Which organ filters blood and forms urine?

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The kidneys are responsible for filtering the blood and forming urine. Blood flows into the kidneys and is filtered by tiny units called nephrons, with the initial filtration occurring in the glomerulus. The resulting filtrate is then processed along the tubules, where water, electrolytes, and useful substances are reabsorbed and wastes are secreted, ultimately becoming urine. This urine is collected in the renal pelvis, moves down the ureters, and is excreted from the body. Through this process the kidneys regulate fluid and electrolyte balance and help maintain acid-base homeostasis, while also removing waste products.

Other organs have different primary roles: the liver processes nutrients and detoxifies various substances; the spleen helps filter blood and supports immune functions; the lungs exchange gases. They don’t form urine, which is why the kidneys uniquely fit the description.