Editing Heart (section)

==Other animals==
{{See also|Circulatory system}}

===Vertebrates===
The size of the heart varies among the different animal [[phylum|groups]], with hearts in [[vertebrate]]s ranging from those of the smallest mice (12&nbsp;mg) to the blue whale (600&nbsp;kg).<ref>{{cite journal|last1=Dobson|first1=Geoffrey P|title=On Being the Right Size: Heart Design, Mitochondrial Efficiency and Lifespan Potential|journal=Clinical and Experimental Pharmacology and Physiology|date=August 2003|volume=30|issue=8|pages=590–597|doi=10.1046/j.1440-1681.2003.03876.x|pmid=12890185|s2cid=41815414|doi-access=}}</ref> In vertebrates, the heart lies in the middle of the ventral part of the body, surrounded by a [[pericardium]].<ref name="Hyman1992">{{cite book|author=Hyman, L. Henrietta|title=Hyman's Comparative Vertebrate Anatomy|url=https://books.google.com/books?id=VKlWjdOkiMwC&pg=PA448|year=1992|publisher=University of Chicago Press|isbn=978-0-226-87013-7|pages=448–|url-status=live|archive-url=https://web.archive.org/web/20161206001900/https://books.google.com/books?id=VKlWjdOkiMwC&pg=PA448|archive-date=6 December 2016}}</ref> which in some fish may be connected to the [[peritoneum]].<ref>{{cite book|editor=Shuttleworth, Trevor J.|title=Physiology of Elasmobranch Fishes|date=1988|publisher=Springer Berlin Heidelberg|location=Berlin, Heidelberg|isbn=978-3-642-73336-9|page=3|url=https://books.google.com/books?id=4VLoCAAAQBAJ&pg=PA3|access-date=16 August 2020|archive-date=14 April 2021|archive-url=https://web.archive.org/web/20210414054657/https://books.google.com/books?id=4VLoCAAAQBAJ&pg=PA3|url-status=live}}</ref> In all vertebrates, the heart has an asymmetric orientation, almost always on the left side. According to one theory, this is caused by a [[axial twist theory|developmental axial twist]] in the early embryo.<ref name="Lussanet2012">{{cite journal | last1=de Lussanet|first1=Marc H.E. | last2=Osse|first2=Jan W.M. | year=2012 | title=An ancestral axial twist explains the contralateral forebrain and the optic chiasm in vertebrates | journal=Animal Biology | volume=62 | issue=2|pages=193–216 | doi=10.1163/157075611X617102 | arxiv=1003.1872|s2cid=7399128|issn = 1570-7555 }}</ref><ref name="lussanet2019">{{cite journal | last=de Lussanet|first=M.H.E. | doi=10.7717/peerj.7096 | journal=PeerJ | pages=e7096 | title=Opposite asymmetries of face and trunk and of kissing and hugging, as predicted by the axial twist hypothesis | volume=7 | year=2019 | pmid=31211022 | pmc=6557252 | doi-access=free}}</ref>

The sinoatrial node is found in all [[amniote]]s but not in more primitive vertebrates. In these animals, the muscles of the heart are relatively continuous, and the sinus venosus coordinates the beat, which passes in a wave through the remaining chambers. Since the sinus venosus is incorporated into the right atrium in amniotes, it is likely [[homology (biology)|homologous]] with the SA node. In teleosts, with their vestigial sinus venosus, the main centre of coordination is, instead, in the atrium. The rate of heartbeat varies enormously between different species, ranging from around 20 beats per minute in [[codfish]] to around 600 in [[hummingbird]]s<ref name=VB /> and up to 1,200 bpm in the [[ruby-throated hummingbird]].<ref>{{cite book |first=June |last=Osborne |title=The Ruby-Throated Hummingbird |year=1998 |publisher=University of Texas Press |isbn=978-0-292-76047-9 |page=[https://archive.org/details/rubythroatedhumm0000osbo/page/14 14] |url=https://archive.org/details/rubythroatedhumm0000osbo/page/14 }}</ref>

