Which organ filters blood and regulates fluid balance?

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

Answer: (A)

The kidneys are the organs that filter blood and regulate fluid balance. In the kidneys, tiny filtering units called nephrons run the process of glomerular filtration, where plasma is filtered to form a filtrate. As this filtrate travels through the tubules, essential water and solutes are reabsorbed back into the bloodstream and wastes are secreted into the filtrate to be excreted as urine. Through this, the kidneys control blood volume, electrolyte levels, and acid-base balance, tuning urine output in response to hydration and hormonal signals such as ADH and aldosterone.

Other organs have different primary roles. The lungs exchange gases and help manage acid-base balance, but they don’t filter blood to regulate fluid balance. The liver processes nutrients and detoxifies, while the heart pumps blood but doesn’t filter it to control body fluid levels.