View source for Circulatory system

{{short description|Organ system for circulating blood in animals}}
{{hatnote group|
{{other uses}}
{{redirect-several|dab=no|Bloodstream (disambiguation)|Cardio-Vascular (album)}}
}}
{{pp-semi-indef|small=yes}}
{{Use British English|date=July 2023}}
{{Use dmy dates|date=July 2023}}
{{Infobox anatomy
| Name        = Circulatory system
| Latin       =
| Image       = Circulatory System en.svg
| Caption     = The human circulatory system (simplified). Red indicates [[oxygenated blood]] carried in [[Artery|arteries]]. Blue indicates [[deoxygenated blood]] carried in [[vein]]s. [[Capillary|Capillaries]] join the arteries and veins.
| Width       =
| Image2      =
| Caption2    =
| Precursor   =
| System      =
| Artery      =
| Vein        =
| Nerve       =
| Lymph       =
}}

In [[vertebrate]]s, the '''circulatory system''' is a [[organ system|system of organs]] that includes the [[heart]], [[blood vessel]]s, and [[blood]] which is circulated throughout the body. It includes the [[cardiovascular system]], or vascular system, that consists of the heart and blood vessels (from Greek {{lang|el|kardia}} meaning ''heart'', and Latin {{lang|la|vascula}} meaning ''vessels''). The circulatory system has two divisions, a [[systemic circulation|systemic circulation or circuit]], and a [[pulmonary circulation|pulmonary circulation or circuit]]. Some sources use the terms ''cardiovascular system'' and ''vascular system'' interchangeably with ''circulatory system''.

The network of blood vessels are the [[great vessels]] of the heart including large [[elastic arteries]], and large [[vein]]s; other arteries, smaller [[arteriole]]s, [[capillaries]] that join with [[venule]]s (small veins), and other veins. The [[Closed circulatory system|circulatory system is closed]] in vertebrates, which means that the blood never leaves the network of blood vessels. Many [[invertebrate]]s such as [[arthropod]]s have an [[open circulatory system]] with a heart that pumps a hemolymph which returns via the body cavity rather than via blood vessels. [[Diploblast]]s such as [[sponges]] and [[comb jellies]] lack a circulatory system.

Blood is a [[Body fluid|fluid]] consisting of [[blood plasma|plasma]], [[red blood cell]]s, [[white blood cell]]s, and [[platelets]]; it is circulated around the body carrying [[oxygen]] and [[nutrient]]s to the tissues and collecting and disposing of [[Metabolic waste|waste materials]]. Circulated nutrients include [[Protein (nutrient)|proteins]] and [[Mineral (nutrient)|minerals]] and other components include  [[hemoglobin]], [[hormone]]s, and [[gas]]es such as oxygen and  [[carbon dioxide]]. These substances provide nourishment, help the [[immune system]] to fight [[diseases]], and help maintain [[homeostasis]] by [[Thermoregulation|stabilizing temperature]] and [[pH#pH of various body fluids|natural pH]].

In vertebrates, the [[lymphatic system]] is complementary to the circulatory system. The lymphatic system carries excess plasma ([[Capillary filtration|filtered]] from the circulatory system [[capillaries]] as [[interstitial fluid]] between cells) away from the body tissues via accessory routes that return excess fluid back to blood circulation as [[lymph]].<ref name="Sherwood, Lauralee 2011 401">{{cite book |author=Sherwood, Lauralee |title=Human Physiology: From Cells to Systems |url=https://books.google.com/books?id=I9qH3eZ1pP0C&pg=PT401 |year=2011 |publisher=Cengage Learning |isbn=978-1-133-10893-1 |pages=401– |access-date=2015-06-27 |archive-date=2020-07-29 |archive-url=https://web.archive.org/web/20200729124855/https://books.google.com/books?id=I9qH3eZ1pP0C&pg=PT401 |url-status=live }}</ref> The lymphatic system is a subsystem that is essential for the functioning of the blood circulatory system; without it the blood would become depleted of fluid.

