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Aquatic respiration is the process whereby an aquatic organism exchanges respiratory gases with water, obtaining oxygen from oxygen dissolved in water and excreting carbon dioxide and some other metabolic waste products into the water.

Unicellular and simple small organisms

In very small animals, plants and bacteria, simple diffusion of gaseous metabolites is sufficient for respiratory function and no special adaptations are found to aid respiration. Passive diffusion or active transport are also sufficient mechanisms for many larger aquatic animals such as many worms, jellyfish, sponges, bryozoans and similar organisms. In such cases, no specific respiratory organs or organelles are found.

Higher plants

Although higher plants typically use carbon dioxide and excrete oxygen during photosynthesis, they also respire and, particularly during darkness, many plants excrete carbon dioxide and require oxygen to maintain normal functions. In fully submerged aquatic higher plants specialised structures such as stoma on leaf surfaces to control gas interchange. In many species, these structures can be controlled to be open or closed depending on environmental conditions. In conditions of high light intensity and relatively high carbonate ion concentrations, oxygen may be produced in sufficient quantities to form gaseous bubbles on the surface of leaves and may produce oxygen super-saturation in the surrounding water body.

Animals

All animals that practice truly aquatic respiration are poikilothermic. All aquatic homeothermic animals and birds including cetaceans and penguins are air breathing despite a fully aquatic life-style.

Echinoderms

Echinoderms have a specialised water vascular system which provides a number of functions including providing the hydraulic power for tube feet but also serves to convey oxygenated sea water into the body and carry waste water out again. In many genera, the water enters through a madreporite, a sieve like structure on the upper surface but may also enter via ciliary action in the tube feet or via special cribiform organelles.^[1]

Molluscs

Molluscs commonly possess gills that allow exchange of respiratory gases from an aqueous environment into the circulatory system. These animals possess a heart that pumps blood which contains hemocyanin as its oxygen-capturing molecule. The respiratory system of gastropods can include either gills or a lung.

Arthropods

Aquatic arthropods generally possess some form of gills in which gas exchange takes place by diffusing through the exoskeleton. Others may breathe atmospheric air while remaining submerged, via breathing tubes or trapped air bubbles, though some aquatic insects may remain submerged indefinitely and respire using a plastron. A number of insects have an aquatic juvenile phase and an adult phase on land. In these case adaptions for life in water are lost at the final ecdysis. A number of orders of insects such as mayflies, caddis flies and stone flies have aquatic juvenile stages while some orders such as Lepidoptera have just a few examples such as China mark moths. A very few arachnids have adopted an aquatic life style including the diving bell spider. In all cases, oxygen is provided from air trapped by hairs^[2] around the animal's body.

Fish

Most fish exchange gases using gills on either side of the pharynx (throat), forming the splanchnocranium, the portion of the skeleton where the cartilage of the cranium converges into the cartilage of the pharynx and its associated parts.^[3] Gills are tissues which consist of threadlike structures called filaments. These filaments have many functions and are involved in ion and water transfer as well as oxygen, carbon dioxide, acid and ammonia exchange.^[4] Each filament contains a capillary network that provides a large surface area for the exchange of gases and ions. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. In species like the spiny dogfish and other sharks and rays, a spiracle exists near the top of the head that pumps water into the gills when the animal is not in motion.^[5] In some fish, capillary blood flows in the opposite direction to the water, causing countercurrent exchange. The muscles on the sides of the pharynx push the oxygen-depleted water out the gill openings. In bony fish, the pumping of oxygen-poor water is aided by a bone that surrounds the gills called the operculum.^[6]

Amphibians

Both the lungs and the skin serve as respiratory organs in amphibians. The skin of these animals is highly vascularized and moist, with moisture maintained via secretion of mucus from specialized cells. While the lungs are of primary importance to breathing control, the unique properties of cutaneous respiration supplements rapid gas exchange when amphibians are submerged in oxygen-rich water.^[7]

Aquatic reptiles, birds and mammals

All aquatic amniotes (reptiles, birds and mammals) have thick and impermeable cutes that preclude cutaneous respiration, and thus rely solely on the lungs to breathe air. When underwater, the animal is essentially holding its breath and has to routinely return to the surface to breathe in new air. Therefore, all amniote animals, even those that spend more time in water than out, are susceptible to drowning if they cannot reach the surface to breath.

