Blood enters your kidneys through your renal arteries. Your kidneys remove excess fluid and waste material from your blood through units called nephrons. Each nephron contains a filter (glomerulus) that has a network of tiny blood vessels called capillaries. The glomeruli filter waste products and substances your body needs — such as sodium, phosphorus and potassium — which then pass through tiny tubules. The substances your body needs are reabsorbed into your bloodstream. The waste products and excess fluid form urine, which flows through the ureters — the tubes that lead to the bladder.