Editing Bird (section)

====Heart type and features====
[[File:Didactic model of an avian heart-FMVZ USP-13 (cropped).jpg|thumb|upright=0.65|[[Educational toy|Didactic model]] of an avian heart]]
The avian circulatory system is driven by a four-chambered, myogenic heart contained in a fibrous pericardial sac. This pericardial sac is filled with a [[serous fluid]] for lubrication.<ref name=Whittow>{{cite book |last1=Whittow |first1=G. |year=2000 |title=Sturkie's Avian Physiology |editor-first1=G. Causey |editor-last1=Whittow |location=San Diego |publisher=Academic Press}}</ref> The heart itself is divided into a right and left half, each with an [[atrium (heart)|atrium]] and [[ventricle (heart)|ventricle]]. The atrium and ventricles of each side are separated by [[atrioventricular valves]] which prevent back flow from one chamber to the next during contraction. Being myogenic, the heart's pace is maintained by pacemaker cells found in the sinoatrial node, located on the right atrium.<ref>{{Cite web |last1=Molnar |first1=Charles |last2=Gair |first2=Jane |date=14 May 2015 |title=21.3. Mammalian Heart and Blood Vessels |url=https://opentextbc.ca/biology/chapter/21-3-mammalian-heart-and-blood-vessels/ |language=en}}</ref>

The [[sinoatrial node]] uses calcium to cause a [[Depolarization|depolarising]] [[signal transduction pathway]] from the atrium through right and left atrioventricular bundle which communicates contraction to the ventricles. The avian heart also consists of muscular arches that are made up of thick bundles of muscular layers. Much like a mammalian heart, the avian heart is composed of [[endocardial]], [[myocardial]] and [[epicardial]] layers.<ref name=Whittow /> The atrium walls tend to be thinner than the ventricle walls, due to the intense ventricular contraction used to pump oxygenated blood throughout the body. Avian hearts are generally larger than mammalian hearts when compared to body mass. This adaptation allows more blood to be pumped to meet the high metabolic need associated with flight.<ref name="Hoagstrom">{{cite journal |last1=Hoagstrom |first1=C.W. |year=2002 |title=Vertebrate Circulation |journal=Magill's Encyclopedia of Science: Animal Life |volume=1 |pages=217–219 |location=Pasadena, California |publisher=Salem Press}}</ref>