall homeostasis. Blood flows into the kidneys and is filtered through the glomeruli, producing a filtrate. As this filtrate moves through the tubules, much of the water, along with essential ions and nutrients, is reabsorbed back into the bloodstream, while wastes and any excess ions are concentrated into urine to be excreted. This process also helps maintain acid-base balance by adjusting hydrogen ion and bicarbonate levels.

Other organs have important jobs, but they don’t form urine. The liver detoxifies chemicals and produces bile; the spleen filters blood and supports the immune system; and the lungs exchange gases and help regulate pH, but urine formation is a function mainly of the kidneys.