Editing Urea (section)

==Physiology==
Amino acids from ingested food (or produced from catabolism of muscle protein) that are used for the synthesis of proteins and other biological substances can be oxidized by the body as an alternative source of energy, yielding urea and [[carbon dioxide]].<ref>{{cite journal | vauthors = Sakami W, Harrington H | title = Amino acid metabolism | journal = Annual Review of Biochemistry | volume = 32 | issue = 1 | pages = 355–98 | year = 1963 | pmid = 14144484 | doi = 10.1146/annurev.bi.32.070163.002035 }}</ref> The oxidation pathway starts with the removal of the amino group by a [[transaminase]]; the amino group is then fed into the [[urea cycle]]. The first step in the conversion of amino acids into [[Metabolic waste#Nitrogen wastes|metabolic waste]] in the liver is removal of the alpha-amino nitrogen, which produces [[ammonia]]. Because ammonia is toxic, it is excreted immediately by fish, converted into [[uric acid]] by birds, and converted into urea by mammals.<ref>{{cite web | title = Urea | publisher = [[Imperial College London]] | url = http://www.ch.ic.ac.uk/rzepa/mim/environmental/html/urea_text.htm | access-date = 2015-03-23 }}</ref>

Ammonia ({{chem2|NH3}}) is a common byproduct of the metabolism of nitrogenous compounds. Ammonia is smaller, more volatile, and more mobile than urea. If allowed to accumulate, ammonia would raise the [[pH]] in cells to toxic levels. Therefore, many organisms convert ammonia to urea, even though this synthesis has a net energy cost. Being practically neutral and highly soluble in water, urea is a safe vehicle for the body to transport and excrete excess nitrogen.

Urea is synthesized in the body of many organisms as part of the [[urea cycle]], either from the oxidation of [[amino acid]]s or from [[ammonia]]. In this cycle, [[amino]] groups donated by ammonia and {{sc|L}}-[[aspartate]] are converted to urea, while {{sc|L}}-[[ornithine]], [[citrulline]], {{sc|L}}-[[argininosuccinate]], and {{sc|L}}-[[arginine]] act as intermediates. Urea production occurs in the [[liver]] and is regulated by [[N-acetylglutamate|''N''-acetylglutamate]]. Urea is then dissolved into the blood (in the [[reference ranges for blood tests|reference range]] of 2.5 to 6.7&nbsp;mmol/L) and further transported and excreted by the kidney as a component of [[urine]]. In addition, a small amount of urea is excreted (along with [[sodium chloride]] and water) in [[sweat]].

In water, the amine groups undergo slow displacement by water molecules, producing ammonia, [[ammonium ion]]s, and [[bicarbonate ion]]s. For this reason, old, stale urine has a stronger odor than fresh urine.

===Humans===
The [[renal urea handling|cycling of and excretion of urea by the kidneys]] is a vital part of mammalian metabolism. Besides its role as carrier of waste nitrogen, urea also plays a role in the [[countercurrent exchange system]] of the [[nephron]]s, that allows for reabsorption of water and critical ions from the excreted [[urine]]. Urea is reabsorbed in the [[inner medullary collecting duct]]s of the nephrons,<ref name=boron837>{{cite book |author=Walter F. Boron |title=Medical Physiology: A Cellular And Molecular Approach |publisher=Elsevier/Saunders |isbn=1-4160-2328-3 |year=2005 }} Page 837</ref> thus raising the [[osmolarity]] in the [[renal interstitium|medullary interstitium]] surrounding the [[thin descending limb of the loop of Henle]], which makes the water reabsorb.

By action of the [[urea transporter 2]], some of this reabsorbed urea eventually flows back into the thin descending limb of the tubule,<ref>{{cite book | pmid = 23737200| year = 2011| vauthors = Klein J, Blount MA, Sands JM | title = Comprehensive Physiology| volume = 1| issue = 2| pages = 699–729 | doi = 10.1002/cphy.c100030| chapter = Urea Transport in the Kidney| isbn = 9780470650714}}</ref> through the collecting ducts, and into the excreted urine. The body uses this mechanism, which is controlled by the [[antidiuretic hormone]], to create [[hyperosmotic]] urine — i.e., urine with a higher concentration of dissolved substances than the [[blood plasma]]. This mechanism is important to prevent the loss of water, maintain [[blood pressure]], and maintain a suitable concentration of [[sodium]] ions in the blood plasma.

The equivalent nitrogen content (in [[gram]]s) of urea (in [[Mole (unit)|mmol]]) can be estimated by the conversion factor 0.028&nbsp;g/mmol.<ref>Section 1.9.2 (page 76) in: {{cite book |author=Jacki Bishop |author2=Thomas, Briony |title=Manual of Dietetic Practice |publisher=Wiley-Blackwell |year=2007 |isbn=978-1-4051-3525-2}}</ref> Furthermore, 1&nbsp;gram of nitrogen is roughly equivalent to 6.25&nbsp;grams of [[protein]], and 1&nbsp;gram of protein is roughly equivalent to 5&nbsp;grams of [[muscle]] tissue. In situations such as [[muscle wasting]], 1&nbsp;mmol of excessive urea in the urine (as measured by urine volume in litres multiplied by urea concentration in mmol/L) roughly corresponds to a muscle loss of 0.67&nbsp;gram.

===Other species===
In [[marine biology|aquatic]] organisms the most common form of nitrogen waste is ammonia, whereas land-dwelling organisms convert the toxic ammonia to either urea or [[uric acid]]. Urea is found in the urine of [[mammal]]s and [[amphibian]]s, as well as some fish. Birds and [[saurian]] reptiles have a different form of nitrogen metabolism that requires less water, and leads to nitrogen excretion in the form of uric acid. [[Tadpole]]s excrete ammonia, but shift to urea production during [[metamorphosis (biology)|metamorphosis]]. Despite the generalization above, the urea pathway has been documented not only in mammals and amphibians, but in many other organisms as well, including birds, [[invertebrate]]s, insects, plants, [[yeast]], [[fungi]], and even [[microorganism]]s.<ref>{{Cite web|last=PubChem|title=urea cycle|url=https://pubchem.ncbi.nlm.nih.gov/pathway/PlantCyc:TEA_PWY-4984|access-date=2021-06-28|website=pubchem.ncbi.nlm.nih.gov|language=en}}</ref>