Excretion: The removal from organisms of the toxic waste products of metabolism, toxic materials and substances in excess of requirements. It keeps the internal environment safe and balanced.
Egestion: The removal of undigested food (faeces) from the gut through the anus; it is not excretion because faeces is never absorbed into cells.
Metabolism: The sum of all the chemical reactions taking place in the cells of an organism, many of which produce waste products that must be excreted.
Waste product of metabolism: A substance such as carbon dioxide or urea produced by chemical reactions in cells that the body must remove.
Excretory product: Any waste substance removed from the body by excretion, mainly carbon dioxide, urea, excess water and excess ions.
Carbon dioxide: A waste gas produced by aerobic respiration in cells; it is excreted from the body through the lungs.
Urea: A nitrogen-containing waste substance made in the liver from excess amino acids; it is excreted by the kidneys in urine.
Excess water: Water taken in beyond the body's needs, removed mainly by the kidneys in urine and partly by the skin in sweat.