Editing Circulatory system (section)

===Blood vessels===
The [[blood vessel]]s of the circulatory system are the [[arteries]], [[veins]], and [[capillaries]]. The large arteries and veins that take blood to, and away from the heart are known as the [[great vessels]].<ref name="Gray's">{{cite book |last1=Standring |first1=Susan |title=Gray's anatomy : the anatomical basis of clinical practice |date=2016 |location=[Philadelphia] |isbn=9780702052309 |page=1024 |edition=Forty-first |publisher=Elsevier Limited }}</ref>

====Arteries====
{{main|Artery}}
{{See also|Arterial tree}}
[[File:Vein_art_near.png|thumb|Depiction of the heart, major veins and arteries constructed from body scans]]
Oxygenated blood enters the systemic circulation when leaving the left ventricle, via the [[Aortic valve|aortic semilunar valve]].<ref name="Iaizzo">{{cite book|url=https://books.google.com/books?id=4PLuCgAAQBAJ|title=Handbook of Cardiac Anatomy, Physiology, and Devices|publisher=Springer|author=Iaizzo, Paul A|year=2015|page=93|isbn=978-3-31919464-6|access-date=2022-01-28|archive-date=2017-10-11|archive-url=https://web.archive.org/web/20171011044440/https://books.google.com/books?id=4PLuCgAAQBAJ|url-status=live}}</ref> The first part of the systemic circulation is the aorta, a massive and thick-walled artery. The aorta arches and gives branches supplying the upper part of the body after passing through the aortic opening of the diaphragm at the level of thoracic ten vertebra, it enters the abdomen.<ref name="Iaizzo two">{{cite book|url=https://books.google.com/books?id=4PLuCgAAQBAJ|title=Handbook of Cardiac Anatomy, Physiology, and Devices|publisher=Springer|author=Iaizzo, Paul A|year=2015|pages=5, 77|isbn=978-3-31919464-6|access-date=2022-01-28|archive-date=2017-10-11|archive-url=https://web.archive.org/web/20171011044440/https://books.google.com/books?id=4PLuCgAAQBAJ|url-status=live}}</ref> Later, it descends down and supplies branches to abdomen, pelvis, perineum and the lower limbs.<ref name="Iaizzo three">{{cite book|url=https://books.google.com/books?id=4PLuCgAAQBAJ|title=Handbook of Cardiac Anatomy, Physiology, and Devices|publisher=Springer|author=Iaizzo, Paul A|year=2015|pages=5, 41–43|isbn=978-3-31919464-6|access-date=2022-01-28|archive-date=2017-10-11|archive-url=https://web.archive.org/web/20171011044440/https://books.google.com/books?id=4PLuCgAAQBAJ|url-status=live}}</ref>

The walls of the aorta are elastic. This elasticity helps to maintain the [[blood pressure]] throughout the body.<ref name="Vaz">{{cite book|url=https://books.google.com/books?id=J_HQDwAAQBAJ&pg=PA255|title=Guyton & Hall Textbook of Medical Physiology – E-Book: A South Asian Edition|publisher=Elsevier Health Sciences|author1=Vaz, Mario|author2=Raj, Toni|author3=Anura, Kurpad|year=2016|page=255|isbn=978-8-13-124665-8|access-date=2022-01-28|archive-date=2022-01-28|archive-url=https://web.archive.org/web/20220128215455/https://books.google.com/books?id=J_HQDwAAQBAJ&pg=PA255|url-status=live}}</ref> When the aorta receives almost five litres of blood from the heart, it recoils and is responsible for pulsating blood pressure. As the aorta branches into smaller arteries, their elasticity goes on decreasing and their compliance goes on increasing.<ref name="Vaz"/>

====Capillaries====
Arteries branch into small passages called [[arteriole]]s and then into the [[Capillary|capillaries]].<ref>{{cite web | url =https://www.nhlbi.nih.gov/health/health-topics/topics/hlw/printall-index.html | title =What Are the Lungs? | author =National Institutes of Health | publisher =nih.gov | url-status=dead | archive-url =https://web.archive.org/web/20141004200807/https://www.nhlbi.nih.gov/health/health-topics/topics/hlw/printall-index.html | archive-date =2014-10-04 | author-link =National Institutes of Health }}</ref> The capillaries merge to bring blood into the venous system.<ref>{{cite web | url =https://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20100/Bio%20100%20Lectures/Organ%20Systems/Circulatory%20System/Circulatory%20System.htm | title =The Circulatory System | author =State University of New York | publisher =suny.edu | date =February 3, 2014 | url-status=dead | archive-url =https://archive.today/20140203084650/http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20100/Bio%20100%20Lectures/Organ%20Systems/Circulatory%20System/Circulatory%20System.htm | archive-date =February 3, 2014 | author-link =State University of New York }}</ref> The total length of muscle capillaries in a 70 kg human is estimated to be between 9,000 and 19,000 km.<ref>{{Cite journal |last1=Poole |first1=David C. |last2=Kano |first2=Yutaka |last3=Koga |first3=Shunsaku |last4=Musch |first4=Timothy I. |date=March 2021 |title=August Krogh: Muscle capillary function and oxygen delivery |journal=Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology |language=en |volume=253 |issue=110852 |url=https://europepmc.org/backend/ptpmcrender.fcgi?accid=PMC7867635&blobtype=pdf |doi=10.1016/j.cbpa.2020.110852 |pmc=7867635 |pmid=33242636}}</ref>

