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Gas exchange
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==Other vertebrates== ===Fish=== [[File:Tuna Gills in Situ 01.jpg|300 px|thumb|left|'''Fig. 8.''' Gills of tuna showing filaments and lamellae]] The dissolved oxygen content in [[fresh water]] is approximately 8β10 milliliters per liter compared to that of air which is 210 milliliters per liter.<ref name="Advanced Biology">{{cite book|title=Advanced Biology|author1=M. b. v. Roberts |author2=Michael Reiss |author3=Grace Monger |pages=164β165|publisher=Nelson|year=2000|location=London, UK}}</ref> Water is 800 times more dense than air<ref name=tyson>{{cite book|last1=Tyson|first1=P. D.|last2=Preston-White|first2=R.A.|title=The Weather and Climate of Southern Africa|edition=Second|date=2013|publisher=Oxford University Press| location=Cape Town, South Africa|page=14|isbn=9780195718065}}</ref> and 100 times more viscous.<ref name="Advanced Biology"/> Therefore, oxygen has a diffusion rate in air 10,000 times greater than in water.<ref name="Advanced Biology"/> The use of sac-like lungs to remove oxygen from water would therefore not be efficient enough to sustain life.<ref name="Advanced Biology"/> Rather than using lungs, gaseous exchange takes place across the surface of highly vascularized [[Fish gill|gill]]s. Gills are specialised organs containing [[Gill filament|filaments]], which further divide into [[lamella (anatomy)|lamellae]]. The lamellae contain [[capillaries]] that provide a large surface area and short diffusion distances, as their walls are extremely thin.<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=396β428| doi=10.1007/bf00335484| pmid=5598734| s2cid=20771899}}</ref> Gill rakers are found within the exchange system in order to filter out food, and keep the gills clean. Gills use a [[countercurrent flow]] system that increases the efficiency of oxygen-uptake (and waste gas loss).<ref name=campbell /><ref name="Hughes1972" /><ref name=storer/> Oxygenated water is drawn in through the mouth and passes over the gills in one direction while blood flows through the lamellae in the opposite direction. This [[countercurrent exchange|countercurrent]] maintains steep concentration gradients along the entire length of each capillary (see the diagram in the [[#Interaction with circulatory systems|"Interaction with circulatory systems"]] section above). Oxygen is able to continually diffuse down its gradient into the blood, and the carbon dioxide down its gradient into the water.<ref name="Hughes1972"/> The deoxygenated water will eventually pass out through the [[Operculum (fish)|operculum]] (gill cover). 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=campbell /> ===Amphibians=== Amphibians have three main organs involved in gas exchange: the lungs, the skin, and the gills, which can be used singly or in a variety of different combinations. The relative importance of these structures differs according to the age, the environment and species of the amphibian. The skin of amphibians and their larvae are highly vascularised, leading to relatively efficient gas exchange when the skin is moist. The larvae of amphibians, such as the pre-metamorphosis [[tadpole]] stage of [[frog]]s, also have external [[gills]]. The gills are absorbed into the body during [[metamorphosis]], after which the lungs will then take over. The lungs are usually simpler than in the [[amniote|other land vertebrates]], with few internal septa and larger alveoli; however, toads, which spend more time on land, have a larger alveolar surface with more developed lungs. To increase the rate of gas exchange by diffusion, amphibians maintain the concentration gradient across the respiratory surface using a process called [[buccal pumping]].