===Double circulatory systems===
{{Further|Reptile#Circulation|Snake#Internal organs}}
[[File:Copy of Ap Bio 2.svg|thumb|A cross section of a three-chambered adult amphibian heart. Note the single ventricle. The purple regions represent areas where mixing of oxygenated and de-oxygenated blood occurs. {{ordered list |Pulmonary vein |Left atrium |Right atrium |Ventricle |Conus arteriosus |Sinus venosus}}]]
Adult [[amphibian]]s and most [[reptile]]s have a [[double circulatory system]], meaning a circulatory system divided into arterial and venous parts. However, the heart itself is not completely separated into two sides. Instead, it is separated into three chambers—two atria and one ventricle. Blood returning from both the systemic circulation and the lungs is returned, and blood is pumped simultaneously into the systemic circulation and the lungs. The double system allows blood to circulate to and from the lungs which deliver oxygenated blood directly to the heart.<ref name=GRIMM2015>{{Cite book|url=https://books.google.com/books?id=WG9uBwAAQBAJ|title=Veterinary Anesthesia and Analgesia|last1=Grimm|first1=Kurt A.|last2=Lamont|first2=Leigh A.|last3=Tranquilli|first3=William J.|last4=Greene|first4=Stephen A.|last5=Robertson|first5=Sheilah A.|page=418|year=2015|publisher=John Wiley & Sons|isbn=978-1-118-52620-0|language=en|url-status=live|archive-url=https://web.archive.org/web/20161206212053/https://books.google.com/books?id=WG9uBwAAQBAJ|archive-date=6 December 2016}}</ref>

In reptiles, other than [[snake]]s, the heart is usually situated around the middle of the thorax. In terrestrial and arboreal snakes, it is usually located nearer to the head; in aquatic species the heart is more centrally located.<ref name="Seymour"/> There is a heart with three chambers: two atria and one ventricle. The form and function of these hearts are different from mammalian hearts due to the fact that snakes have an elongated body, and thus are affected by different environmental factors. In particular, the snake's heart relative to the position in their body has been influenced greatly by gravity. Therefore, snakes that are larger in size tend to have a higher [[blood pressure]] due to gravitational change.<ref name="Seymour">{{cite journal|last1=Seymour|first1=Roger S.|year=1987|title=Scaling of Cardiovascular Physiology in Snakes|journal=Integrative and Comparative Biology|language=en|volume=27|issue=1|pages=97–109|doi=10.1093/icb/27.1.97|issn=1540-7063|doi-access=free}}</ref> The ventricle is incompletely separated into two-halves by a wall ([[:wikt:septum|septum]]), with a considerable gap near the pulmonary artery and aortic openings. In most reptilian species, there appears to be little, if any, mixing between the bloodstreams, so the aorta receives, essentially, only oxygenated blood.<ref name=VB /><ref name=GRIMM2015 /> The exception to this rule is [[crocodile]]s, which have a four-chambered heart.<ref name=COLVILLE2015>{{Cite book|url=https://books.google.com/books?id=9DMwBwAAQBAJ|title=Clinical Anatomy and Physiology for Veterinary Technicians|last1=Colville|first1=Thomas P.|last2=Bassert|first2=Joanna M.|page=547|year=2015|publisher=Elsevier Health Sciences|isbn=978-0-323-35620-6|language=en|url-status=live|archive-url=https://web.archive.org/web/20161206110844/https://books.google.com/books?id=9DMwBwAAQBAJ|archive-date=6 December 2016}}</ref>

In the heart of [[lungfish]], the septum extends partway into the ventricle. This allows for some degree of separation between the de-oxygenated bloodstream destined for the lungs and the oxygenated stream that is delivered to the rest of the body. The absence of such a division in living amphibian species may be partly due to the amount of respiration that occurs through the skin; thus, the blood returned to the heart through the venae cavae is already partially oxygenated. As a result, there may be less need for a finer division between the two bloodstreams than in lungfish or other [[tetrapod]]s. Nonetheless, in at least some species of amphibian, the spongy nature of the ventricle does seem to maintain more of a separation between the bloodstreams. Also, the original valves of the [[conus arteriosus]] have been replaced by a spiral valve that divides it into two parallel parts, thereby helping to keep the two bloodstreams separate.<ref name=VB />