The anatomical structure of the lungs is less complex in reptiles than in mammals, with reptiles lacking the very extensive bronchial tree found in mammalian lungs. Gas exchange in reptiles still occurs in alveoli, but reptiles do not possess a diaphragm, therefore ventilation occurs via a change in the volume of the body cavity which is controlled by contraction of intercostal muscles in all reptiles except turtles. In turtles, contraction of specific pairs of flank muscles governs inspiration or expiration.^[8]

Diving birds and pelagic seabirds breath air using lungs like reptiles and mammals, but avian lungs are fairly rigid structures that do not expand and contract as elastically. Instead, the structures that act as bellows that ventilate the lungs are the avascular air sacs, which are distributed throughout much of the birds' bodies^[9] and move air unidirectionally through the parabronchi, where gas exchange happens.^[10]^[11] Although bird lungs are smaller than those of mammals of comparable size, the air sacs account for 15% of the total body volume, whereas in mammals, the alveoli (which act as the bellows) constitute only 7% of the total body volume.^[12] Like their reptilian cousins, birds also lack a diaphragm and thus rely on the intercostal and abdominal muscles to change the volume of the entire thoracoabdominal cavity. The active phase of respiration in birds is exhalation, which requires contracting of respiratory muscles,^[11] while the relaxation of these muscles causes inhalation.

Gills

Many aquatic animals have developed gills for respiration which are specifically adapted to their function. In fish, for example, they have:

A large surface area to allow as much oxygen to enter the gills as possible because more of the gas comes into contact with the membrane
Good blood supply to maintain the concentration gradient needed
Thin membrane to allow for a short diffusion pathway
Each gill arch has two rows (hemibranchs) of gill filaments
Each gill filament has many lamellae

In osteichthyes, the gills contain 4 gill arches on each side of the head, two on each side for chondrichthyes or seven gill baskets on each side of the fish's head in lampreys. In fish, the long bony cover for the gill (the operculum) can be used for pushing water. Some fish pump water using the operculum. Without an operculum, other methods, such as ram ventilation, are required. Some species of sharks use this system. When they swim, water flows into the mouth and across the gills. Because these sharks rely on this technique, they must keep swimming in order to respire.

Bony fish use countercurrent flow to maximize the intake of oxygen that can diffuse through the gill. Countercurrent flow occurs when deoxygenated blood moves through the gill in one direction while oxygenated water moves through the gill in the opposite direction. This mechanism maintains the concentration gradient thus increasing the efficiency of the respiration process as well and prevents the oxygen levels from reaching an equilibrium. Cartilaginous fish do not have a countercurrent flow system as they lack bones which are needed to have the opened out gill that bony fish have.

Control of respiration

In fish neurons located in the brainstem of fish are responsible for the genesis of the respiratory rhythm.^[13] The position of these neurons is slightly different from the centers of respiratory genesis in mammals but they are located in the same brain compartment, which has caused debates about the homology of respiratory centers between aquatic and terrestrial species. In both aquatic and terrestrial respiration, the exact mechanisms by which neurons can generate this involuntary rhythm are still not completely understood (see Involuntary control of respiration).

The respiratory rhythm is modulated to adapt to the oxygen consumption of the body. As observed in mammals, fish “breathe” faster and heavier when they do physical exercise. The mechanisms by which these changes occur have been subject to debate .^[14] The views can be classified as either that the major part of the respiratory changes are pre-programmed in the brain, which would imply that neurons from locomotion centers of the brain connect to respiratory centers in anticipation of movements, or that the major part of the respiratory changes result from the detection of muscle contraction, and that respiration is adapted as a consequence of muscular contraction and oxygen consumption. The latter view would imply that the brain possesses some kind of detection mechanisms that would trigger a respiratory response when muscular contraction occurs.

Many now agree that both mechanisms are probably present and complementary, or working alongside a mechanism that can detect changes in oxygen and/or carbon dioxide blood saturation.

Notes

^ Nichols, Davd (1967). Echinodermms (third ed.). Hutchinson University Library. p. 44.
^ "Getting Oxygen" (PDF). British Ecological Society. united kingdom. April 2016.
^ "Introduction to the skeletal system". www.shsu.edu. Retrieved 2019-06-07.
^ Evans, David H. (2010-06-18). "A Brief History of the Study of Fish Osmoregulation: The Central Role of the Mt. Desert Island Biological Laboratory". Frontiers in Physiology. 1: 13. doi:10.3389/fphys.2010.00013. ISSN 1664-042X. PMC 3059943. PMID 21423356.
^ Wischnitzer, Saul (1967). Atlas and Dissection Guide for Comparative Anatomy. United States of America. p. 22. ISBN 0-7167-0691-1.{{cite book}}: CS1 maint: location missing publisher (link)
^ Kimmel, Charles B.; Aguirre, Windsor E.; Ullmann, Bonnie; Currey, Mark; Cresko, William A. (2008). "Allometric Change Accompanies Opercular Shape Evolution in Alaskan Threespine Sticklebacks". Behaviour. 145 (4/5): 669–691. doi:10.1163/156853908792451395. ISSN 0005-7959. JSTOR 40295944. S2CID 53466588.
^ Gottlieb, G; Jackson DC (1976). "Importance of pulmonary ventilation in respiratory control in the bullfrog". Am J Physiol. 230 (3): 608–13. doi:10.1152/ajplegacy.1976.230.3.608. PMID 4976.
^ "reptile - animal". Retrieved 8 September 2016.
^ Calder, William A. (1996). Size, Function, and Life History. Mineola, New York: Courier Dove Publications. p. 91. ISBN 978-0-486-69191-6.
^ Maina, John N. (2005). The lung air sac system of birds development, structure, and function; with 6 tables. Berlin: Springer. pp. 3.2–3.3 "Lung", "Airway (Bronchiol) System" 66–82. ISBN 978-3-540-25595-6.
^ ^a ^b Krautwald-Junghanns, Maria-Elisabeth; et al. (2010). Diagnostic Imaging of Exotic Pets: Birds, Small Mammals, Reptiles. Germany: Manson Publishing. ISBN 978-3-89993-049-8.
^ Whittow, G. Causey (2000). Sturkie's Avian Physiology. San Diego, California: Academic Press. pp. 233–241. ISBN 978-0-12-747605-6.
^ Russell, David F. (1986). "Respiratory pattern generation in adult lampreys (Lampetra fluviatilis): interneurons and burst resetting". Journal of Comparative Physiology A. 158 (1): 91–102. doi:10.1007/BF00614523. PMID 3723432. S2CID 19436421.
^ Waldrop, Tony G.; Gary A. Iwamoto; Philippe Haouzi (10 November 2005). "Point:Counterpoint: Supraspinal locomotor centers do/do not contribute significantly to the hyperpnea of dynamic exercise". Journal of Applied Physiology. 100 (3): 1077–1083. doi:10.1152/japplphysiol.01528.2005. PMID 16467394.

