Excretion

File:Urinary System Large Unlabeled.jpg

Mammals excrete urine through the urinary system.

Excretion is elimination of metabolic waste, which is an essential process in all organisms. In vertebrates, this is primarily carried out by the lungs, kidneys, and skin.<ref name='beckett'>Template:Cite book</ref> This is in contrast with secretion, where the substance may have specific tasks after leaving the cell. For example, placental mammals expel urine from the bladder through the urethra,<ref name="Wake1992">Template:Cite book</ref> which is part of the excretory system. Unicellular organisms discharge waste products directly through the surface of the cell.

During life activities such as cellular respiration, several chemical reactions take place in the body. These are known as metabolism. These chemical reactions produce waste products such as carbon dioxide, water, salts, urea and uric acid. Accumulation of these wastes beyond a level inside the body is harmful to the body. The excretory organs remove these wastes. This process of removal of metabolic waste from the body is known as excretion.

Processes across various types of life

Plants

In green plants, oxygen is a byproduct generated during photosynthesis, and exits through stomata, root cell walls, and other routes.<ref name="eb">Template:Cite book</ref> Other materials that are exuded by some plants — resin, saps, latex, are forced from the interior of the plant by hydrostatic pressures inside the plant and by absorptive forces of plant cells. These latter processes do not need added energy, as they act passively.<ref name=eb/> During the pre-abscission phase, deciduous plants excrete by leaf-fall.<ref name=eb/><ref name="ford">Template:Cite journal</ref>

Animals

File:Uric Acid.svg

Chemical structure of uric acid.

In animals, the main excretory products are carbon dioxide, ammonia (in ammoniotelics), urea (in ureotelics), uric acid (in uricotelics), guanine (in Arachnida), and creatine. The liver and kidneys clear many substances from the blood (for example, in renal excretion), and the cleared substances are then excreted from the body in the urine and feces.<ref>Template:Cite journal</ref>

Aquatic animals usually excrete ammonia directly into the external environment, as this compound has high solubility and there is ample water available for dilution. In terrestrial animals, ammonia-like compounds are converted into other nitrogenous materials, i.e. urea, that are less harmful as there is less water in the environment and ammonia itself is toxic. This process is called detoxification.<ref>Template:Cite web</ref>

Birds

File:Feces and uric-acid.jpg

White cast of uric acid defecated along with the dark feces by a lizard. Insects, birds and some other reptiles also use a similar mechanism.

Birds excrete their nitrogenous wastes as uric acid in the form of a paste. Although this process is metabolically more expensive, it allows more efficient water retention and it can be stored more easily in the egg. Many avian species, especially seabirds, can also excrete salt via specialized nasal salt glands, the saline solution leaving through nostrils in the beak.<ref>Template:Cite web</ref>

Insects

In insects, a system involving Malpighian tubules is used to excrete metabolic waste. Metabolic waste diffuses or is actively transported into the tubule, which transports the wastes to the intestines. The metabolic waste is then released from the body along with fecal matter.<ref>Template:Cite web</ref>

The excreted material may be called ejecta.<ref name="EOS1887">Template:Cite journal</ref> In pathology the word ejecta is more commonly used.<ref>Template:Cite book</ref>

External links

• UAlberta.ca, Animation of excretion
• Brian J Ford on leaf fall in Nature