Editing Heart (section)

===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 />