Excretion

Excretionis one of the most basicfunctionsof life. It is the process of eliminatingwasteproducts ofmetabolismand other non-useful materials.It is an essential process in all forms of life. It contrasts withsecretion,where the substance may have specific tasks after leaving thecell.

Insingle-celled organisms,waste products are discharged directly through the surface of the cell.Multicellular organismsuse more complex methods. Higherplantseliminate gasses through thestomataon the surface of leaves. Animals have special excretory organs.

Mammalian excretion

Inmammals,excretion is the formation ofurinein thekidneys,and expellingcarbon dioxidefrom the lungs. These waste products are eliminated byurinationand breathing out respectively. If excretion does not occur in an organism, waste products accumulate, which eventually kill the organism.

Also, food waste is expelled through theanus.Other substances are secreted into thebile,and then eliminated in thefaeces.Theskinandlungsalso have excretory functions: the skin eliminates metabolic wastes likeureaandlactic acidthroughsweating,^p395and the lungs expelcarbon dioxide.

Some terms

• Mucociliary excretionis the excretion ofmucusin therespiratory system.Ciliawork to clear the small passages, andcoughingremovesphlegm.
• Biliary excretiongoes through thebile ductto theduodenumand removed in the faeces.
• Perspirationis the excretion of salts and water from the body, although the main purpose is to cool the body.

Other vertebrates

Aquaticanimals usually excreteammoniadirectly into the water, as this compound issolubleand there is ample water available for dilution. Interrestrialanimals ammonia-like compounds are converted into other nitrogenous materials as there is less water in theenvironment,and ammonia itself istoxic.

Reptilesandbirdsexcrete their nitrogenous wastes asuric acidin the form of a white paste. This allows efficientwaterretention and it can be stored more easily in theegg.Both uric acid and faeces are expelled through a common opening called thecloaca.Their waste is usually two-coloured: part white (uric acid) and part green or black (organic waste).

Many bird species, especiallyseabirds,can also excrete salt via specialized nasal salt glands, the salinesolutionleaving through nostrils in thebeak.