Your blood flows through your kidneys, which are the key organs in the complex system that removes excess fluid and waste material from the blood. Blood that flows into your kidneys is diffused through filtering structures called nephrons. Each nephron contains a tuft of capillary blood vessels (glomerulus) and tiny tubules that lead to larger collecting tubes. The glomeruli filter fluid from your blood, extracting both waste products and substances your body needs — such as sodium, phosphorus and potassium. The substances your body needs are reabsorbed into your bloodstream. Waste products are excreted in the urine.