The lymphatic system also works with the immune system. The circulation of lymph takes much longer than that of blood<ref name="CRU">{{cite web |url=https://www.cancerresearchuk.org/what-is-cancer/body-systems-and-cancer/the-lymphatic-system-and-cancer? |title=The lymphatic system and cancer &#124; Cancer Research UK |format= |date=29 October 2014 |accessdate=January 30, 2022 |archive-date=30 January 2022 |archive-url=https://web.archive.org/web/20220130075814/https://www.cancerresearchuk.org/what-is-cancer/body-systems-and-cancer/the-lymphatic-system-and-cancer |url-status=live }}</ref> and, unlike the closed (blood) circulatory system, the lymphatic system is an open system. Some sources describe it as a ''secondary circulatory system''.

The circulatory system can be affected by many [[cardiovascular disease]]s. [[Cardiologist]]s are medical professionals which specialise in the heart, and [[cardiothoracic surgeon]]s specialise in operating on the heart and its surrounding areas. [[Vascular surgeon]]s focus on disorders of the blood vessels, and lymphatic vessels.

==Structure==
{{anchor|Human cardiovascular system}}
[[File:2101 Blood Flow Through the Heart.jpg|thumb|350px|Blood flow in the pulmonary and systemic circulations showing capillary networks in the torso sections]]
The circulatory system includes the [[heart]], [[blood vessel]]s, and [[blood]].<ref name="Saladin3" /><ref name="Hall">{{cite book |last=Hall |first=John E. |title=Guyton and Hall textbook of medical physiology |date=2011 |isbn=9781416045748 |edition=Twelfth |location=Philadelphia, Pennsylvania |page=4}}</ref> The cardiovascular system in all vertebrates, consists of the heart and blood vessels. Some sources use the terms ''cardiovascular system'' and ''vascular system'' interchangeably with ''circulatory system''.<ref name="ncbibooks">{{cite book |url=https://www.ncbi.nlm.nih.gov/books/NBK279250/ |title=How does the blood circulatory system work? – InformedHealth.org – NCBI Bookshelf |date=31 January 2019 |publisher=Institute for Quality and Efficiency in Health Care |archive-url=https://web.archive.org/web/20220129064012/https://www.ncbi.nlm.nih.gov/books/NBK279250/ |archive-date=29 January 2022 |url-status=live |accessdate=}}</ref>

The circulatory system is further divided into two major circuits – a [[pulmonary circulation]], and a [[systemic circulation]].<ref>{{MeshName|Cardiovascular System}}</ref><ref name="Hall" /><ref name="Saladin3">{{cite book |last1=Saladin |first1=Kenneth S. |title=Human anatomy |date=2011 |publisher=McGraw-Hill |location=New York |isbn=9780071222075 |page=540 |edition=3rd}}</ref> The pulmonary circulation is a circuit loop from the [[right heart]] taking deoxygenated blood to the [[lung]]s where it is oxygenated and returned to the [[left heart]]. The systemic circulation is a circuit loop that delivers oxygenated blood from the left heart to the rest of the body, and returns deoxygenated blood back to the right heart via [[Great vessels|large veins]] known as the [[venae cavae]]. The systemic circulation can also be defined as two parts – a ''macrocirculation'' and a ''[[microcirculation]]''. An average adult contains five to six quarts (roughly 4.7 to 5.7 liters) of blood, accounting for approximately 7% of their total body weight.<ref>{{cite web|last=Pratt|first=Rebecca|title=Cardiovascular System: Blood|url=https://app.anatomyone.com/systemic/cardiovascular-system/blood|archive-url=https://web.archive.org/web/20170224023239/https://app.anatomyone.com/systemic/cardiovascular-system/blood|url-status=dead|archive-date=2017-02-24|work=AnatomyOne|publisher=Amirsys, Inc.}}</ref> Blood consists of [[Blood plasma|plasma]], [[red blood cells]], [[white blood cells]], and [[platelets]]. The [[digestive system]] also works with the circulatory system to provide the nutrients the system needs to keep the [[heart]] pumping.<ref name="Guyton">{{cite book |title=Guyton Textbook of Medical Physiology |edition=10 |author1=Guyton, Arthur |author2=Hall, John |year=2000 |publisher=Saunders |isbn=978-0-7216-8677-6}}</ref>