====Veins====
{{main|Vein}}
Capillaries merge into [[venule]]s, which merge into veins.<ref name="Mcconnell">{{cite book|url=https://books.google.com/books?id=BeLcDwAAQBAJ&pg=PA432|title=Human Form, Human Function: Essentials of Anatomy & Physiology, Enhanced Edition|publisher=Jones & Bartlett Learning|author1=Mcconnell, Thomas H.|author2=Hull, Kerry L.|year=2020|page=432|isbn=978-1-28-421805-3|access-date=2022-01-28|archive-date=2022-01-28|archive-url=https://web.archive.org/web/20220128215457/https://books.google.com/books?id=BeLcDwAAQBAJ&pg=PA432|url-status=live}}</ref> The [[venous system]] feeds into the two major veins: the superior vena cava – which mainly drains tissues above the heart – and the inferior vena cava – which mainly drains tissues below the heart. These two large veins empty into the right atrium of the heart.<ref name="Floyd">{{cite book|url=https://books.google.com/books?id=V__aAAAAMAAJ|title=Understanding Pathophysiology|publisher=Mosby|author1=Parkinson, Clayton Floyd|author2=Huether, Sue E.|author3=McCance, Kathryn L.|year=2000|page=161|isbn=978-0-32-300792-4}}</ref>

====Portal veins====
{{Main|Portal venous system}}

The general rule is that arteries from the heart branch out into capillaries, which collect into veins leading back to the heart. [[Portal venous system|Portal veins]] are a slight exception to this. In humans, the only significant example is the [[hepatic portal vein]] which combines from capillaries around the [[gastrointestinal tract]] where the blood absorbs the various products of digestion; rather than leading directly back to the heart, the hepatic portal vein branches into a second capillary system in the [[liver]].

====Coronary circulation====
{{Main|Coronary circulation}}
The heart itself is supplied with oxygen and nutrients through a small "loop" of the systemic circulation and derives very little from the blood contained within the four chambers.
The coronary circulation system provides a blood supply to the [[myocardium|heart muscle]] itself. The coronary circulation begins near the origin of the aorta by two [[coronary arteries]]: the [[right coronary artery]] and the [[left coronary artery]]. After nourishing the heart muscle, blood returns through the coronary veins into the [[coronary sinus]] and from this one into the right atrium. Backflow of blood through its opening during [[atrial systole]] is prevented by the [[Thebesian valve]]. The [[smallest cardiac veins]] drain directly into the heart chambers.<ref name="Guyton"/>

====Cerebral circulation====
{{Main|Cerebral circulation}}
The brain has a dual blood supply, an ''anterior'' and a ''posterior circulation'' from arteries at its front and back. The anterior circulation arises from the [[Internal carotid artery|internal carotid arteries]] to supply the front of the brain. The posterior circulation arises from the [[Vertebral artery|vertebral arteries]], to supply the back of the brain and [[brainstem]]. The circulation from the front and the back join ([[anastomise]]) at the [[circle of Willis]]. The [[neurovascular unit]], composed of various cells and vasculature channels within the brain, regulates the flow of blood to activated neurons in order to satisfy their high energy demands.<ref>{{Cite journal |last=Iadecola |first=Costantino |date=2017-09-27 |title=The Neurovascular Unit Coming of Age: A Journey through Neurovascular Coupling in Health and Disease |journal=Neuron |volume=96 |issue=1 |pages=17–42 |doi=10.1016/j.neuron.2017.07.030 |issn=1097-4199 |pmc=5657612 |pmid=28957666 }}</ref>

====Renal circulation====
The [[renal circulation]] is the blood supply to the [[kidney]]s, contains many specialized blood vessels and receives around 20% of the cardiac output. It branches from the [[abdominal aorta]] and returns blood to the ascending [[inferior vena cava]].