Editing Swim bladder (section)

==Structure and function==
[[File:Oste023c labelled.png|thumb|right|360px|{{center|Swim bladder from a bony (teleost) fish}}]]
[[File:GasbladderpumpingEng.png|thumb|right|360px|How gas is pumped into the swim bladder using [[counter-current exchange]].]]

The swim bladder normally consists of two gas-filled sacs located in the [[Dorsum (biology)|dorsal]] portion of the fish, although in a few primitive species, there is only a single sac. It has flexible walls that contract or expand according to the ambient [[pressure]]. The walls of the bladder contain very few [[blood vessels]] and are lined with [[guanine]] crystals, which make them impermeable to gases. By adjusting the gas pressurising organ using the gas gland or oval window, the fish can obtain neutral buoyancy and ascend and descend to a large range of depths. Due to the dorsal position it gives the fish lateral stability.

In [[physostome|physostomous]] swim bladders, a connection is retained between the swim bladder and the [[Gut (zoology)|gut]], the pneumatic duct, allowing the fish to fill up the swim bladder by "gulping" air. Excess gas can be removed in a similar manner.

In more derived varieties of fish (the [[physoclisti]]), the connection to the digestive tract is lost. In early life stages, these fish must rise to the surface to fill up their swim bladders; in later stages, the pneumatic duct disappears, and the [[gas gland]] has to introduce gas (usually [[oxygen]]) to the bladder to increase its [[volume]] and thus increase [[buoyancy]]. This process begins with the acidification of the blood in the ''[[rete mirabile]]'' when the gas gland excretes [[lactic acid]] and produces [[carbon dioxide]], the latter of which acidifies the blood via the [[bicarbonate buffer system]]. The resulting acidity causes the [[hemoglobin]] of the blood to lose its oxygen ([[Root effect]]) which then [[diffusion|diffuses]] partly into the swim bladder. Before returning to the body, the blood re-enters the ''rete mirabile'', and as a result, virtually all the excess carbon dioxide and oxygen produced in the gas gland diffuses back to the arteries supplying the gas gland via a [[countercurrent exchange|countercurrent multiplication loop]]. Thus a very high gas pressure of oxygen can be obtained, which can even account for the presence of gas in the swim bladders of deep sea fish like the [[eel]], requiring a pressure of hundreds of [[bar (unit)|bars]].<ref name=Pelster2001>{{cite journal |author=Pelster B |title=The generation of hyperbaric oxygen tensions in fish |journal=News Physiol. Sci. |volume=16 |issue= 6 |pages=287–91 |date=December 2001 |pmid=11719607 |doi= 10.1152/physiologyonline.2001.16.6.287 |s2cid=11198182 }}</ref> Elsewhere, at a similar structure known as the 'oval window', the bladder is in contact with blood and the oxygen can diffuse back out again. Together with oxygen, other gases are salted out{{clarify|date=May 2014}}<!--what does salted out mean?--> in the swim bladder which accounts for the high pressures of other gases as well.<ref name="biolbull.org">{{cite web |url=http://www.biolbull.org/cgi/content/abstract/161/3/440 |title=Secretion Of Nitrogen Into The Swimbladder Of Fish. Ii. Molecular Mechanism. Secretion Of Noble Gases |publisher=Biolbull.org |date=1981-12-01 |access-date=2013-06-24}}</ref>

The combination of gases in the bladder varies. In shallow water fish, the ratios closely approximate that of the [[Earth's atmosphere|atmosphere]], while deep sea fish tend to have higher percentages of oxygen. For instance, the [[eel]] ''[[Synaphobranchus]]'' has been observed to have 75.1% oxygen, 20.5% [[nitrogen]], 3.1% [[carbon dioxide]], and 0.4% [[argon]] in its swim bladder.

Physoclist swim bladders have one important disadvantage: they prohibit fast rising, as the bladder would burst. [[Physostome]]s can "burp" out gas, though this complicates the process of re-submergence.

The swim bladder in some species, mainly fresh water fishes ([[common carp]], [[catfish]], [[bowfin]]) is interconnected with the [[inner ear]] of the fish. They are connected by four bones called the [[Weberian ossicles]] from the [[Weberian apparatus]]. These bones can carry the vibrations to the [[saccule]] and the [[lagena (anatomy)|lagena]]. They are suited for detecting sound and vibrations due to its low density in comparison to the density of the fish's body tissues. This increases the ability of sound detection.<ref name=":0">{{Cite book |title=Vertebrates: Comparative Anatomy, Function, Evolution |last=Kardong |first=Kenneth |publisher=New York: McGraw-Hill Education |isbn=9780073524238 |pages=701 |date=2011-02-16}}</ref> The swim bladder can radiate the pressure of sound which help increase its sensitivity and expand its hearing. In some deep sea fishes like the ''[[Antimora]]'', the swim bladder maybe also connected to the [[macula of saccule]] in order for the inner ear to receive a sensation from the sound pressure.<ref>{{Cite journal |last1=Deng |first1=Xiaohong |last2=Wagner |first2=Hans-Joachim |last3=Popper |first3=Arthur N. |date=2011-01-01 |title=The inner ear and its coupling to the swim bladder in the deep-sea fish Antimora rostrata (Teleostei: Moridae) |journal=Deep Sea Research Part I: Oceanographic Research Papers |volume=58 |issue=1 |pages=27–37 |doi=10.1016/j.dsr.2010.11.001 |pmc=3082141 |pmid=21532967 |bibcode=2011DSRI...58...27D }}</ref>
In [[red-bellied piranha]], the swim bladder may play an important role in sound production as a resonator. The sounds created by piranhas are generated through rapid contractions of the sonic muscles and is associated with the swim bladder.<ref>{{cite journal|last=Onuki |first=A |author2=Ohmori Y. |author3=Somiya H. |title=Spinal Nerve Innervation to the Sonic Muscle and Sonic Motor Nucleus in Red Piranha, ''Pygocentrus nattereri'' (Characiformes, Ostariophysi) |journal=Brain, Behavior and Evolution |date=January 2006 |volume=67 |issue=2 |pages=11–122 |doi=10.1159/000089185 |pmid=16254416 |s2cid=7395840 }}</ref>

[[Teleost]]s are thought to lack a sense of absolute [[hydrostatic pressure]], which could be used to determine absolute depth.<ref>{{cite book |last1=Bone |first1=Q. |last2=Moore |first2=Richard H. |title=Biology of fishes |publisher=Taylor & Francis |isbn=9780415375627 |edition=3rd., Thoroughly updated and rev |year=2008 }}</ref> However, it has been suggested that teleosts may be able to determine their depth by sensing the rate of change of swim-bladder volume.<ref>{{cite journal |last1=Taylor |first1=Graham K. |last2=Holbrook |first2=Robert Iain |last3=de Perera |first3=Theresa Burt |title=Fractional rate of change of swim-bladder volume is reliably related to absolute depth during vertical displacements in teleost fish |journal=Journal of the Royal Society Interface |date=6 September 2010 |volume=7 |issue=50 |pages=1379–1382 |doi=10.1098/rsif.2009.0522 |pmid=20190038 |pmc=2894882 }}</ref>