Further circulatory routes are associated, such as the [[coronary circulation]] to the heart itself, the [[cerebral circulation]] to the [[brain]], [[renal circulation]] to the [[kidney]]s, and [[bronchial circulation]] to the [[bronchi]] in the lungs. The human circulatory system is [[closed circulatory system|closed]], meaning that the blood is contained within the [[blood vessels|vascular network]].<ref name="Lawton">{{cite book|url=https://books.google.com/books?id=mj4mEAAAQBAJ&pg=PA6|title=The Human Circulatory System|publisher=Cavendish Square Publishing|author=Lawton, Cassie M.|year=2019|page=6|isbn=978-1-50-265720-6|access-date=2022-01-28|archive-date=2022-01-28|archive-url=https://web.archive.org/web/20220128204010/https://books.google.com/books?id=mj4mEAAAQBAJ&pg=PA6|url-status=live}}</ref> Nutrients travel through tiny blood vessels of the microcirculation to reach organs.<ref name="Lawton" /> The [[lymphatic system]] is an essential subsystem of the circulatory system consisting of a network of [[lymphatic vessel]]s, [[lymph nodes]], [[lymphatic organs|organs]], [[lymphatic tissues|tissues]] and circulating [[lymph]]. This subsystem is an [[open circulatory system|open system]].<ref name="Gartner">{{cite book|url=https://books.google.com/books?id=pGTtd5i2fR8C&pg=PA166|title=Concise Histology E-Book|publisher=Elsevier Health Sciences|author1=Gartner, Leslie P.|author2=Hiatt, James L.|year=2010|page=166|isbn=978-1-43-773579-6|access-date=2022-01-28|archive-date=2022-01-28|archive-url=https://web.archive.org/web/20220128204011/https://books.google.com/books?id=pGTtd5i2fR8C&pg=PA166|url-status=live}}</ref> A major function is to carry the lymph, draining and returning [[interstitial fluid]] into the [[lymphatic duct]]s back to the heart for return to the circulatory system. Another [[Lymphatic system#Function|major function]] is working together with the [[immune system]] to provide defense against [[pathogen]]s.<ref name="Alberts">{{cite book |last1=Alberts |first1=B. |last2=Johnson |first2=A. |last3=Lewis |first3=J. |last4=Raff |first4=M. |last5=Roberts |first5=K. |last6=Walters |first6=P. |title=Molecular Biology of the Cell |edition=4th |publisher=Garland Science |year=2002 |location=New York and London |url=https://www.ncbi.nlm.nih.gov/books/NBK21054/?depth=2 |isbn=978-0-8153-3218-3 |access-date=2017-08-30 |archive-date=2006-08-17 |archive-url=https://www.webcitation.org/5IDSbXuy2?url=http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowTOC |url-status=live }}</ref><ref name="Saladin3" />

===Heart===
{{Main|Heart}}
[[File:Blood oxygenation to the pulmonary and systemic circulation.svg|thumb|Diagram of the human [[heart]] showing blood oxygenation to the pulmonary and systemic circulation|border]]
The heart pumps blood to all parts of the body. In the human heart there is one [[Atrium (heart)|atrium]] and one [[Ventricle (heart)|ventricle]] for each circulation, and with both a systemic and a pulmonary circulation there are four chambers in total: [[left atrium]], [[left ventricle]], [[right atrium]] and [[right ventricle]].

====Pulmonary circulation====
{{Main|Pulmonary circulation}}
[[File:2119 Pulmonary Circuit.jpg|thumb|370px|right|The [[pulmonary circulation]] as it passes from the [[heart]]. Showing both the [[Pulmonary artery|pulmonary]] and [[Bronchial artery|bronchial arteries]].]]
Oxygen-deprived blood from the superior and inferior [[vena cava]] enters the right atrium of the heart and flows through the [[tricuspid valve]] (right atrioventricular valve) into the right ventricle, from which it is then pumped through the [[pulmonary semilunar valve]] into the pulmonary artery to the lungs. [[Gas exchange]] occurs in the lungs, whereby {{CO2|link=yes}} is released from the blood, and oxygen is absorbed. The pulmonary vein returns the now oxygen-rich blood to the [[left atrium]].<ref name="Guyton" />

A separate circuit from the systemic circulation, the [[bronchial circulation]] supplies blood to the tissue of the larger airways of the lung.