[1] Nichols, Davd (1967). Echinodermms (third ed.). Hutchinson University Library. p. 44.

[2] "Getting Oxygen" (PDF). British Ecological Society. united kingdom. April 2016.

[3] "Introduction to the skeletal system". www.shsu.edu. Retrieved 2019-06-07.

[4] Evans, David H. (2010-06-18). "A Brief History of the Study of Fish Osmoregulation: The Central Role of the Mt. Desert Island Biological Laboratory". Frontiers in Physiology. 1: 13. doi:10.3389/fphys.2010.00013. ISSN 1664-042X. PMC 3059943. PMID 21423356.

[5] Wischnitzer, Saul (1967). Atlas and Dissection Guide for Comparative Anatomy. United States of America. p. 22. ISBN 0-7167-0691-1.{{cite book}}: CS1 maint: location missing publisher (link)

[6] Kimmel, Charles B.; Aguirre, Windsor E.; Ullmann, Bonnie; Currey, Mark; Cresko, William A. (2008). "Allometric Change Accompanies Opercular Shape Evolution in Alaskan Threespine Sticklebacks". Behaviour. 145 (4/5): 669–691. doi:10.1163/156853908792451395. ISSN 0005-7959. JSTOR 40295944. S2CID 53466588.

[7] Gottlieb, G; Jackson DC (1976). "Importance of pulmonary ventilation in respiratory control in the bullfrog". Am J Physiol. 230 (3): 608–13. doi:10.1152/ajplegacy.1976.230.3.608. PMID 4976.

[8] "reptile - animal". Retrieved 8 September 2016.

[9] Calder, William A. (1996). Size, Function, and Life History. Mineola, New York: Courier Dove Publications. p. 91. ISBN 978-0-486-69191-6.

[Maina2005-10] Maina, John N. (2005). The lung air sac system of birds development, structure, and function; with 6 tables. Berlin: Springer. pp. 3.2–3.3 "Lung", "Airway (Bronchiol) System" 66–82. ISBN 978-3-540-25595-6.

[Krautwald-Junghanns,_et_al._2010-11] Krautwald-Junghanns, Maria-Elisabeth; et al. (2010). Diagnostic Imaging of Exotic Pets: Birds, Small Mammals, Reptiles. Germany: Manson Publishing. ISBN 978-3-89993-049-8.

[Whittow_2000_233–241-12] Whittow, G. Causey (2000). Sturkie's Avian Physiology. San Diego, California: Academic Press. pp. 233–241. ISBN 978-0-12-747605-6.

[13] Russell, David F. (1986). "Respiratory pattern generation in adult lampreys (Lampetra fluviatilis): interneurons and burst resetting". Journal of Comparative Physiology A. 158 (1): 91–102. doi:10.1007/BF00614523. PMID 3723432. S2CID 19436421.

[14] Waldrop, Tony G.; Gary A. Iwamoto; Philippe Haouzi (10 November 2005). "Point:Counterpoint: Supraspinal locomotor centers do/do not contribute significantly to the hyperpnea of dynamic exercise". Journal of Applied Physiology. 100 (3): 1077–1083. doi:10.1152/japplphysiol.01528.2005. PMID 16467394.

