The excretory system is a collection of organs that allow for the removal of waste from the body. The liver and kidneys function to remove a variety of waste by-products from the body.
As nutrients from food get absorbed by the small intestine, anything not required passes through to the large intestine. Within the large intestine, any excess water, minerals and vitamins are absorbed before the solidified waste is passed through to the rectum to leave as feces. The blood itself may have excess levels of certain chemicals, in particular amino acids, which are harmful to the body if large amounts build up. The liver works to convert these amino acids into urea which is then processed by the kidneys to leave the body as urine.