Illustration showing kidney cross section

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 flow through the ureters — the tubes that lead to the bladder.