Excretion is the removal of the waste products of metabolism either by storing them in insoluble forms or by removing them from the body. Excretory organs are also responsible for maintaining the correct balance of body fluids (see excretory system). In vertebrates the excretory organs are the kidneys: blood flows through these and waste products are removed as urine.
Other forms of excretory organs include the Malpighian tubes of insects, arachnids, and myriapods, the contractile vacuole of protozoans, and the nephridia of Annelida. In plants, excretion usually takes the form of producing insoluble salts of waste products within the cells.