Editing Shark (section)

==Physiology==

===Buoyancy===
Unlike bony fish, sharks do not have gas-filled swim bladders for buoyancy. Instead, sharks rely on a large liver filled with oil that contains [[squalene]], and their cartilage, which is about half the normal density of bone.<ref name=Cartilagious/> Their liver constitutes up to 30% of their total body mass.<ref name="Collins">{{cite book
|last1=Compagno |first1=Leonard
|last2=Dando |first2=Marc
|last3=Fowler |first3=Sarah
|title=Sharks of the World
|publisher=Collins Field Guides
|year=2005
|isbn=978-0-00-713610-0
|oclc=183136093
}}</ref> The liver's effectiveness is limited, so sharks employ [[dynamic lift (fish)|dynamic lift]] to maintain depth while swimming. [[Sand tiger shark]]s store air in their stomachs, using it as a form of swim bladder. Bottom-dwelling sharks, like the [[nurse shark]], have negative buoyancy, allowing them to rest on the ocean floor.

Some sharks, if inverted or stroked on the nose, enter a natural state of [[tonic immobility]]. Researchers use this condition to handle sharks safely.<ref name=ParttGruberTaniuchi>{{cite book
| last1=Pratt|first1=H. L. Jr|last2=Gruber |first2=S. H. |last3=Taniuchi |first3=T
| title=Elasmobranchs as living resources: Advances in the biology, ecology, systematics, and the status of the fisheries
| publisher=NOAA Tech Rept.
| year=1990}}</ref>

===Respiration===
Like other fish, sharks extract oxygen from seawater as it passes over their [[gill]]s. Unlike other fish, shark gill slits are not covered, but lie in a row behind the head. A modified slit called a [[Spiracle (vertebrates)|spiracle]] lies just behind the eye, which assists the shark with taking in water during [[Aquatic respiration|respiration]] and plays a major role in bottom–dwelling sharks. Spiracles are reduced or missing in active [[pelagic]] sharks.<ref name="Gilbertson">{{cite book
| last = Gilbertson| first = Lance
|title = Zoology Laboratory Manual
| publisher = McGraw-Hill Companies, Inc.
| year = 1999
| location = New York
|isbn= 978-0-07-237716-3}}</ref> While the shark is moving, water passes through the mouth and over the gills in a process known as "ram ventilation". While at rest, most sharks pump water over their gills to ensure a constant supply of oxygenated water. A small number of species have lost the ability to pump water through their gills and must swim without rest. These species are ''obligate ram ventilators'' and would presumably [[asphyxiate]] if unable to move. Obligate ram ventilation is also true of some pelagic bony fish species.<ref>{{cite web
| url = http://www.textbookleague.org/73shark.htm
| title = Deep Breathing
| first = William J.
| last = Bennetta
| year = 1996
| access-date = 2007-08-28
| archive-url = https://web.archive.org/web/20070814075030/http://www.textbookleague.org/73shark.htm
| archive-date = 2007-08-14
| url-status = usurped
}}</ref><ref>{{cite web|url=http://www.flmnh.ufl.edu/fish/education/questions/basics.html#sleep |title=Do sharks sleep |publisher=Flmnh.ufl.edu <!--|access-date=2010-09-23--> |archive-url=https://web.archive.org/web/20100918164840/http://www.flmnh.ufl.edu/fish/education/questions/basics.html#sleep |archive-date=2010-09-18|date=2017-05-02 }}</ref>

The [[respiratory]] and [[circulatory]] process begins when deoxygenated [[venous blood]] travels to the shark's two-chambered [[heart]]. Here, the shark pumps blood to its gills via the ventral [[aorta]] where it branches into [[wikt:afferent#Adjective|afferent]] [[branchial arteries]]. [[Gas exchange]] takes place in the gills and the reoxygenated blood flows into the [[wikt:efferent#Adjective|efferent]] branchial arteries, which come together to form the [[dorsal aorta]]. The blood flows from the dorsal aorta throughout the body. The deoxygenated blood from the body then flows through the [[posterior cardinal vein]]s and enters the posterior cardinal [[sinus (anatomy)|sinus]]es. From there venous blood re-enters the heart [[Ventricle (heart)|ventricle]] and the cycle repeats.<ref>{{cite web|url=http://www.seaworld.org/animal-info/info-books/sharks-&-rays/anatomy.htm|title=SHARKS & RAYS, SeaWorld/Busch Gardens ANIMALS, CIRCULATORY SYSTEM|publisher=Busch Entertainment Corporation|access-date=2009-09-03|archive-url=https://web.archive.org/web/20090424033204/http://www.seaworld.org/animal-info/info-books/sharks-%26-rays/anatomy.htm|archive-date=2009-04-24|url-status=dead}}</ref>

