The cells in the body perform various activities. Waste products such as urea, uric acid and excess water are formed and have to be removed from the body. The removal of waste products is termed excretion. The organs that help in the process of excretion constitute the excretory system, and include the kidneys, the ureters, the urinary bladder and the urethra. The useful and harmful substances in blood enter the kidneys, where the capillaries filter it. The useful substances are reabsorbed, but the harmful substances are removed along with water in the form of urine. The urine from the kidneys comes down through long tubes called the ureters, which open into the urinary bladder where the urine is stored temporarily. When the bladder fills, the urine is disposed.