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@@ Line 2: / Line 2: @@
 [[File:Pleurobranchaea meckelii.jpg|thumb|300px|right|[[Sea slug]]s respire through a gill (or ctenidium)]]
-'''Aquatic respiration''' is the [[Biological process|process]] whereby an [[aquatic animal]] obtains [[oxygen]] from [[water]].
+'''Aquatic respiration''' is the [[Biological process|process]] whereby an [[aquatic animal|aquatic]] organism exchanges [[Respiration (physiology)|respiratory]] gases with [[water]], obtaining [[oxygen]] from oxygen dissolved in water and [[excretion|excreting]] [[carbon dioxide]] and some other metabolic waste products into the water.
+==Unicellular and simple small organisms==
-==Respiratory systems==
+In very small animals, plants and bacteria, simple diffusion of gaseous metabolites is sufficient for respiratory function and no special adaptations are found to aid respiration. Passive diffusion or [[active transport]] are also sufficient mechanisms for many larger aquatic animals such as many [[worm]]s, [[jellyfish]], [[sponge]]s, [[bryozoan]]s and similar organisms. In such cases, no specific respiratory organs or organelles are found.
-===Fish===
+==Higher plants==
+Although higher plants typically use carbon dioxide and excrete oxygen during photosynthesis, they also respire and, particularly during darkness, many plants excrete carbon dioxide and require oxygen to maintain normal functions.  In fully submerged aquatic higher plants specialised structures such as [[stoma]] on leaf surfaces to control gas interchange. In many species, these structures can be controlled to be open or closed depending on environmental conditions. In conditions of high light intensity and relatively high carbonate ion concentrations, oxygen may be produced in sufficient quantities to form gaseous bubbles on the surface of leaves and may produce oxygen [[Oxygen saturation|super-saturation]] in the surrounding water body.
-{{see also|Fish respiration}}
+==Animals==
+All animals that practice truly aquatic respiration are [[poikilothermy|poikilothermic]]. All aquatic [[homeothermy|homeothermic]] animals and birds including [[cetacean]]s and [[penguin]]s are air breathing despite a fully aquatic life-style.
+===Echinoderms===
-Most fish exchange gases using [[Gill|gills]] on either side of the [[pharynx]] (throat), forming the [[Splanchnocranium]]; the Splanchnocranium being the portion of the skeleton where the cartilage of the cranium converges into the cartilage of the pharynx and its associated parts.<ref>{{Cite web|url=https://www.shsu.edu/~bio_mlt/Chapter7.html|title=Introduction to the skeletal system|website=www.shsu.edu|access-date=2019-06-07}}</ref> Gills are tissues which consist of threadlike structures called [[Protein filament|filaments]]. These filaments have many functions and are involved in ion and water transfer as well as oxygen, carbon dioxide, acid and ammonia exchange.<ref>{{Cite journal|last=Evans|first=David H.|date=2010-06-18|title=A Brief History of the Study of Fish Osmoregulation: The Central Role of the Mt. Desert Island Biological Laboratory|journal=Frontiers in Physiology|volume=1|pages=13|doi=10.3389/fphys.2010.00013|issn=1664-042X|pmc=3059943|pmid=21423356}}</ref> Each filament contains a [[capillary]] network that provides a large [[surface area]] for the exchange of gases and ions. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. In species like the [[Spiny dogfish]] and other sharks and rays, a [[Spiracle (vertebrates)|spiracle]] exists near the top of the head that pumps water into the gills when the animal is not in motion.<ref>{{Cite book|title=Atlas and Dissection Guide for Comparative Anatomy|last=Wischnitzer|first=Saul|year=1967|isbn=0-7167-0691-1|location=United States of America|pages=22}}</ref> In some fish, capillary blood flows in the opposite direction to the water, causing [[countercurrent exchange]]. The muscles on the sides of the pharynx push the oxygen-depleted water out the gill openings. In bony fish, the pumping of oxygen-poor water is aided by a bone that surrounds the gills called the [[Operculum (fish)]].<ref>{{Cite journal|last1=Kimmel|first1=Charles B.|last2=Aguirre|first2=Windsor E.|last3=Ullmann|first3=Bonnie|last4=Currey|first4=Mark|last5=Cresko|first5=William A.|s2cid=53466588|date=2008|title=Allometric Change Accompanies Opercular Shape Evolution in Alaskan Threespine Sticklebacks|journal=Behaviour|volume=145|issue=4/5|pages=669–691|issn=0005-7959|jstor=40295944|doi=10.1163/156853908792451395}}</ref>
+[[Echinoderm]]s have  a specialised [[water vascular system]] which provides a number of functions including providing the hydraulic power for [[tube feet]] but also serves to convey oxygenated sea water into the body and carry waste water out again. In many genera, the water enters through a [[madreporite]], a sieve like structure on the upper surface but may also enter via ciliary action in the tube feet or via special cribiform organelles.<ref>{{cite book|title=Echinodermms|first=Davd|last=Nichols|publisher=Hutchinson University Library|date=1967|edition=third|page=44}}</ref>
