Editing Respiratory system (section)

==Fish==
{{Main |Fish physiology#Respiration}}
[[File:Gills_(esox).jpg|250 px|thumb|left|'''Fig. 21.''' The [[Operculum (fish)|operculum]] or gill cover of a [[Northern pike|pike]] has been pulled open to expose the [[gill arch]]es bearing [[Gill filament|filaments]].]]
[[File:Comparison of con- and counter-current flow exchange.jpg|250px|thumb|right|'''Fig. 22.'''  A comparison between the operations and effects of a '''cocurrent and a countercurrent flow exchange system''' is depicted by the upper and lower diagrams respectively. In both, it is assumed that red has a higher value (e.g. of temperature or the partial pressure of a gas) than blue and that the property being transported in the channels, therefore, flows from red to blue. In fish a countercurrent flow (lower diagram) of blood and water in the gills is used to extract oxygen from the environment.<ref name=campbell3 /><ref name="Hughes1972" /><ref name=storer />]]
[[File:breathing in fish.jpg|thumb|left|250 px|'''Fig. 23''' The respiratory mechanism in bony fish. The inhalatory process is on the left, the exhalatory process on the right. The movement of water is indicated by the blue arrows.]]
Oxygen is poorly soluble in water. Fully aerated [[fresh water]] therefore contains only 8–10&nbsp;ml O<sub>2</sub>/liter compared to the O<sub>2</sub> concentration of 210&nbsp;ml/liter in the air at sea level.<ref name="Advanced Biology">{{cite book|title=Advanced Biology|author1=M. b. v. Roberts |author2=Michael Reiss |author3=Grace Monger |pages=164–165|publisher=Nelson|year=2000|location=London, UK}}</ref> Furthermore, the [[Mass diffusivity|coefficient of diffusion]] (i.e. the rate at which a substances diffuses from a region of high concentration to one of low concentration, under standard conditions) of the respiratory gases is [[Mass diffusivity#Example values|typically 10,000 faster in air than in water]].<ref name="Advanced Biology"/> Thus oxygen, for instance, has a diffusion coefficient of 17.6&nbsp;mm<sup>2</sup>/s in air, but only 0.0021&nbsp;mm<sup>2</sup>/s in water.<ref name="Cussler">{{cite book |first=E. L. |last=Cussler |title=Diffusion: Mass Transfer in Fluid Systems |edition=2nd |publisher=Cambridge University Press |location=New York |year=1997 |isbn=0-521-45078-0 }}</ref><ref name="Welty">{{cite book |first1=James R. |last1=Welty |first2=Charles E. |last2=Wicks |first3=Robert E. |last3=Wilson |first4=Gregory |last4=Rorrer |title=Fundamentals of Momentum, Heat, and Mass Transfer |publisher=Wiley |year=2001 |isbn=978-0-470-12868-8 }}</ref><ref name=crc>{{Cite web |url=http://www.crcpress.com/product/isbn/9781439820773 |title=CRC Press Online: CRC Handbook of Chemistry and Physics, Section 6, 91st Edition |access-date=2017-08-06 |archive-date=2011-07-16 |archive-url=https://web.archive.org/web/20110716073635/http://www.crcpress.com/product/isbn/9781439820773 |url-status=dead }}</ref><ref name=caltech>[http://www.cco.caltech.edu/~brokawc/Bi145/Diffusion.html Diffusion<!-- Bot generated title -->]</ref> The corresponding values for carbon dioxide are 16&nbsp;mm<sup>2</sup>/s in air and 0.0016&nbsp;mm<sup>2</sup>/s in water.<ref name=crc /><ref name=caltech />  This means that when oxygen is taken up from the water in contact with a gas exchanger, it is replaced considerably more slowly by the oxygen from the oxygen-rich regions small distances away from the exchanger than would have occurred in air. Fish have developed [[Fish gill|gills]] deal with these problems. Gills are specialized organs containing [[Gill filament|filaments]], which further divide into [[lamella (anatomy)|lamellae]]. The lamellae contain a dense [[capillary|thin walled capillary network]] that exposes a large gas exchange surface area to the very large volumes of water passing over them.<ref name="Newstead1967">{{Cite journal| author=Newstead James D | title=Fine structure of the respiratory lamellae of teleostean gills| journal=[[Cell and Tissue Research]]| volume=79| issue=3| year=1967| pages=396–428| doi=10.1007/bf00335484| pmid=5598734| s2cid=20771899}}</ref>

