Excretion is the process by which metabolic wastes and other non-useful materials are eliminated from an organism. In vertebrates this is primarily carried out by the lungs,kidneys and skin. This is in contrast withsecretion, where the substance may have specific tasks after leaving the cell. Excretion is an essential process in all forms of life. For example, in mammals urine is expelled through the urethra, which is part of theexcretory system. In unicellular organisms, waste products are discharged directly through the surface of the cell.