Editing Reptile (section)

===Circulation===
[[File:wiki varano.jpg|thumb|Thermographic image of [[monitor lizard]]s]]

All [[Lepidosauria|lepidosaurs]] and [[turtle]]s have a three-chambered [[heart]] consisting of two [[atrium (heart)|atria]], one variably partitioned [[ventricle (heart)|ventricle]], and two aortas that lead to the [[systemic circulation]]. The degree of mixing of [[oxygen]]ated and deoxygenated blood in the three-chambered heart varies depending on the species and physiological state. Under different conditions, deoxygenated blood can be shunted back to the body or oxygenated blood can be shunted back to the lungs. This variation in blood flow has been hypothesized to allow more effective thermoregulation and longer diving times for aquatic species, but has not been shown to be a [[fitness (biology)|fitness]] advantage.<ref>{{cite journal |last=Hicks |first=James |year=2002 |title=The Physiological and Evolutionary Significance of Cardiovascular Shunting Patterns in Reptiles |journal=News in Physiological Sciences |volume=17 |issue=6 |pages=241–245 |pmid=12433978|doi=10.1152/nips.01397.2002 |s2cid=20040550 }}</ref>

[[File:Bisected Iguana Heart Image.png|thumb|Juvenile [[iguana]] [[heart]] bisected through the ventricle, bisecting the left and right atrium]]
For example,&nbsp;[[iguana]]&nbsp;hearts, like the majority of the&nbsp;[[Squamata|squamate]]&nbsp;hearts, are composed of three chambers with two aorta and one ventricle, cardiac involuntary muscles.<ref>{{Cite news|url=http://veterinarycalendar.dvm360.com/reptilian-cardiovascular-anatomy-and-physiology-evaluation-and-monitoring-proceedings?id=&pageID=1&sk=&date=|title=Reptilian cardiovascular anatomy and physiology: evaluation and monitoring (Proceedings)|last=DABVP|first=Ryan S. De Voe DVM MSpVM DACZM|work=dvm360.com|access-date=2017-04-22|archive-date=2018-11-06|archive-url=https://web.archive.org/web/20181106205150/http://veterinarycalendar.dvm360.com/reptilian-cardiovascular-anatomy-and-physiology-evaluation-and-monitoring-proceedings?id=&pageID=1&sk=&date=|url-status=dead}}</ref> The main structures of the heart are the&nbsp;[[sinus venosus]], the pacemaker, the&nbsp;[[Atrium (heart)|left atrium]], the&nbsp;[[Atrium (heart)|right atrium]], the&nbsp;[[Heart valve|atrioventricular valve]], the cavum venosum, cavum arteriosum, the cavum pulmonale, the muscular ridge, the ventricular ridge,&nbsp;[[pulmonary vein]]s, and paired&nbsp;[[aortic arches]].<ref>{{Cite news|url=http://reptile-parrots.com/forums/showthread.php/825-Iguana-Internal-Body-Parts|title=Iguana Internal Body Parts|work=Reptile & Parrots Forum|access-date=2017-04-22|language=en|archive-date=2017-04-22|archive-url=https://web.archive.org/web/20170422123904/http://reptile-parrots.com/forums/showthread.php/825-Iguana-Internal-Body-Parts|url-status=dead}}</ref>

Some squamate species (e.g., pythons and monitor lizards) have three-chambered hearts that become functionally four-chambered hearts during contraction. This is made possible by a muscular ridge that subdivides the ventricle during [[cardiac cycle|ventricular diastole]] and completely divides it during [[systole (medicine)|ventricular systole]]. Because of this ridge, some of these [[squamata|squamates]] are capable of producing ventricular pressure differentials that are equivalent to those seen in mammalian and avian hearts.<ref>{{cite journal | last=Wang | first=Tobias |author2=Altimiras, Jordi |author3=Klein, Wilfried |author4= Axelsson, Michael | title=Ventricular haemodynamics in Python molurus: separation of pulmonary and systemic pressures | journal=The Journal of Experimental Biology | year=2003 | volume=206 | pages=4242–4245 | doi=10.1242/jeb.00681 | pmid=14581594 | issue=Pt 23| doi-access=free | bibcode=2003JExpB.206.4241W }}</ref>

[[Crocodilia]]ns have an anatomically four-chambered heart, similar to [[bird]]s, but also have two systemic aortas and are therefore capable of bypassing their [[pulmonary circulation]].<ref>{{cite journal |last=Axelsson |first=Michael |author2=Craig E. Franklin |year=1997 |title=From anatomy to angioscopy: 164 years of crocodilian cardiovascular research, recent advances, and speculations |journal=Comparative Biochemistry and Physiology A |volume=188 |issue=1 |pages=51–62 |doi=10.1016/S0300-9629(96)00255-1}}</ref> In turtles, the ventricle is not perfectly divided, so a mix of aerated and nonaerated blood can occur.<ref>{{cite web |last1=Zug |first1=George R. |title=Reptile |url=https://www.britannica.com/animal/reptile. Accessed 30 September 2024. |website=Encyclopedia Britannica |publisher=Britannica |access-date=30 September 2024}}</ref>