Editing Heart (section)

===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 />