Excretion refers to the system of excretion of body wastes. The system that helps in excretion is called excretory system. As a result of metabolism, pollutants like water , carbon dioxide , urea etc. are produced in the body. If these are not given regularly, there is loss of health. These contaminants accumulate in the body and cause poisoning and can lead to death. All these wastes are excreted mainly through respiration , air , sweat and urine. The three excretory organs are the lungs , the skin and the tree. The kidneys release nitrogen-containing fluids , contaminants from the human body . Urine excretes 80% of nitrogenous wastes. Therefore, the nitrogenous fluid-releasing organ is called the main excretory organ.