=== Full division ===
[[Archosaur]]s ([[crocodilia]]ns and [[bird]]s) and [[mammal]]s show complete separation of the heart into two pumps for a total of four heart chambers; it is thought that the four-chambered heart of archosaurs evolved independently from that of mammals. In crocodilians, there is a small opening, the [[foramen of Panizza]], at the base of the arterial trunks and there is some degree of mixing between the blood in each side of the heart, during a dive underwater;<ref name=nonimportantshunt1>{{cite journal |last1=Crigg |first1=Gordon |last2=Johansen |first2=Kjell |s2cid=28733499 |year=1987 |title=Cardiovascular Dynamics in Crocodylus Porosus Breathing Air And During Voluntary Aerobic Dives |journal=Journal of Comparative Physiology B |volume=157 |issue=3 |pages=381–392 |doi=10.1007/BF00693365 |url=http://espace.library.uq.edu.au/view/UQ:9269/_Crocodylus_poro.pdf |access-date=20 February 2019 |archive-date=28 August 2021 |archive-url=https://web.archive.org/web/20210828102937/https://espace.library.uq.edu.au/data/UQ_9269/_Crocodylus_poro.pdf?Expires=1630146664&Key-Pair-Id=APKAJKNBJ4MJBJNC6NLQ&Signature=KyKofJBI4oqlJwIFGuf6sY5Y9iqjzCyhkCo7ZN0mWF1e2rird4irPgGfFpXZ-KA91HxOYNgUd2KPb9x4npGjFTRCcMBOU~DwZ9UZf~NZt4zJhhEbAzKghtgVGp6f1q9XSda~v0UEiKJMALjnOK0dIDAuYRjsrGq8JoCs7zDtamTncowHlU3UmtSKbXOdJH6OwFSjhv7pp7t-mNJT28Cfx8F9sV0W1NH~Fu5UyMJQlfodQC00jL-86iSEKMqUXO-86L6I5SFANGQVl7qSNNwO1yL~f6BzXVWS6jSbDlcJ3soZt-tQrL3VqYZfhFQEJuYbFif-ijW2A~cJML0SujYkrA__ |url-status=live }}</ref><ref name=nonimportantshunt2>{{cite journal |last1=Axelsson |first1=Michael |last2=Craig |first2=Franklin |last3=Löfman |first3=Carl |last4=Nilsson |first4=Stefan |last5=Crigg |first5=Gordon |year=1996 |title=Dynamic Anatomical Study of Cardiac Shunting in Crocodiles Using High-Resolution Angioscopy |journal=The Journal of Experimental Biology |volume=199 |issue=2 |pages=359–365 |doi=10.1242/jeb.199.2.359 |pmid=9317958 |url=http://jeb.biologists.org/content/199/2/359.full.pdf |access-date=3 July 2012 |url-status=live |archive-url=https://web.archive.org/web/20150303181815/http://jeb.biologists.org/content/199/2/359.full.pdf |archive-date=3 March 2015|doi-access=free |bibcode=1996JExpB.199..359A }}</ref> thus, only in birds and mammals are the two streams of blood—those to the pulmonary and systemic circulations—permanently kept entirely separate by a physical barrier.<ref name=VB>{{cite book |last1=Romer |first1=Alfred Sherwood |last2=Parsons |first2=Thomas S. |year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location= Philadelphia|pages= 437–442|isbn= 978-0-03-910284-5}}</ref>