Gills use a [[Gas exchange#Interaction with circulatory systems|countercurrent exchange]] system that increases the efficiency of oxygen-uptake from the water.<ref name=campbell3>{{cite book|last1=Campbell|first1=Neil A.|title= Biology|edition= Second|publisher= Benjamin/Cummings Publishing Company, Inc|location= Redwood City, California|date= 1990|pages=836–838|isbn=0-8053-1800-3}}</ref><ref name="Hughes1972">{{Cite journal| author=Hughes GM| title=Morphometrics of fish gills| journal=Respiration Physiology| volume=14| issue=1–2| year=1972| pages=1–25| doi=10.1016/0034-5687(72)90014-x| pmid=5042155}}</ref><ref name=storer>{{cite book|last1=Storer|first1=Tracy I.|last2=Usinger|first2=R. L.|last3=Stebbins|first3=Robert C.|last4=Nybakken|first4=James W.|title=General Zoology|edition=sixth|publisher=McGraw-Hill|location=New York|date=1997|pages=[https://archive.org/details/generalzoolog00stor/page/668 668–670]|isbn=0-07-061780-5|url=https://archive.org/details/generalzoolog00stor/page/668}}</ref> Fresh oxygenated water taken in through the mouth is uninterruptedly "pumped" through the gills in one direction, while the blood in the lamellae flows in the opposite direction, creating the countercurrent blood and water flow (Fig. 22), on which the fish's survival depends.<ref name=storer />

Water is drawn in through the mouth by closing the [[Operculum (fish)|operculum]] (gill cover), and enlarging the mouth cavity (Fig. 23). Simultaneously the gill chambers enlarge, producing a lower pressure there than in the mouth causing water to flow over the gills.<ref name=storer /> The mouth cavity then contracts, inducing the closure of the passive oral valves, thereby preventing the back-flow of water from the mouth (Fig. 23).<ref name=storer /><ref name=VB>{{cite book |author=Romer, Alfred Sherwood|author2=Parsons, Thomas S.|year=1977|title=The Vertebrate Body |publisher=Holt-Saunders International|location= Philadelphia, PA|pages= 316–327|isbn= 0-03-910284-X|author-link=Alfred Romer}}</ref> The water in the mouth is, instead, forced over the gills, while the gill chambers contract emptying the water they contain through the opercular openings (Fig. 23). Back-flow into the gill chamber during the inhalatory phase is prevented by a membrane along the [[Anatomical terms of location#Axes|ventroposterior]] border of the operculum (diagram on the left in Fig. 23). Thus the mouth cavity and gill chambers act alternately as suction pump and pressure pump to maintain a steady flow of water over the gills in one direction.<ref name=storer /> Since the blood in the lamellar capillaries flows in the opposite direction to that of the water, the consequent [[countercurrent exchange|countercurrent]] flow of blood and water maintains steep concentration gradients for oxygen and carbon dioxide along the entire length of each capillary (lower diagram in Fig.&nbsp;22). Oxygen is, therefore, able to continually diffuse down its gradient into the blood, and the carbon dioxide down its gradient into the water.<ref name="Hughes1972"/> Although countercurrent exchange systems theoretically allow an almost complete transfer of a respiratory gas from one side of the exchanger to the other, in fish less than 80% of the oxygen in the water flowing over the gills is generally transferred to the blood.<ref name=campbell3 />

In certain active [[pelagic]] sharks, water passes through the mouth and over the gills while they are moving, in a process known as "ram ventilation".<ref name="Gilbertson">{{cite book| last = Gilbertson| first = Lance|title = Zoology Laboratory Manual| publisher = McGraw-Hill | year = 1999| location = New York|isbn= 0-07-237716-X}}</ref> While at rest, most sharks pump water over their gills, as most bony fish do, to ensure that oxygenated water continues to flow over their gills. But 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 | author = William J. Bennetta | year = 1996 | access-date = 2007-08-28 | archive-date = 2007-08-14 | archive-url = https://web.archive.org/web/20070814075030/http://www.textbookleague.org/73shark.htm | url-status = usurped }}</ref>

There are a few fish that can obtain oxygen for brief periods of time from air swallowed from above the surface of the water. Thus [[lungfish]] possess one or two lungs, and the [[Anabantoidei|labyrinth fish]] have developed a special  "labyrinth organ", which characterizes this suborder of fish. The labyrinth organ is a much-folded supra[[Branchial arches|branchial]] accessory [[breathing organ]]. It is formed by a [[Blood vessel|vascularized]] expansion of the epibranchial bone of the first gill arch, and is used for [[Respiration (physiology)|respiration]] in air.<ref name ="Pinter">Pinter, H. (1986). Labyrinth Fish. Barron's Educational Series, Inc., {{ISBN|0-8120-5635-3}}</ref> This organ allows labyrinth fish to take in [[oxygen]] directly from the air, instead of taking it from the water in which they reside through the use of [[gills]]. The labyrinth organ helps the oxygen in the inhaled air to be absorbed into the [[bloodstream]]. As a result, labyrinth fish can survive for a short period of time out of water, as they can inhale the air around them, provided they stay moist. Labyrinth fish are not born with functional labyrinth organs. The development of the organ is gradual and most juvenile labyrinth fish breathe entirely with their gills and develop the labyrinth organs when they grow older.<ref name="Pinter" />