<ref name="Brainerd">{{cite journal |last=Brainerd |first=E. L. |date=1999 |title=New perspectives on the evolution of lung ventilation mechanisms in invertebrates|journal=Experimental Biology Online |volume=4 |issue= 2|pages=1β28 |doi=10.1007/s00898-999-0002-1 |bibcode=1999EvBO....4b...1B |s2cid=35368264 }}</ref> The lower floor of the mouth is moved in a "pumping" manner, which can be observed by the naked eye. ===Reptiles=== All [[reptile]]s breathe using lungs. In [[squamate]]s (the [[lizards]] and [[snakes]]) ventilation is driven by the [[core (anatomy)|axial musculature]], but this musculature is also used during movement, so some squamates rely on [[buccal pumping]] to maintain gas exchange efficiency.<ref name ="reptiles">{{cite journal |last1=Taylor |first1=E. W. |last2=Campbell |first2= H. A.|last3=Leite|first3=C|last4=Abe|first4=A. S.|last5=Wang|first5=T|title= Respiration in reptiles |journal= Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology|volume=148 |pages=S110βS111 |doi=10.1016/j.cbpa.2007.06.431 |year=2007}}</ref> Due to the rigidity of [[turtle]] and [[tortoise]] shells, significant expansion and contraction of the chest is difficult. Turtles and tortoises depend on muscle layers attached to their shells, which wrap around their lungs to fill and empty them.<ref name="Klein2003">{{cite journal | last=Klein | first=Wilfied |author2=Abe, Augusto |author3=Andrade, Denis |author4= Perry, Steven | title=Structure of the posthepatic septum and its influence on visceral topology in the tegu lizard, ''Tupinambis merianae'' (Teidae: Reptilia) | journal=Journal of Morphology | volume=258 | issue=2 | year=2003 | pages=151β157 | doi=10.1002/jmor.10136 | pmid=14518009| s2cid=9901649 }}</ref> Some aquatic turtles can also pump water into a highly vascularised mouth or [[cloaca]] to achieve gas-exchange.<ref name="Orenstein 2001">{{cite book | last=Orenstein | first=Ronald | title=Turtles, Tortoises & Terrapins: Survivors in Armor | publisher=Firefly Books | year=2001 | isbn=978-1-55209-605-5 | url-access=registration | url=https://archive.org/details/turtlestortoises0000oren }}</ref><ref name="Feder1985">{{cite journal|last=Feder|first=Martin E.|author2=Burggren, Warren W. |title=Cutaneous gas exchange in vertebrates: design, patterns, control and implications|journal=Biological Reviews|date=1985|volume=60|issue=1|pages=1β45|doi=10.1111/j.1469-185X.1985.tb00416.x|pmid=3919777|s2cid=40158158|url=http://www.biol.unt.edu/~burggren/pdfs/1985/%2849%29Feder,Burggren1985BR.pdf}}</ref> [[Crocodile]]s have a structure similar to the mammalian diaphragm - the diaphragmaticus - but this muscle helps create a unidirectional flow of air through the lungs rather than a tidal flow: this is more similar to the air-flow seen in [[birds]] than that seen in mammals.<ref>{{cite journal|last=Farmer|first=CG|author2=Sanders, K |title=Unidirectional airflow in the lungs of alligators|journal=Science|year=2010|volume=327|issue=5963|pages=338β340|doi=10.1126/science.1180219|pmid=20075253|bibcode=2010Sci...327..338F|s2cid=206522844}}</ref> During inhalation, the diaphragmaticus pulls the liver back, inflating the lungs into the space this creates.<ref name="Farmer2012">{{cite journal |author1=Farmer, C. G. |author2=Carrier D. R. | year=2000 | title=Pelvic aspiration in the American alligator (''Alligator mississippiensis'') | journal=Journal of Experimental Biology | volume=203 |issue=11 | pages=1679β1687 |doi=10.1242/jeb.203.11.1679 |pmid=10804158}}</ref><ref>{{cite journal|author1=Munns, S. L. |author2=Owerkowicz, T. |author3=Andrewartha, S. J. |author4=Frappell, P. B. | year=2012| title=The accessory role of the diaphragmaticus muscle in lung ventilation in the estuarine crocodile ''Crocodylus porosus''| journal=Journal of Experimental Biology| volume=215| pages=845β852|doi=10.1242/jeb.061952| issue=5 | pmid=22323207| doi-access=free}}</ref> Air flows into the lungs from the bronchus during inhalation, but during exhalation, air flows out of the lungs into the bronchus by a different route: this one-way movement of gas is achieved by aerodynamic valves in the airways.