What is Excretion?
'''Excretion''' is one of the most basic [[function (biology)|functions]] of life. It is the process of eliminating [[waste]] products of [[metabolism]] and other non-useful materials.<ref name='beckett'>{{cite book | last = Beckett | first = B. S. | title = Biology: a modern introduction | publisher = Oxford University Press | date = 1986 | page = 110 | isbn = 0199142602 }}</ref> It is an essential process in all forms of life. It contrasts with [[secretion]], where the substance may have specific tasks after leaving the [[Cell (biology)|cell]]. In [[Microorganism|single-celled organisms]], waste products are discharged directly through the surface of the cell. [[Multicellular organism]]s use more complex methods. Higher [[plant]]s eliminate gasses through the [[stomata]] on the surface of leaves. Animals have special excretory organs. == Mammalian excretion == In [[mammals]], excretion is the formation of [[urine]] in the [[kidneys]],<ref name=tiger>{{cite book | last = Tigerstedt | first = Dr. Robert | title = A text-book of human physiology | url = https://archive.org/details/cu31924001044175 | publisher = Appleton | date = 1906 | pages = [https://archive.org/details/cu31924001044175/page/n419 384]–390 }}</ref> and expelling [[carbon dioxide]] from the lungs. These waste products are eliminated by [[urination]] and 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 the [[anus]]. Other substances are secreted into the [[bile]], and then eliminated in the [[faeces]]. The [[skin]] and [[lung]]s also have excretory functions: the skin eliminates metabolic wastes like [[urea]] and [[lactic acid]] through [[sweat]]ing,<ref name=tiger/><sup>p395</sup> and the lungs expel [[carbon dioxide]]. === Some terms === * ''Mucociliary excretion'' is the excretion of [[mucus]] in the [[respiratory system]]. [[Cilia]] work to clear the small passages, and [[cough]]ing removes [[phlegm]]. * ''[[Bile|Biliar