 ===Molluscs===
 {{see also|Respiratory system of gastropods}}
+[[Molluscs]] commonly possess gills that allow exchange of respiratory gases from an aqueous environment into the circulatory system. These animals possess a heart that pumps blood which contains [[hemocyanin]] as its oxygen-capturing molecule. The [[respiratory system of gastropods]] can include either gills or a lung.
+=== Arthropods ===
-[[Molluscs]] generally possess gills that allow exchange of oxygen from an aqueous environment into the circulatory system. These animals also possess a heart that pumps blood which contains hemocyaninine as its oxygen-capturing molecule. Therefore, this respiratory system is similar to that of vertebrate fish. The [[respiratory system of gastropods]] can include either gills or a lung.
+{{See also|Plastron}}
+Aquatic [[arthropod]]s generally possess some form of gills in which gas exchange takes place by diffusing through the [[exoskeleton]]. Others may breathe atmospheric air while remaining submerged, via breathing tubes or trapped air bubbles, though some [[aquatic insect]]s may remain submerged indefinitely and respire using a [[Plastron (arthropod)|plastron]]. A number of insects have  an aquatic juvenile phase and an adult phase on land. In these case adaptions for life in water are lost at the final [[ecdysis]].  A number of orders of insects such as [[Mayfly|mayflies]], [[Caddis fly|caddis flies]] and [[Stone fly|stone flies]] have aquatic juvenile stages while some orders such as [[Lepidoptera]] have just a few examples such as [[Nymphula nitidulata|China mark moth]]s.  A very few [[arachnid]]s have adopted an aquatic life style including the [[diving bell spider]]. In all cases, oxygen is provided from air trapped by hairs<ref>{{Cite web |date=April 2016 |title=Getting Oxygen |url=https://www.britishecologicalsociety.org/wp-content/uploads/2016/04/Getting-oxygen.pdf |website=British Ecological Society |publication-place=united kingdom}}</ref> around the animal's body.
-===Arthropods===
+===Fish===
-{{see also|Gill#Invertebrate gills}}
+{{see also|Fish respiration}}
+Most fish exchange gases using [[Gill|gills]] on either side of the [[pharynx]] (throat), forming the [[splanchnocranium]], the portion of the skeleton where the cartilage of the cranium converges into the cartilage of the pharynx and its associated parts.<ref>{{Cite web|url=https://www.shsu.edu/~bio_mlt/Chapter7.html|title=Introduction to the skeletal system|website=www.shsu.edu|access-date=2019-06-07}}</ref> Gills are tissues which consist of threadlike structures called [[Protein filament|filaments]]. These filaments have many functions and are involved in ion and water transfer as well as oxygen, carbon dioxide, acid and ammonia exchange.<ref>{{Cite journal|last=Evans|first=David H.|date=2010-06-18|title=A Brief History of the Study of Fish Osmoregulation: The Central Role of the Mt. Desert Island Biological Laboratory|journal=Frontiers in Physiology|volume=1|pages=13|doi=10.3389/fphys.2010.00013|issn=1664-042X|pmc=3059943|pmid=21423356|doi-access=free }}</ref> Each filament contains a [[capillary]] network that provides a large [[surface area]] for the exchange of gases and ions. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. In species like the [[spiny dogfish]] and other sharks and rays, a [[Spiracle (vertebrates)|spiracle]] exists near the top of the head that pumps water into the gills when the animal is not in motion.<ref>{{Cite book|title=Atlas and Dissection Guide for Comparative Anatomy|last=Wischnitzer|first=Saul|year=1967|isbn=0-7167-0691-1|location=United States of America|pages=22}}</ref> In some fish, capillary blood flows in the opposite direction to the water, causing [[countercurrent exchange]]. The muscles on the sides of the pharynx push the oxygen-depleted water out the gill openings. In bony fish, the pumping of oxygen-poor water is aided by a bone that surrounds the gills called the [[Operculum (fish)|operculum]].<ref>{{Cite journal|last1=Kimmel|first1=Charles B.|last2=Aguirre|first2=Windsor E.|last3=Ullmann|first3=Bonnie|last4=Currey|first4=Mark|last5=Cresko|first5=William A.|s2cid=53466588|date=2008|title=Allometric Change Accompanies Opercular Shape Evolution in Alaskan Threespine Sticklebacks|journal=Behaviour|volume=145|issue=4/5|pages=669–691|issn=0005-7959|jstor=40295944|doi=10.1163/156853908792451395}}</ref>
-Aquatic [[arthropod]]s generally possess some form of gills in which gas exchange takes place by diffusing through the [[exoskeleton]]. Others may breathe atmospheric air while remaining submerged, via breathing tubes or trapped air bubbles, though some [[aquatic insect]]s may remain submerged indefinitely and respire using a [[Plastron (arthropod)|plastron]]. A very few [[Arachnid]]s have adopted an aquatic life style including the [[Diving bell spider]]. In all cases, oxygen is provided from air trapped by hairs around the animals body.
-===Aquatic reptiles===
