The waste which is present in the blood has to be removed from the body. How can this be done? A mechanism to filter the blood is required. This is done by the blood capillaries in the kidneys. When the blood reaches the two kidneys, it contains both useful and harmful substances. The useful substances are absorbed back into the blood. The wastes dissolved in water are removed as urine. From the kidneys, the urine goes into the urinary bladder through tube-like ureters. It is stored in the bladder and is passed out through the urinary opening at the end of a muscular tube called urethra\xa0. The kindeys, ureters, bladder and urethra form the excretory system.\xa0An adult human being normally passes about 1- 1.8 L of urine in 24 hours. The urine consists of 95% water,2.5% urea and 2.5% other waste products.