Editing Bird (section)

===Respiratory and circulatory systems===
Birds have one of the most complex [[respiratory system]]s of all animal groups.<ref name="Gill"/> Upon inhalation, 75% of the fresh air bypasses the lungs and flows directly into a posterior [[Parabronchi|air sac]] which extends from the lungs and connects with air spaces in the bones and fills them with air. The other 25% of the air goes directly into the lungs. When the bird exhales, the used air flows out of the lungs and the stored fresh air from the posterior air sac is simultaneously forced into the lungs. Thus, a bird's lungs receive a constant supply of fresh air during both inhalation and exhalation.<ref>{{Cite journal |last=Maina |first=John N. |date=2007-01-10 |title=Development, structure, and function of a novel respiratory organ, the lung-air sac system of birds: to go where no other vertebrate has gone |journal=Biological Reviews |language=en |volume=81 |issue=4 |pages=545–579 |doi=10.1111/j.1469-185X.2006.tb00218.x|pmid=17038201 }}</ref> Sound production is achieved using the [[syrinx (biology)|syrinx]], a muscular chamber incorporating multiple tympanic membranes which diverges from the lower end of the trachea;<ref name="Suthers">{{Cite journal|last=Suthers |first=Roderick A. |author2=Sue Anne Zollinger |pmid=15313772 |doi=10.1196/annals.1298.041 |volume=1016 |issue=1 |title=Producing song: the vocal apparatus |date=June 2004 |journal=Ann. N.Y. Acad. Sci. |pages=109–129|bibcode=2004NYASA1016..109S }}</ref> the trachea being elongated in some species, increasing the volume of vocalisations and the perception of the bird's size.<ref name="Fitch">{{Cite journal|last=Fitch |first=W.T. |year=1999 |title=Acoustic exaggeration of size in birds via tracheal elongation: comparative and theoretical analyses |journal=Journal of Zoology |volume=248 |pages=31–48 |doi=10.1017/S095283699900504X}}</ref>

In birds, the main arteries taking blood away from the heart originate from the right [[aortic arches|aortic arch]] (or pharyngeal arch), unlike in the mammals where the left aortic arch forms this part of the [[aorta]].<ref name="Gill"/> The postcava receives blood from the limbs via the renal portal system. Unlike in mammals, the circulating [[red blood cells]] in birds retain their [[cell nucleus|nucleus]].<ref>{{Cite journal|last=Scott |first=Robert B. |date=March 1966 |title=Comparative hematology: The phylogeny of the erythrocyte |journal=Annals of Hematology |volume=12 |issue=6 |pages=340–351 |doi=10.1007/BF01632827 |pmid=5325853 }}</ref>

====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>

====Organisation====
Birds have a very efficient system for diffusing oxygen into the blood; birds have a ten times greater surface area to [[gas exchange]] volume than mammals. As a result, birds have more blood in their capillaries per unit of volume of lung than a mammal.<ref name="Hoagstrom" /> The arteries are composed of thick elastic muscles to withstand the pressure of the ventricular contractions, and become more rigid as they move away from the heart. Blood moves through the arteries, which undergo [[vasoconstriction]], and into arterioles which act as a transportation system to distribute primarily oxygen as well as nutrients to all tissues of the body. As the arterioles move away from the heart and into individual organs and tissues they are further divided to increase surface area and slow blood flow. Blood travels through the arterioles and moves into the capillaries where gas exchange can occur.<ref name=Hill>{{cite book |last1=Hill |first1=Richard W. |year=2012 |title=Animal Physiology |editor-first1=Richard W. |editor-last1=Hill |editor-first2=Gordon A. |editor-last2=Wyse |editor-first3=Margaret |editor-last3=Anderson |edition=Third |pages=647–678 |publisher=Sinauer Associates |location=Sunderland, MA}}</ref>

Capillaries are organised into capillary beds in tissues; it is here that blood exchanges oxygen for carbon dioxide waste. In the capillary beds, blood flow is slowed to allow maximum [[diffusion]] of oxygen into the tissues. Once the blood has become deoxygenated, it travels through venules then veins and back to the heart. Veins, unlike arteries, are thin and rigid as they do not need to withstand extreme pressure. As blood travels through the venules to the veins a funneling occurs called [[vasodilation]] bringing blood back to the heart.<ref name=Hill /> Once the blood reaches the heart, it moves first into the right atrium, then the right ventricle to be pumped through the lungs for further gas exchange of carbon dioxide waste for oxygen. Oxygenated blood then flows from the lungs through the left atrium to the left ventricle where it is pumped out to the body.<ref name=":1" />