===Fish===
{{Main|Fish anatomy#Heart}}
{{See also|Venous heart}}
[[File:Two chamber heart.svg|thumb|{{center|Blood flow through the fish heart: sinus venosus, atrium, ventricle, and outflow tract}}]]
The heart evolved no less than 380&nbsp;million years ago in [[fish]].<ref name="380m">{{Cite web |url=https://www.bbc.com/news/science-environment-62912225 |publisher=[[BBC News]] |date=2022-09-15 |accessdate=2022-09-16 |last=Ghosh |first=Pallab |lang=en-GB |title=World's oldest heart found in prehistoric fish}}</ref> Fish have what is often described as a two-chambered heart,<ref name="Jurd2004">{{cite book|last=Jurd|first=Richard David|title=Instant Notes Animal Biology|url=https://books.google.com/books?id=i9YwWhkHOs4C&pg=PA134|year=2004|publisher=Garland Science|isbn=978-1-85996-325-8|page=134|url-status=live|archive-url=https://web.archive.org/web/20161206110614/https://books.google.com/books?id=i9YwWhkHOs4C&pg=PA134|archive-date=6 December 2016}}</ref> consisting of one atrium to receive blood and one ventricle to pump it.<ref name="Ostrander2000"/> However, the fish heart has entry and exit compartments that may be called chambers, so it is also sometimes described as three-chambered<ref name="Ostrander2000">{{cite book|last=Ostrander|first=Gary Kent|title=The Laboratory Fish|url=https://books.google.com/books?id=Hp4YSFiSD0IC&pg=PT154|year=2000|publisher=Elsevier|isbn=978-0-12-529650-2|pages=154–155|url-status=live|archive-url=https://web.archive.org/web/20161206074402/https://books.google.com/books?id=Hp4YSFiSD0IC&pg=PT154|archive-date=6 December 2016}}</ref> or four-chambered,<ref name=Farrell2011>{{cite book|editor=Farrell, Anthony P|others=Stevens, E Don; Cech, Jr., Joseph J; Richards, Jeffrey G|title=Encyclopedia of Fish Physiology: From Genome to Environment|url=https://books.google.com/books?id=3bsgS125KH0C&pg=PP2315|year=2011|publisher=Academic Press|isbn=978-0-08-092323-9|page=2315|url-status=live|archive-url=https://web.archive.org/web/20161206101409/https://books.google.com/books?id=3bsgS125KH0C&pg=PP2315|archive-date=6 December 2016}}</ref> depending on what is counted as a chamber. The atrium and ventricle are sometimes considered "true chambers", while the others are considered "accessory chambers".<ref name="Shukla">{{cite book|last=Shukla|first=J.P.|title=Fish & Fisheries|url=https://books.google.com/books?id=a05t8fWR2wIC&pg=PA155|publisher=Rastogi Publications|isbn=978-81-7133-800-9|pages=154–155|url-status=live|archive-url=https://web.archive.org/web/20161206134115/https://books.google.com/books?id=a05t8fWR2wIC&pg=PA155|archive-date=6 December 2016}}</ref>

Primitive fish have a four-chambered heart, but the chambers are arranged sequentially so that this primitive heart is quite unlike the four-chambered hearts of mammals and birds. The first chamber is the [[sinus venosus]], which collects deoxygenated blood from the body through the [[hepatic vein|hepatic]] and [[cardinal vein (disambiguation)|cardinal veins]]. From here, blood flows into the atrium and then to the powerful muscular ventricle where the main pumping action will take place. The fourth and final chamber is the [[conus arteriosus]], which contains several valves and sends blood to the ''ventral aorta''. The ventral aorta delivers blood to the gills where it is oxygenated and flows, through the [[descending aorta|dorsal aorta]], into the rest of the body. (In [[tetrapod]]s, the ventral aorta has divided in two; one half forms the [[ascending aorta]], while the other forms the pulmonary artery).<ref name=VB />

In the adult fish, the four chambers are not arranged in a straight row but instead form an S-shape, with the latter two chambers lying above the former two. This relatively simple pattern is found in [[cartilaginous fish]] and in the [[ray-finned fish]]. In [[teleost]]s, the conus arteriosus is very small and can more accurately be described as part of the aorta rather than of the heart proper. The conus arteriosus is not present in any [[amniote]]s, presumably having been absorbed into the ventricles over the course of evolution. Similarly, while the sinus venosus is present as a vestigial structure in some reptiles and birds, it is otherwise absorbed into the right atrium and is no longer distinguishable.<ref name=VB />