<ref>{{Cite journal | author1=Farmer, C. G. | author2=Sanders, K. | year=2010 | title=Unidirectional airflow in the lungs of alligators | journal=Science | volume=327 | issue=5963 | pages=338β340 | doi=10.1126/science.1180219 | pmid=20075253 | url=http://faculty.bennington.edu/~sherman/comp.%20anim.%20physiol./Unidirectional%20Airflow%20in%20the%20Lungs%20of%20Alligators.pdf | bibcode=2010Sci...327..338F | s2cid=206522844 | access-date=2017-04-20 | archive-url=https://web.archive.org/web/20160624213901/http://faculty.bennington.edu/~sherman/comp.%20anim.%20physiol./Unidirectional%20Airflow%20in%20the%20Lungs%20of%20Alligators.pdf | archive-date=2016-06-24 | url-status=dead }}</ref><ref>{{cite journal |author1=Schachner, E. R. |author2=Hutchinson, J. R. |author3=Farmer, C. | year=2013 | title=Pulmonary anatomy in the Nile crocodile and the evolution of unidirectional airflow in Archosauria | journal=PeerJ |volume=1 |page=e60 |doi=10.7717/peerj.60 |pmid=23638399 |pmc=3628916 |doi-access=free }}</ref> ===Birds=== {{Main|Bird anatomy#Respiratory system}} [[File:BirdRespiration.svg|thumb|right|'''Fig. 10.''' Inhalation-exhalation cycle in birds.]] [[File:Cross-current exchanger.jpg|thumb|300 px|left|'''Fig. 9.''' A diagrammatic representation of the cross-current respiratory gas exchanger in the lungs of birds. Air is forced from the air sacs unidirectionally (from right to left in the diagram) through the parabronchi. The pulmonary capillaries surround the parabronchi in the manner shown (blood flowing from below the parabronchus to above it in the diagram).<ref name= graham /> Blood or air with a high oxygen content is shown in red; oxygen-poor air or blood is shown in various shades of purple-blue.]] Birds have [[Bird anatomy#Respiratory system|lungs but no diaphragm]]. They rely mostly on [[air sacs]] for [[Ventilation (physiology)|ventilation]]. These air sacs do not play a direct role in gas exchange, but help to move air unidirectionally across the gas exchange surfaces in the lungs. During inhalation, fresh air is taken from the trachea down into the posterior air sacs and into the [[parabronchi]] which lead from the posterior air sacs into the lung. The air that enters the lungs joins the air which is already in the lungs, and is drawn forward across the gas exchanger into anterior air sacs. During exhalation, the posterior air sacs force air into the same [[parabronchi]] of the lungs, flowing in the same direction as during inhalation, allowing continuous gas exchange irrespective of the breathing cycle. Air exiting the lungs during exhalation joins the air being expelled from the anterior air sacs (both consisting of "spent air" that has passed through the gas exchanger) entering the trachea to be exhaled (Fig. 10).<ref name=AvResp /> Selective [[bronchoconstriction]] at the various bronchial branch points ensures that the air does not ebb and flow through the bronchi during inhalation and exhalation, as it does in mammals, but follows the paths described above. The unidirectional airflow through the parabronchi exchanges respiratory gases with a ''crosscurrent'' blood flow (Fig. 9).<ref name="graham"/><ref name="AvResp"/> The partial pressure of O<sub>2</sub> (<math>P_{{\mathrm{O}}_2}</math>) in the parabronchioles declines along their length as O<sub>2</sub> diffuses into the blood. The capillaries leaving the exchanger near the entrance of airflow take up more O<sub>2</sub> than capillaries leaving near the exit end of the parabronchi. When the contents of all capillaries mix, the final <math>P_{{\mathrm{O}}_2}</math> of the mixed pulmonary venous blood is higher than that of the exhaled air, but lower than that of the inhaled air.<ref name= graham /><ref name=AvResp />
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