-All aquatic reptiles breathe air into lungs. The [[anatomy|anatomical structure]] of the [[lungs]] is less complex in [[reptiles]] than in [[mammals]], with reptiles lacking the very extensive  airway tree structure found in mammalian lungs. [[Gas exchange]] in reptiles still occurs in [[Pulmonary alveolus|alveoli]]; however, reptiles do not possess a [[thoracic diaphragm|diaphragm]]. Thus, breathing occurs via a change in the volume of the body cavity which is controlled by contraction of [[intercostal muscle]]s in all reptiles except [[turtle]]s. In turtles, contraction of specific pairs of flank muscles governs [[inhalation|inspiration]] or [[Exhalation|expiration]].<ref>{{cite web|url=http://www.britannica.com/EBchecked/topic/498684/reptile/38473/Respiratory-system|title=reptile - animal|accessdate=8 September 2016}}</ref>
-See also [[Reptiles#Respiration|reptiles]] for more detailed descriptions of the respiratory system in these animals.
 ===Amphibians===
 {{see also|Amphibian#Respiratory system}}
+Both the [[lung]]s and the [[Frog#Morphology and physiology|skin]] serve as respiratory organs in [[amphibian]]s. The skin of these animals is highly vascularized and moist, with moisture maintained via secretion of [[mucus]] from specialized cells. While the lungs are of primary importance to breathing control, the unique properties of [[cutaneous respiration]] supplements rapid gas exchange when amphibians are submerged in oxygen-rich water.<ref>{{cite journal
-Both the lungs and the [[Frog#Morphology and physiology|skin]] serve as respiratory organs in [[amphibians]]. The skin of these animals is highly vascularized and moist, with moisture maintained via secretion of [[mucus]] from specialized cells. While the lungs are of primary importance to breathing control, the skin's unique properties aid rapid gas exchange when amphibians are submerged in oxygen-rich water.<ref>{{cite journal
  | last = Gottlieb
  | first = G
@@ Line 38: / Line 39: @@
  | pages = 608–13
  | year = 1976
- | accessdate =
  | pmid = 4976
  | issue = 3
- | doi=10.1152/ajplegacy.1976.230.3.608| doi-access = free
+ | doi=10.1152/ajplegacy.1976.230.3.608| doi-access =
  }}</ref>
-===Aquatic birds===
+===Aquatic reptiles, birds and mammals===
+{{See also|Physiology of underwater diving|Cetacean surfacing behaviour}}
-The respiratory system of birds differs significantly from that found in mammals, containing unique anatomical features such as [[air sacs]]. The lungs of birds also do not have the capacity to inflate as birds lack a [[Thoracic diaphragm|diaphragm]] and a [[pleural cavity]]. Gas exchange in birds occurs between air capillaries and [[capillary|blood capillaries]], rather than in [[Pulmonary alveolus|alveoli]]. See [[Bird anatomy#Respiratory system|Avian respiratory system]] for a detailed description of these and other features.
+All aquatic [[amniote]]s ([[reptile]]s, [[bird]]s and [[mammal]]s) have thick and impermeable [[cutis (anatomy)|cutes]] that preclude [[cutaneous respiration]], and thus rely solely on the lungs to breathe air. When underwater, the animal is essentially [[breath holding|holding its breath]] and has to routinely return to the surface to breathe in new air.  Therefore, all amniote animals, even those that spend more time in water than out, are susceptible to [[drowning]] if they cannot reach the surface to breath.
+The [[anatomy|anatomical structure]] of the lungs is less complex in reptiles than in mammals, with reptiles lacking the very extensive [[bronchial tree]] found in mammalian lungs. [[Gas exchange]] in reptiles still occurs in [[pulmonary alveolus|alveoli]], but reptiles do not possess a [[thoracic diaphragm|diaphragm]], therefore [[ventilation (physiology)|ventilation]] occurs via a change in the volume of the body cavity which is controlled by contraction of [[intercostal muscle]]s in all reptiles except [[turtle]]s. In turtles, contraction of specific pairs of flank muscles governs [[inhalation|inspiration]] or [[Exhalation|expiration]].<ref>{{cite web|url=http://www.britannica.com/EBchecked/topic/498684/reptile/38473/Respiratory-system|title=reptile - animal|access-date=8 September 2016}}</ref>
+[[Diving bird]]s and pelagic seabirds breath air using lungs like reptiles and mammals, but avian lungs are fairly rigid structures that do not expand and contract as elastically. Instead, the structures that act as [[bellows]] that ventilate the lungs are the avascular [[air sacs]], which are distributed throughout much of the birds' bodies<ref>{{cite book|last=Calder|first=William A.|title=Size, Function, and Life History|year=1996|publisher=Courier Dove Publications|location=Mineola, New York|isbn=978-0-486-69191-6 |page=91|url=https://books.google.com/books?id=-iBS6-2OO3wC}}</ref> and move air ''unidirectionally'' through the [[parabronchi]], where [[gas exchange]] happens.<ref name="Maina2005">{{cite book|last1=Maina|first1=John N.