===Invertebrates===
[[File:Structural organization of the heart of the mosquito Anopheles gambiae - image.ppat.v08.i11.g001.png|thumb|The tube-like heart (green) of the mosquito ''[[Anopheles gambiae]]'' extends horizontally across the body, interlinked with the diamond-shaped [[insect wing#Muscles|wing muscles]] (also green) and surrounded by [[pericardium|pericardial cells]] (red). Blue depicts [[cell nuclei]].]]
[[File:Arthropod body struct 01.png|thumb|left|Basic [[arthropod]] body structure – heart shown in red]]
[[Arthropod]]s and most [[mollusk]]s have an open circulatory system. In this system, deoxygenated blood collects around the heart in cavities ([[:wikt:sinus|sinuses]]). This blood slowly permeates the heart through many small one-way channels. The heart then pumps the blood into the [[hemocoel]], a cavity between the organs. The heart in arthropods is typically a muscular tube that runs the length of the body, under the back and from the base of the head. Instead of blood the circulatory fluid is [[haemolymph]] which carries the most commonly used [[respiratory pigment]], copper-based [[haemocyanin]] as the oxygen transporter. Haemoglobin is only used by a few arthropods.<ref name=SOLOMON2010>{{Cite book|url=https://books.google.com/books?id=qdQ8AAAAQBAJ|title=Biology|last1=Solomon|first1=Eldra|last2=Berg|first2=Linda|last3=Martin|first3=Diana W.|year=2010|page=939|publisher=Cengage Learning|isbn=978-1-133-17032-7|language=en|url-status=live|archive-url=https://web.archive.org/web/20161206054642/https://books.google.com/books?id=qdQ8AAAAQBAJ|archive-date=6 December 2016}}</ref>
[[File:Cephalopod systemic heart.svg|thumb|left|Schematic of [[cephalopod]] heart]]
In some other invertebrates such as [[earthworm]]s, the circulatory system is not used to transport oxygen and so is much reduced, having no veins or arteries and consisting of two connected tubes. Oxygen travels by diffusion and there are five small muscular vessels that connect these vessels that contract at the front of the animals that can be thought of as "hearts".<ref name=SOLOMON2010/>

[[Cephalopod|Squids and other cephalopods]] have two "gill hearts" also known as [[branchial heart]]s, and one "systemic heart".<ref>Schipp, R., von Boletzky, S., Jakobs, P. et al. "A congenital malformation of the systemic heart complex in Sepia officinalis L. (Cephalopoda)". Helgoländer Meeresunters. 52, 29–40 (1998). {{doi|10.1007/BF02908733}}</ref> The branchial hearts have two atria and one ventricle each, and pump to the [[gill]]s, whereas the systemic heart pumps to the body.<ref>{{cite web|title=Meet our animals|url=https://nationalzoo.si.edu/Animals/Invertebrates/facts/cephalopods/default.cfm|website=Smithsonian National Zoological Park|access-date=14 August 2016|url-status=dead|archive-url=https://web.archive.org/web/20160729081825/https://nationalzoo.si.edu/Animals/Invertebrates/Facts/cephalopods/default.cfm|archive-date=29 July 2016}}</ref><ref name="Prosser1991">{{cite book|vauthors=Ladd, Prosser C|title=Comparative Animal Physiology, Environmental and Metabolic Animal Physiology|url=https://books.google.com/books?id=7fQvbFlQBaQC&pg=PA537|year=1991|publisher=John Wiley & Sons|isbn=978-0-471-85767-9|pages=537–|url-status=live|archive-url=https://web.archive.org/web/20161206215610/https://books.google.com/books?id=7fQvbFlQBaQC&pg=PA537|archive-date=6 December 2016}}</ref>
{{clear}}

Only the [[chordates]] (including vertebrates) and the [[hemichordates]] have a central "heart", which is a vesicle formed from the thickening of the [[aorta]] and contracts to pump blood. This suggests a presence of it in the last [[deuterostome|common ancestor of these groups]] (may have been lost in the [[echinoderms]]).