====Systemic circulation====
[[File:2105 Capillary Bed.jpg|thumb|upright=1.4|Capillary bed]]
[[File:Capillary system CERT.jpg|thumb|Diagram of capillary network joining the arterial system with the venous system]]

The systemic circulation is a circuit loop that delivers oxygenated blood from the left heart to the rest of the body through the [[aorta]]. Deoxygenated blood is returned in the systemic circulation to the right heart via two large veins, the [[inferior vena cava]] and [[superior vena cava]], where it is pumped from the right atrium into the pulmonary circulation for oxygenation. The systemic circulation can also be defined as having two parts – a macrocirculation and a [[microcirculation]].<ref name="Guyton"/>

===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]].

==Development==
{{Main|Heart development|Vasculogenesis|Vascular remodelling in the embryo|Fetal circulation}}
The development of the circulatory system starts with [[vasculogenesis]] in the [[embryo]]. The human arterial and venous systems develop from different areas in the embryo. The arterial system develops mainly from the [[aortic arches]], six pairs of arches that develop on the upper part of the embryo. The venous system arises from three bilateral veins during weeks 4&nbsp;– 8 of [[Human embryogenesis|embryogenesis]]. [[Fetal circulation]] begins within the 8th week of development. Fetal circulation does not include the lungs, which are bypassed via the [[truncus arteriosus]]. Before birth the [[fetus]] obtains [[oxygen]] (and [[nutrient]]s) from the mother through the [[placenta]] and the [[umbilical cord]].<ref name="Whitaker">{{cite book |last=Whitaker |first=Kent |chapter=Fetal Circulation |chapter-url=https://books.google.com/books?id=R3WK8XyAHYgC&pg=PA18 |title=Comprehensive Perinatal and Pediatric Respiratory Care |publisher=Delmar Thomson Learning |year=2001 |isbn=978-0-7668-1373-1 |pages=18–20 }}{{Dead link|date=August 2021 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

===Arteries===
{{Main|Aortic arches}}
[[File:Erytrocyte deoxy to oxy v0.7.gif|thumb|Animation of a typical human red blood cell cycle in the circulatory system. This animation occurs at a faster rate (~20 seconds of the average [[Red blood cell#Human|60-second cycle]]) and shows the red blood cell deforming as it enters capillaries, as well as the bars changing color as the cell alternates in states of oxygenation along the circulatory system.]]

The human arterial system originates from the [[aortic arches]] and from the [[dorsal aortae]] starting from week 4 of embryonic life. The first and second aortic arches regress and form only the [[Maxillary artery|maxillary arteries]] and [[Stapedial artery|stapedial arteries]] respectively. The arterial system itself arises from aortic arches 3, 4 and 6 (aortic arch 5 completely regresses).

The dorsal aortae, present on the [[:wikt:dorsal|dorsal]] side of the embryo, are initially present on both sides of the embryo. They later fuse to form the basis for the aorta itself. Approximately thirty smaller arteries branch from this at the back and sides. These branches form the [[intercostal arteries]], arteries of the arms and legs, lumbar arteries and the lateral sacral arteries. Branches to the sides of the aorta will form the definitive [[Renal artery|renal]], [[Inferior suprarenal artery|suprarenal]] and [[Gonadal artery|gonadal arteries]]. Finally, branches at the front of the aorta consist of the [[vitelline arteries]] and [[umbilical arteries]]. The vitelline arteries form the [[Celiac artery|celiac]], [[Superior mesenteric artery|superior]] and [[Inferior mesenteric artery|inferior mesenteric arteries]] of the gastrointestinal tract. After birth, the umbilical arteries will form the [[Internal iliac artery|internal iliac arteries]].

===Veins===
{{Further|Vasculogenesis}}
The human venous system develops mainly from the [[vitelline vein]]s, the [[umbilical vein]]s and the [[Posterior cardinal vein|cardinal veins]], all of which empty into the [[sinus venosus]].

==Function==
{{Main|Blood#Oxygen transport}}
About 98.5% of the oxygen in a sample of [[arterial blood]] in a healthy human, breathing air at sea-level pressure, is chemically combined with [[hemoglobin]] molecules. About 1.5% is physically dissolved in the other blood liquids and not connected to hemoglobin. The hemoglobin molecule is the primary transporter of oxygen in vertebrates.