|title=The lung air sac system of birds development, structure, and function; with 6 tables|date=2005|publisher=Springer|location=Berlin|isbn=978-3-540-25595-6|pages=3.2–3.3 "Lung", "Airway (Bronchiol) System" 66–82|url=https://books.google.com/books?id=-wtoEg7fcjkC&q=neopulmonic+parabronchi&pg=PA66}}</ref><ref name="Krautwald-Junghanns, et al. 2010">{{cite book|last=Krautwald-Junghanns|first=Maria-Elisabeth|title=Diagnostic Imaging of Exotic Pets: Birds, Small Mammals, Reptiles|year=2010|publisher=Manson Publishing|location=Germany|isbn=978-3-89993-049-8|display-authors=etal}}</ref>  Although bird lungs are smaller than those of mammals of comparable size, the air sacs account for 15% of the total body volume, whereas in mammals, the [[pulmonary alveolus|alveoli]] (which act as the bellows) constitute only 7% of the total body volume.<ref name="Whittow 2000 233–241">{{cite book|last=Whittow|first=G. Causey|title=Sturkie's Avian Physiology|url=https://archive.org/details/sturkiesavianphy00whit|url-access=limited|year=2000|publisher=Academic Press|location=San Diego, California|isbn=978-0-12-747605-6 |pages=[https://archive.org/details/sturkiesavianphy00whit/page/n230 233]–241}}</ref> Like their reptilian cousins, birds also lack a diaphragm and thus rely on the [[intercostal muscle|intercostal]] and [[abdominal muscles]] to change the volume of the entire thoracoabdominal cavity. The active phase of respiration in birds is exhalation, which requires contracting of [[respiratory muscles]],<ref name="Krautwald-Junghanns, et al. 2010" /> while the relaxation of these muscles causes inhalation.
 ==Gills==
 [[File:Tuna Gills in Situ 01.jpg|thumb|right|Posterior view of the gills of a [[tuna]]]]
+Many aquatic animals have developed gills for respiration which are specifically adapted to their function. In fish, for example, they have:
+*A large [[surface area]] to allow as much oxygen to enter the gills as possible because more of the gas comes into contact with the [[Biological membrane|membrane]]
-Many aquatic animals have developed [[gill]]s for [[Respiration (physiology)|respiration]] which are specifically adapted to their function. In fish, for example, they have:
-*A large [[surface area]] to allow as much [[oxygen]] to enter the gills as possible because more of the gas comes into contact with the [[Biological membrane|membrane]]
 *Good [[Blood circulation|blood supply]] to maintain the concentration gradient needed
 *Thin membrane to allow for a short diffusion pathway
-*each gill arch has two rows (hemibranchs) of [[gill filament]]s
+*Each gill arch has two rows (hemibranchs) of [[gill filament]]s
-*each gill filament has many [[lamella (zoology)|lamella]]e
+*Each gill filament has many [[lamella (zoology)|lamella]]e
-In [[osteichthyes]], the gills contain 4 [[gill arch]]es on each side of the head, two on each side for [[chondrichthyes]] or 7 gill baskets on each side of the fish's head in [[Lampreys]].
+In [[osteichthyes]], the gills contain 4 [[gill arch]]es on each side of the head, two on each side for [[chondrichthyes]] or seven gill baskets on each side of the fish's head in [[lampreys]].
-In fish, the long bony cover for the gill (the [[operculum (fish)|operculum]]) can be used for pushing water. Some fish pump water using the operculum.  Without an operculum, other methods, such as ram [[Ventilation (physiology)|ventilation]], are required. Some species of [[shark]]s use this system.  When they swim, water flows into the mouth and across the gills.  Because these sharks rely on this technique, they must keep swimming in order to respire.
+In fish, the long bony cover for the gill (the operculum) can be used for pushing water. Some fish pump water using the operculum.  Without an operculum, other methods, such as ram [[Ventilation (physiology)|ventilation]], are required. Some species of [[shark]]s use this system.  When they swim, water flows into the mouth and across the gills.  Because these sharks rely on this technique, they must keep swimming in order to respire.
-Bony fish use [[Countercurrent exchange|countercurrent flow]] to maximize the intake of oxygen that can [[Diffusion|diffuse]] through the gill.  Countercurrent flow occurs when deoxygenated blood moves through the gill in one direction while oxygenated water moves through the gill in the opposite direction. This mechanism maintains the [[concentration gradient]] thus increasing the efficiency of the respiration process as well and prevents the oxygen levels from reaching an [[Chemical equilibrium|equilibrium]]. Cartilaginous fish do not have a countercurrent flow system as they lack bones which are needed to have the opened out gill that bony fish have.
+Bony fish use countercurrent flow to maximize the intake of oxygen that can [[Diffusion|diffuse]] through the gill.  Countercurrent flow occurs when deoxygenated blood moves through the gill in one direction while oxygenated water moves through the gill in the opposite direction. This mechanism maintains the [[concentration gradient]] thus increasing the efficiency of the respiration process as well and prevents the oxygen levels from reaching an [[Chemical equilibrium|equilibrium]]. Cartilaginous fish do not have a countercurrent flow system as they lack bones which are needed to have the opened out gill that bony fish have.
