The excretory system in all types of organisms has one main function: to maintain homeostasis within a given organism. Homeostasis is that condition in which all internal systems and chemicals of that organism are in consistent balance. In order to maintain homeostasis, an excretory system has to be capable of three main functions: it must be able to rid the organisms of waste products; it must be able to keep both the fluid and the salt content of the organism within normal parameters; and it must keep the concentration of other substances in body fluids at normal levels.

The primary waste products of all organisms are carbon dioxide, nitrogen-based substances (created by the breakdown of proteins into amino acids), and water. Carbon dioxide and some water excretion is performed by the respiratory system. Nitrogen and water are processed and released (excreted) by...