==Clinical significance==
{{Further|List of circulatory system conditions}}
{{anchor|Health and disease}}
Many diseases affect the circulatory system. These include a number of [[cardiovascular disease]]s, affecting the heart and blood vessels; [[hematologic disease]]s that affect the blood, such as [[anemia]], and [[lymphatic disease]]s affecting the lymphatic system. [[Cardiologist]]s are medical professionals which specialise in the heart, and [[cardiothoracic surgeon]]s specialise in operating on the heart and its surrounding areas. [[Vascular surgeon]]s focus on the blood vessels.

===Cardiovascular disease===
{{Main|Cardiovascular disease}}
Diseases affecting the cardiovascular system are called ''cardiovascular disease''.

Many of these diseases are called "[[lifestyle disease]]s" because they develop over time and are related to a person's exercise habits, diet, whether they smoke, and other lifestyle choices a person makes. [[Atherosclerosis]] is the precursor to many of these diseases. It is where small [[Atheroma|atheromatous plaques]] build up in the walls of medium and large arteries. This may eventually grow or rupture to occlude the arteries. It is also a risk factor for [[acute coronary syndrome]]s, which are diseases that are characterised by a sudden deficit of oxygenated blood to the heart tissue. Atherosclerosis is also associated with problems such as [[aneurysm]] formation or splitting ("dissection") of arteries.

Another major cardiovascular disease involves the creation of a [[Thrombus|clot, called a "thrombus"]]. These can originate in veins or arteries. [[Deep venous thrombosis]], which mostly occurs in the legs, is one cause of clots in the veins of the legs, particularly when a person has been stationary for a long time. These clots may [[Embolus|embolise]], meaning travel to another location in the body. The results of this may include [[pulmonary embolus]], [[transient ischaemic attack]]s, or [[stroke]].

Cardiovascular diseases may also be congenital in nature, such as [[Congenital heart defect|heart defects]] or [[persistent fetal circulation]], where the circulatory changes that are supposed to happen after birth do not. Not all congenital changes to the circulatory system are associated with diseases, a large number are [[anatomical variation]]s.

===Investigations===
[[File:Arteria-lusoria MRA MIP.gif|thumb|[[Magnetic resonance angiography]] of [[aberrant subclavian artery]]]]
The function and health of the circulatory system and its parts are measured in a variety of manual and automated ways. These include simple methods such as those that are part of the [[cardiovascular examination]], including the taking of a person's [[pulse]] as an indicator of a person's [[heart rate]], the taking of [[blood pressure]] through a [[sphygmomanometer]] or the use of a [[stethoscope]] to listen to the heart for [[cardiac murmur|murmurs]] which may indicate problems with the [[heart valve|heart's valves]]. An [[electrocardiogram]] can also be used to evaluate the way in which electricity is conducted through the heart.

Other more invasive means can also be used. A [[cannula]] or [[catheter]] inserted into an artery may be used to measure [[pulse pressure]] or [[pulmonary wedge pressure]]s. Angiography, which involves injecting a dye into an artery to visualise an arterial tree, can be used in the heart ([[coronary angiography]]) or brain. At the same time as the arteries are visualised, blockages or narrowings may be fixed through the insertion of [[stent]]s, and active bleeds may be managed by the insertion of coils. An MRI may be used to image arteries, called an [[magnetic resonance angiography|MRI angiogram]]. For evaluation of the blood supply to the lungs a [[CT pulmonary angiogram]] may be used. [[Doppler ultrasonography#Blood vessels|Vascular ultrasonography]] may be used to investigate [[vascular disease]]s affecting the [[venous system]] and the [[arterial system]] including the diagnosis of [[stenosis]], [[Ultrasonography of deep venous thrombosis|thrombosis]] or [[Ultrasonography of chronic venous insufficiency of the legs|venous insufficiency]]. An [[intravascular ultrasound]] using a [[catheter]] is also an option.