===Thermoregulation===
Most sharks are "cold-blooded" or, more precisely, [[poikilotherm]]ic, meaning that their internal [[Thermoregulation|body temperature]] matches that of their ambient environment. Members of the family [[Lamnidae]] (such as the [[shortfin mako shark]] and the [[great white shark]]) are [[homeothermy|homeothermic]] and maintain a higher body temperature than the surrounding water. In these sharks, a strip of [[aerobic metabolism|aerobic]] red muscle located near the center of the body generates the heat, which the body retains via a [[countercurrent exchange]] mechanism by a system of [[blood vessel]]s called the [[rete mirabile]] ("miraculous net"). The [[common thresher]] and [[bigeye thresher]] sharks have a similar mechanism for maintaining an elevated body temperature.<ref>{{cite web
|url=http://elasmo-research.org/education/topics/p_warm_body_1.htm
|title=Fire in the Belly of the Beast
|last=Martin
|first=R. Aidan
|date=April 1992
|publisher=ReefQuest Centre for Shark Research
|access-date=2009-08-21
|archive-url=https://web.archive.org/web/20090917074708/http://www.elasmo-research.org/education/topics/p_warm_body_1.htm
|archive-date=2009-09-17
|url-status=live
}}</ref>

Larger species, like the whale shark, are able to conserve their body heat through sheer size when they dive to colder depths. The [[scalloped hammerhead]] closes its mouth and gills when diving to depths of around 800 metres, holding its breath until it reaches warmer waters again.<ref>{{Cite journal|url=https://www.nature.com/articles/d41586-023-01569-x|title=Hammerhead sharks are first fish found to 'hold their breath'|first=Bianca|last=Nogrady|date=May 11, 2023|journal=Nature|volume=617|issue=7962|pages=663|via=www.nature.com|doi=10.1038/d41586-023-01569-x|pmid=37169849 |bibcode=2023Natur.617..663N |s2cid=258639015 }}</ref>

===Osmoregulation===
In contrast to bony fish, with the exception of the [[coelacanth]],<ref>{{cite journal|jstor=35431|doi=10.1098/rspb.1980.0054|pmid=6106196|title=Chemistry of the Body Fluids of the Coelacanth, Latimeria chalumnae|journal=Proceedings of the Royal Society B: Biological Sciences|volume=208|issue=1172|pages=329–347|year=1980|last1=Griffith|first1=R. W|bibcode=1980RSPSB.208..329G|s2cid=38498079}}</ref> the blood and other tissue of sharks and [[Chondrichthyes]] is generally [[Isotonicity|isotonic]] to their marine environments because of the high concentration of [[urea]] (up to 2.5%<ref>{{cite web|url=http://www.sharkproject.org/haiothek/index_e.php?site=anatomie_2|title=Sharkproject|access-date=31 December 2016|url-status=dead|archive-url=https://web.archive.org/web/20160304031932/http://www.sharkproject.org/haiothek/index_e.php?site=anatomie_2|archive-date=4 March 2016}}</ref>) and [[trimethylamine]] N-oxide (TMAO), allowing them to be in [[osmotic]] balance with the seawater. This adaptation prevents most sharks from surviving in freshwater, and they are therefore confined to [[Marine (ocean)|marine]] environments. A few exceptions exist, such as the [[bull shark]], which has developed a way to change its [[kidney]] function to excrete large amounts of urea.<ref name="Collins" /> When a shark dies, the urea is broken down to ammonia by bacteria, causing the dead body to gradually smell strongly of ammonia.<ref>{{cite web
| url=http://www.fao.org/docrep/009/a0212e/A0212E18.htm
| title=Management techniques for elasmobranch fisheries: 14. Shark Utilization
| first=John A.
| last=Musick
| publisher=FAO: Fisheries and Aquaculture Department
| access-date=2008-03-16
| year=2005
| archive-url=https://web.archive.org/web/20110722151220/http://www.fao.org/docrep/009/a0212e/A0212E18.htm
| archive-date=2011-07-22
| url-status=live
}}</ref><ref>{{cite web
| url=http://www.ocean.udel.edu/mas/seafood/mako.html
| title=MAKO SHARK Isurus oxyrinchus
| first=Thomas
| last=Batten
| publisher=Delaware Sea Grant, University of Delaware
| access-date=2008-03-16
| archive-url=https://web.archive.org/web/20080311091749/http://www.ocean.udel.edu/mas/seafood/mako.html
| archive-date=2008-03-11
| url-status=dead
}}</ref>

