The four organs of excretion are the kidneys, the lungs, the skin and the liver. Excretion is the process of elimination of toxic waste products and substances in excess of requirements from the human body.
The kidneys are the primary excretory organs that eliminate waste products in the form of urine. The main components of urine are surplus water and nitrogenous wastes obtained from urea, uric acid and breakdown of proteins.
The liver's principal function is to produce bile, through which it excretes cholesterol, drugs and steroid hormones. The liver also purifies any toxic substance that may enter the body through the alimentary canal.
The lungs help to remove water vapor and carbon dioxide by means of exhalation and diffusion. The skin's sweat glands excrete perspiration. Foreign elements are also excreted in the hair and nails.