===Surgery===
{{expand section|date=March 2015}}

{{Further|Vascular surgery|Vascular bypass}}
There are a number of surgical procedures performed on the circulatory system:
* [[Coronary artery bypass surgery]]
* [[Coronary stent]] used in [[angioplasty]]
* [[Vascular surgery]]
* [[Vein stripping]]
* Cosmetic procedures

Cardiovascular procedures are more likely to be performed in an inpatient setting than in an ambulatory care setting; in the United States, only 28% of cardiovascular surgeries were performed in the ambulatory care setting.<ref>{{cite journal | vauthors = Wier LM, Steiner CA, Owens PL | title = Surgeries in Hospital-Owned Outpatient Facilities, 2012 | journal =HCUP Statistical Brief |issue=188 | publisher = Agency for Healthcare Research and Quality | location = Rockville, MD | date = April 17, 2015 | url = https://www.hcup-us.ahrq.gov/reports/statbriefs/sb188-Surgeries-Hospital-Outpatient-Facilities-2012.jsp | archive-url = https://web.archive.org/web/20150312035159/http://www.hcup-us.ahrq.gov/reports/statbriefs/sb188-Surgeries-Hospital-Outpatient-Facilities-2012.jsp | url-status = dead | archive-date = March 12, 2015 }}</ref>

==Other animals==
[[File:Open Circulatroy.gif|thumb|The open circulatory system of the grasshopper – made up of a heart, vessels and hemolymph. The hemolymph is pumped through the heart, into the aorta, dispersed into the head and throughout the hemocoel, then back through the ostia in the heart and the process repeated.]]

{{multiple image
 | align             = right
 | direction         = vertical
 | width             = 220
 | header            = Circulation in vertebrates
 | header_align      = center
 | header_background =
 | footer            =
 | footer_align      = <!-- left/right/center -->
 | footer_background =
 | background color  =
 | image1            =
 | alt1              =
 | caption1          = {{center|''Fish, amphibians and mammals''}}'''1''': heart{{right|{{bg|#000066|&nbsp; &nbsp;}} venous blood &nbsp;}}<br />'''2''': [[Systemic circulation|systemic]]{{right|{{bg|#FF0000|&nbsp; &nbsp;}} arterial blood &nbsp;&nbsp;}}<br />'''3''': [[Pulmonary circulation|pulmonary]]{{right|{{bg|#00CCFF|&nbsp; &nbsp;}} mixed blood &nbsp; &nbsp;}}
}}
While humans, as well as other [[vertebrates]], have a closed blood circulatory system (meaning that the blood never leaves the network of arteries, veins and capillaries), some [[invertebrate]] groups have an open circulatory system containing a heart but limited blood vessels. The most primitive, [[diploblastic]] animal [[phylum|phyla]] lack circulatory systems.

An additional transport system, the lymphatic system, which is only found in animals with a closed blood circulation,  is an open system providing an accessory route for excess interstitial fluid to be returned to the blood.<ref name="Sherwood, Lauralee 2011 401"/>

The blood vascular system first appeared probably in an ancestor of the [[triploblasts]] over 600 million years ago, overcoming the time-distance constraints of diffusion, while [[endothelium]] evolved in an ancestral vertebrate some 540–510 million years ago.<ref>{{cite journal|doi=10.1111/jth.12253|pmid=23809110|pmc=5378490|title=Evolutionary origins of the blood vascular system and endothelium|year=2013|last1=Monahan-Earley|first1=R.|last2=Dvorak|first2=A. M.|last3=Aird|first3=W. C.|journal=Journal of Thrombosis and Haemostasis|volume=11|issue=Suppl 1 |pages=46–66}}</ref>

===Open circulatory system===
{{See also|Hemolymph}}
In [[arthropod]]s, the open circulatory system is a system in which a fluid in a [[body cavity|cavity]] called the '''hemocoel''' or '''haemocoel''' bathes the organs directly with oxygen and nutrients, with there being no distinction between blood and interstitial fluid; this combined fluid is called [[hemolymph]] or haemolymph.<ref>{{cite web|last=Bailey|first=Regina|title=Circulatory System|url=https://biology.about.com/od/organsystems/a/circulatorysystem.htm|work=biology.about.com|access-date=2022-02-23|archive-date=2016-11-29|archive-url=https://web.archive.org/web/20161129050935/http://biology.about.com/od/organsystems/a/circulatorysystem.htm|url-status=live}}</ref> Muscular movements by the animal during [[Animal locomotion|locomotion]] can facilitate hemolymph movement, but diverting flow from one area to another is limited. When the heart relaxes, blood is drawn back toward the heart through open-ended pores (ostia).