 ==Control of respiration==
+In fish [[neuron]]s located in the [[brainstem]] of fish are responsible for the genesis of the [[respiratory rhythm]].<ref>{{cite journal|last=Russell|first=David F.|title=Respiratory pattern generation in adult lampreys (Lampetra fluviatilis): interneurons and burst resetting|journal=Journal of Comparative Physiology A|date=1986|volume=158|issue=1|pages=91–102|doi=10.1007/BF00614523|pmid=3723432|s2cid=19436421 }}</ref>  The position of these neurons is slightly different from the centers of respiratory genesis in mammals but they are located in the same brain compartment, which has caused debates about the [[homology (biology)|homology]] of [[respiratory center]]s between aquatic and terrestrial species. In both aquatic and terrestrial respiration, the exact mechanisms by which neurons can generate this involuntary rhythm are still not completely understood (see [[Involuntary control of respiration]]).
-Scientists have investigated what part of the body is responsible for maintaining the respiratory rhythm. They found that [[neuron]]s located in the
-[[brainstem]] of fish are responsible for the genesis of the [[respiratory rhythm]].<ref>{{cite journal|last=Russell|first=David F.|title=Respiratory pattern generation in adult lampreys (Lampetra fluviatilis): interneurons and burst resetting|journal=Journal of Comparative Physiology A|date=1986|volume=158|issue=1|pages=91–102|doi=10.1007/BF00614523|pmid=3723432}}</ref>  The position of these neurons is slightly different from the centers of respiratory genesis in mammals but they are located in the same brain compartment, which has caused debates about the [[homology (biology)|homology]] of [[respiratory center]]s between aquatic and terrestrial species. In both aquatic and terrestrial respiration, the exact mechanisms by which neurons can generate this involuntary rhythm are still not completely understood (see [[Involuntary control of respiration]]).
-Another important feature of the respiratory rhythm is that it is modulated to adapt to the oxygen consumption of the body. As observed in mammals, fish “breathe” faster and heavier when they do [[exercise|physical exercise]]. The mechanisms by which these changes occur have been strongly debated over more than 100 years between scientists.<ref>{{cite journal|last=Waldrop|first=Tony G.|author2=Gary A. Iwamoto|author3=Philippe Haouzi|title=Point:Counterpoint: Supraspinal locomotor centers do/do not contribute significantly to the hyperpnea of dynamic exercise|journal=Journal of Applied Physiology|date=10 November 2005|volume=100|issue=3|pages=1077–1083|doi=10.1152/japplphysiol.01528.2005|pmid=16467394}}</ref> The authors can be classified in 2 schools:
-. Those who think that the major part of the respiratory changes are pre-programmed in the brain, which would imply that neurons from [[animal locomotion|locomotion]] centers of the brain connect to respiratory centers in anticipation of movements.
-. Those who think that the major part of the respiratory changes result from the detection of muscle contraction, and that respiration is adapted as a consequence of muscular contraction and oxygen consumption. This would imply that the brain possesses some kind of detection mechanisms that would trigger a respiratory response when muscular contraction occurs.
+The respiratory rhythm is modulated to adapt to the oxygen consumption of the body. As observed in mammals, fish “breathe” faster and heavier when they do [[exercise|physical exercise]]. The mechanisms by which these changes occur have been subject to debate .<ref>{{cite journal|last=Waldrop|first=Tony G.|author2=Gary A. Iwamoto|author3=Philippe Haouzi|title=Point:Counterpoint: Supraspinal locomotor centers do/do not contribute significantly to the hyperpnea of dynamic exercise|journal=Journal of Applied Physiology|date=10 November 2005|volume=100|issue=3|pages=1077–1083|doi=10.1152/japplphysiol.01528.2005|pmid=16467394}}</ref> The views can be classified as either  that the major part of the respiratory changes are pre-programmed in the brain, which would imply that neurons from [[animal locomotion|locomotion]] centers of the brain connect to respiratory centers in anticipation of movements, or that the major part of the respiratory changes result from the detection of muscle contraction, and that respiration is adapted as a consequence of muscular contraction and oxygen consumption. The latter view would imply that the brain possesses some kind of detection mechanisms that would trigger a respiratory response when muscular contraction occurs.
 Many now agree that both mechanisms are probably present and complementary, or working alongside a mechanism that can detect changes in oxygen and/or carbon dioxide blood saturation.
@@ Line 79: / Line 78: @@
 * [[Ecosystem respiration]]
 * [[Maintenance respiration]]
-* [[Respiration (physiology)]]
 * [[Respiratory system]]