Excretion is the process by which waste products of
metabolism 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 with
secretion, where the substance may have specific tasks after leaving the
cell. Excretion is an essential process in all forms of life. In mammals
urine is carried out through the urethra and that is part of the
excretory system.