Hemolymph fills all of the interior hemocoel of the body and surrounds all [[Cell (biology)|cells]]. Hemolymph is composed of [[water]], [[Inorganic chemistry|inorganic]] [[Salt (chemistry)|salts]] (mostly [[sodium]], [[chloride]], [[potassium]], [[magnesium]], and [[calcium]]), and [[Organic chemistry|organic compounds]] (mostly carbohydrates, [[protein]]s, and [[lipid]]s). The primary oxygen transporter molecule is [[hemocyanin]].

There are free-floating cells, the [[Hemocyte (invertebrate immune system cell)|hemocytes]], within the hemolymph. They play a role in the arthropod [[immune system]].

[[File:Pseudoceros liparus - Blue Pseudoceros Flatworm.jpg|thumb|left|Flatworms, such as this ''[[Pseudoceros bifurcus]]'', lack specialized circulatory organs.]]

===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}}

===No circulatory system===
Circulatory systems are absent in some animals, including [[flatworm]]s. Their [[body cavity]] has no lining or enclosed fluid. Instead, a muscular [[pharynx]] leads to an extensively branched [[digestive system]] that facilitates direct [[diffusion]] of nutrients to all cells. The flatworm's dorso-ventrally flattened body shape also restricts the distance of any cell from the digestive system or the exterior of the organism. [[Oxygen]] can diffuse from the surrounding water into the cells, and carbon dioxide can diffuse out. Consequently, every cell is able to obtain nutrients, water and oxygen without the need of a transport system.

Some animals, such as [[jellyfish]], have more extensive branching from their [[gastrovascular cavity]] (which functions as both a place of digestion and a form of circulation), this branching allows for bodily fluids to reach the outer layers, since the digestion begins in the inner layers.

==History==

[[File:Charta ex qva figvram parare convenit, illi qvae nervorvm seriem exprimit appendendam, 1543..JPG |thumb |right |Human anatomical chart of blood vessels, with heart, lungs, liver and kidneys included. Other organs are numbered and arranged around it. Before cutting out the figures on this page, [[Vesalius]] suggests that readers glue the page onto parchment and gives instructions on how to assemble the pieces and paste the multilayered figure onto a base "muscle man" illustration. "Epitome", fol.14a. HMD Collection, WZ 240 V575dhZ 1543.]]

The earliest known writings on the circulatory system are found in the [[Ebers Papyrus]] (16th century BCE), an [[Ancient Egyptian medicine |ancient Egyptian medical papyrus]] containing over 700 prescriptions and remedies, both physical and spiritual. In the [[papyrus]], it acknowledges the connection of the heart to the arteries. The Egyptians thought air came in through the mouth and into the lungs and heart. From the heart, the air travelled to every member through the arteries. Although this concept of the circulatory system is only partially correct, it represents one of the earliest accounts of scientific thought.{{cn|date=January 2025}}

In the 6th century BCE, the knowledge of circulation of vital fluids through the body was known to the [[Ayurveda |Ayurvedic]] physician [[Sushruta]] in [[History of India |ancient India]].<ref name=Dwivedi&Dwivedi07/> He also seems to have possessed knowledge of the arteries, described as 'channels' by Dwivedi & Dwivedi (2007).<ref name=Dwivedi&Dwivedi07>Dwivedi, Girish & Dwivedi, Shridhar (2007). [https://medind.nic.in/iae/t07/i4/iaet07i4p243.pdf "History of Medicine: Sushruta&nbsp;– the Clinician&nbsp;– Teacher par Excellence"] {{webarchive |url=https://web.archive.org/web/20081010045900/https://medind.nic.in/iae/t07/i4/iaet07i4p243.pdf |date=October 10, 2008 }}, ''Indian J Chest Dis Allied Sci'' Vol. 49 pp. 243–244, [[National Informatics Centre |National Informatics Centre (Government of India)]].</ref> The first major ancient Greek research into the circulatory system was completed by Plato in the ''Timaeus'', who argues that blood circulates around the body in accordance with the general rules that govern the motions of the elements in the body; accordingly, he does not place much importance in the heart itself.<ref>See ''Timaeus'' 77a–81e. For a scholarly discussion, see Douglas R. Campbell, "Irrigating Blood: Plato on the Circulatory System, the Cosmos, and Elemental Motion," ''Journal of the History of Philosophy'' 62 (4): 519–541. 2024. See also Francis Cornford, ''Plato's Cosmology: The Timaeus of Plato,'' Indianapolis: Hackett, 1997.</ref> The [[Heart valve |valves of the heart]] were discovered by a physician of the [[Hippocrates |Hippocratic]] school around the early 3rd century BC.<ref>The central text here is the Hippocratic text ''On The Heart'', which Elizabeth Craik argues was written between 300 and 250 BC. See Craik, Elizabeth. 2015. ''The ‘Hippocratic’ Corpus: Content and Context''. New York: Routledge.</ref> However, their function was not properly understood then. Because blood pools in the veins after death, arteries look empty. Ancient anatomists assumed they were filled with air and that they were for the transport of air.{{cn|date=January 2025}}

