The waste products and unwanted substances are removed from the body through excretory system. During the digestion of food, certain substances like ammonia, urea and uric acid are produced. Metabolic activities of the body produce wastes such as carbon dioxide and water. Since these waste products are poisonous in nature, they should be removed from the body.
Water vapor and carbon dioxide come out of the body in the process of respiration. About 0.2 liter of carbon dioxide is given out every minute by a person. Ammonia, urea and uric acid reach the liver where ammonia is converted into urea. Urea and uric acid get mixed in the blood stream and go to the kidneys form the liver. The kidneys work as filters and separate out these substances form the blood. About 120 ml of blood of filtered by the kidney every minute. The whole blood is filtered by the kidneys about 30 times a day.
The filtered substances do not contain urea and uric acid only but some other harmful substances also. All these substances are excreted from the body as urine. A person passes out about one liter of urine in a day.
Our skin is also an important excretory organ. Water and salts come out from the skin in the form of sweat. About 0.7 liter of sweat along with small amount of salt is excreted from the human body each day through perspiration.
The Human Excretory System