Editing Circulatory system (section)

===Closed circulatory system===

[[File:Two chamber heart.svg |thumb |left |Two-chambered heart of a fish]]

The circulatory systems of all vertebrates, as well as of [[annelid]]s (for example, [[earthworm]]s) and [[cephalopod]]s ([[squid]]s, [[octopus]]es and relatives) always keep their circulating blood enclosed within heart chambers or blood vessels and are classified as ''closed'', just as in humans. Still, the systems of [[fish]], [[amphibian]]s, [[reptile]]s, and [[bird]]s show various stages of the [[evolution]] of the circulatory system.<ref>{{cite journal |doi=10.1016/j.ydbio.2004.09.026 |pmid=15572135 |title=The evolutionary origin of cardiac chambers |year=2005 |last1=Simões-Costa |first1=Marcos S. |last2=Vasconcelos |first2=Michelle |last3=Sampaio |first3=Allysson C. |last4=Cravo |first4=Roberta M. |last5=Linhares |first5=Vania L. |last6=Hochgreb |first6=Tatiana |last7=Yan |first7=Chao Y.I. |last8=Davidson |first8=Brad |last9=Xavier-Neto |first9=José |journal=Developmental Biology |volume=277 |issue=1 |pages=1–15 |doi-access=}}</ref> Closed systems permit blood to be directed to the organs that require it.

In fish, the system has only one circuit, with the blood being pumped through the capillaries of the [[gill]]s and on to the capillaries of the body tissues. This is known as ''single cycle'' circulation. The heart of fish is, therefore, only a single pump (consisting of two chambers).{{cn |date=January 2025}}

In amphibians and most reptiles, a double circulatory system is used, but the heart is not always completely separated into two pumps. Amphibians have a three-chambered heart.{{cn |date=January 2025}}

In reptiles, the [[ventricular septum]] of the heart is incomplete and the [[pulmonary artery]] is equipped with a [[sphincter muscle]]. This allows a second possible route of blood flow. Instead of blood flowing through the pulmonary artery to the lungs, the sphincter may be contracted to divert this blood flow through the incomplete ventricular septum into the left ventricle and out through the [[aorta]]. This means the blood flows from the capillaries to the heart and back to the capillaries instead of to the lungs. This process is useful to [[ectothermic]] (cold-blooded) animals in the regulation of their body temperature.{{cn |date=January 2025}}

Mammals, birds and [[crocodilia]]ns 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 birds and crocodilians evolved independently from that of mammals.<ref>{{cite web | title=Crocodilian Hearts | date=October 24, 2008 | access-date=October 3, 2015 | website=National Center for Science Education | url=https://ncse.com/creationism/analysis/crocodilian-hearts | archive-date=September 26, 2015 | archive-url=https://web.archive.org/web/20150926104854/http://ncse.com/creationism/analysis/crocodilian-hearts | url-status=live }}</ref> Double circulatory systems permit blood to be repressurized after returning from the lungs, speeding up delivery of oxygen to tissues.{{cn |date=January 2025}}