The function of many animal systems is to contribute toward homeostasis, or the maintenance of stable, internal conditions within narrow limits. In general, excretory systems help maintain homeostasis in organisms by regulating water balance and by removing harmful substances.

Osmoregulation is the absorption and excretion of water and dissolved substances (solutes) so that proper water balance (and osmotic pressure) is maintained between the organism and its surroundings. Here are two examples:

• Marine fish. The body of a marine fish is hypoosmotic with its environment — that is, it is less salty than the surrounding water. Thus, water is constantly lost by osmosis. In order to maintain their proper internal environment, marine fish constantly drink, rarely urinate, and secrete accumulated salts (that they acquire when they drink) out through their gills.

• Fresh water fish. The body of a fresh water fish is hyperosmotic, or saltier than the surrounding water. Thus, water constantly diffuses into the fish. In response, fresh water fish rarely drink, constantly urinate, and absorb salts through their gills.

Various excretory mechanisms have evolved in animals for the purpose of osmoregulation and for the removal of toxic substances. Toxic substances include by-products of cellular metabolism, such as the nitrogen products of protein breakdown.

• Contractile vacuoles are found in the cytoplasm of various protists, such as paramecia and amoebas. These vacuoles accumulate water, merge with the plasma membrane, and release the water to the environment.

• Flame cells are found in various Platyhelminthes, such as planaria. The flame cells are distributed along a branched tube system that permeates the flatworm. Body fluids are filtered across the flame cells, whose internal cilia move the fluids through the tube system. Wastes (water and salts) are excreted from the tube system through pores that exit the body.

• Nephridia (or metanephridia) occur in pairs within each segment of most annelids, such as earthworms. Interstitial fluids enter a nephridium through a ciliated opening called a nephrostome. Fluids are concentrated as they pass through the collecting tubule due to selective secretion of materials into the surrounding coelomic fluid. Blood capillaries that surround the tubule reabsorb the secreted materials. At the end of the collecting tubule, the concentrated waste materials are excreted through an excretory pore. Nephridia exemplify a tube-type excretory system, where body fluids are selectively filtered as they pass through the tube. Materials to be retained are secreted back into the body fluids, while concentrated wastes continue through the tube to be excreted at the far end.

• Malpighian tubules occur in many arthropods, such as terrestrial insects. Tubes attached to the midsection of the digestive tract of insects (midgut) collect body fluids from the hemolymph that bathe the cells. The fluids, which include both nitrogen wastes and materials to be retained (salts and water), are deposited into the midgut. As the fluids passthrough the hindgut of the insect (along with digested food), materials to be retained pass back out though the walls of the digestive tract. Wastes continue in the tract and are excreted through the anus.

• The vertebrate kidney consists of about a million individual filtering tubes called nephrons. Two kidneys produce waste fluids, or urine, which pass through ureters to the bladder for temporary storage. From the bladder, the urine is excreted through the urethra.