The excretory system removes the potentially toxic substances that accumulate from metabolic processes. In mammals, the primary organs used to filter the blood of excess chemicals, particularly nitrogenous wastes, are the kidneys. Other organs involved include the ureters that carry urine to the bladder, which stores the wastes until they can be excreted via the urethra.
In mammals, urine is used to collect nitrogenous waste in the form of either uric acid or, as in humans, urea. Urea is a water-soluble chemical, so the body can easily use excess water to help get rid of it. Uric acid, on the other hand, is not water soluble and is excreted as a solid by species such as birds with very little water. This is also used by many species in arid climates where using water for excretion would be too wasteful.
Very large amounts of water and chemicals are taken in by the kidneys, but a large majority is returned to the blood. Kidneys are at least as necessary for regulating the fluid balance in the body as they are for excreting waste. Other organs involved in the excretory system include the liver and the skin, which removes some toxic and waste materials via sweating.