Hadith 31-131

[37] Their function is to process the waste products and excess fluid in the blood. Our bodies produce several kinds of wastes, including sweat, carbon dioxide gas, feces (also known as stool), and urine. These wastes exit the body in different ways. Sweat is released through pores (tiny holes) in the skin. Water vapor and carbon dioxide are exhaled (breathed out) from the lungs. And undigested food materials are formed into feces in the intestines and excreted from the body as solid waste in bowel movements. Urine, which is produced by the kidneys, contains the by-products of our body's metabolism - salts, toxins, and water - that end up in our blood. The kidneys and urinary tract (which includes the ureters, bladder, urethra, and the kidneys) filter and eliminate these waste substances from our blood. Without the kidneys, waste products and toxins would soon build up in the blood to dangerous levels.