Research in 1930 by [[Homer W. Smith]] showed that sharks' urine does not contain sufficient sodium to avoid [[hypernatremia]], and it was postulated that there must be an additional mechanism for salt secretion. In 1960 it was discovered at the [[Mount Desert Island Biological Laboratory]] in [[Salsbury Cove, Maine]] that sharks have a type of [[salt gland]] located at the end of the intestine, known as the "rectal gland", whose function is the secretion of chlorides.<ref>{{cite journal |last1=Forrest |first1=John N. (Jnr.) |title=The Shark Rectal Gland Model: A Champion of Receptor Mediated Chloride Secretion Through CFTR |date=2016 |journal=Transactions of the American Clinical Climatological Association |volume=127 |pages=162–175 |pmid=28066051 |pmc=5216465 }}</ref>

===Digestion===
Digestion can take a long time. The food moves from the mouth to a J-shaped stomach, where it is stored and initial digestion occurs.<ref name="Digestion">{{cite web
|url=http://elasmo-research.org/education/white_shark/digestion.htm
|title=No Guts, No Glory
|last=Martin
|first=R. Aidan
|publisher=ReefQuest Centre for Shark Research
|access-date=2009-08-22
|archive-url=https://web.archive.org/web/20090811032322/http://www.elasmo-research.org/education/white_shark/digestion.htm
|archive-date=2009-08-11
|url-status=live
}}</ref> Unwanted items may never get past the stomach, and instead the shark either vomits or turns its stomachs inside out and ejects unwanted items from its mouth.<ref>{{cite news|last1=Potenza|first1=Alessandra|title=Sharks literally puke their guts out—here's why|url=https://www.theverge.com/tldr/2017/6/2/15725932/shark-puking-behavior-stress|access-date=21 June 2017|work=[[The Verge]]|date=20 June 2017|archive-url=https://web.archive.org/web/20170619112058/https://www.theverge.com/tldr/2017/6/2/15725932/shark-puking-behavior-stress|archive-date=19 June 2017|url-status=live}}</ref>

One of the biggest differences between the digestive systems of sharks and mammals is that sharks have much shorter intestines. This short length is achieved by the [[spiral valve]] with multiple turns within a single short section instead of a long tube-like intestine. The valve provides a long surface area, requiring food to circulate inside the short gut until fully digested, when remaining waste products pass into the [[cloaca]].<ref name="Digestion"/>

=== Fluorescence ===
A few sharks appear [[fluorescent]] under blue light, such as the [[swell shark]] and the [[chain catshark]], where the [[fluorophore]] derives from a [[metabolite]] of [[kynurenic acid]].<ref>{{cite journal |last1=Park |first1=Hyun Bong |last2=Lam |first2=Yick Chong |last3=Gaffney |first3=Jean P. |last4=Weaver |first4=James C. |last5=Krivoshik |first5=Sara Rose |last6=Hamchand |first6=Randy |last7=Pieribone |first7=Vincent |last8=Gruber |first8=David F. |last9=Crawford |first9=Jason M. |title=Bright Green Biofluorescence in Sharks Derives from Bromo-Kynurenine Metabolism |journal=iScience |date=27 September 2019 |volume=19 |pages=1291–1336 |doi=10.1016/j.isci.2019.07.019|pmid=31402257 |pmc=6831821 |bibcode=2019iSci...19.1291P |doi-access=free }}</ref>