Excretion is a process by which animals remove nitrogen containing waste products from their bodies. Other forms of waste removal that are not covered under this definition of excretion include blood-gas transport which involves the removal of carbon dioxide waste and body-temperature regulation which considers the removal of heat. These topics are examined in discussions of respiration and thermoregulation.

The basic physiological problem that excretion has evolved to overcome is the problem of maintaining a consist internal environment. This process known as homeostasis, involves the removal and gain of equal amounts of material. The most important compounds involved in excretion are water and nitrogen. Water regulation is crucial to maintaining osmotic pressures in the cell. When too much is present, a cell will burst. If too little is present, the chemical reactions in the cell will not proceed correctly. Nitrogen waste is a result of the chemical breakdown...