The [[Ancient Greek Medicine |Greek physician]], [[Herophilus]], distinguished veins from arteries but thought that the [[pulse]] was a property of arteries themselves. Greek anatomist [[Erasistratus]] observed that arteries that were cut during life bleed. He ascribed the fact to the phenomenon that air escaping from an artery is replaced with blood that enters between veins and arteries by very small vessels. Thus he apparently postulated capillaries but with reversed flow of blood.{{citation needed |date=February 2022}}

In 2nd-century AD [[Rome]], the [[Ancient Greek Medicine |Greek]] physician [[Galen]] knew that blood vessels carried blood and identified venous (dark red) and arterial (brighter and thinner) blood, each with distinct and separate functions. Growth and energy were derived from venous blood created in the liver from chyle, while arterial blood gave vitality by containing pneuma (air) and originated in the heart. Blood flowed from both creating organs to all parts of the body where it was consumed and there was no return of blood to the heart or liver. The heart did not pump blood around, the heart's motion sucked blood in during diastole and the blood moved by the pulsation of the arteries themselves.{{cn|date=January 2025}} Galen believed that the arterial blood was created by venous blood passing from the left ventricle to the right by passing through 'pores' in the interventricular septum, air passed from the lungs via the pulmonary artery to the left side of the heart. As the arterial blood was created 'sooty' vapors were created and passed to the lungs also via the pulmonary artery to be exhaled.{{cn|date=January 2025}}

In 1025, ''[[The Canon of Medicine]]'' by the [[Ancient Iranian Medicine |Persian physician]], [[Avicenna]], "erroneously accepted the Greek notion regarding the existence of a hole in the ventricular septum by which the blood traveled between the ventricles." Despite this, Avicenna "correctly wrote on the [[cardiac cycle]]s and valvular function", and "had a vision of blood circulation" in his ''Treatise on Pulse''.<ref>{{Cite journal |last1=Shoja |first1=M.M. |last2=Tubbs |first2=R.S. |last3=Loukas |first3=M. |last4=Khalili |first4=M. |last5=Alakbarli |first5=F. |last6=Cohen-Gadol |first6=A.A. |doi=10.1016/j.ijcard.2009.02.035 |title=Vasovagal syncope in the Canon of Avicenna: The first mention of carotid artery hypersensitivity |journal=International Journal of Cardiology |volume=134 |issue=3 |pages=297–301 |year=2009 |pmid=19332359}}</ref> While also refining Galen's erroneous theory of the pulse, Avicenna provided the first correct explanation of pulsation: "Every beat of the pulse comprises two movements and two pauses. Thus, expansion : pause : contraction : pause. [...] The pulse is a movement in the heart and arteries ... which takes the form of alternate expansion and contraction."<ref name="Chamsi-Pasha2014">{{cite journal |last1=Chamsi-Pasha |first1=MA |last2=Chamsi-Pasha |first2=H |title=Avicenna's contribution to cardiology. |journal=Avicenna Journal of Medicine |date=January 2014 |volume=4 |issue=1 |pages=9–12 |doi=10.4103/2231-0770.127415 |doi-access=free |pmid=24678465|pmc=3952394 }}</ref>

In 1242, the [[Medicine in medieval Islam |Arabian physician]], [[Ibn al-Nafis]] described the process of [[pulmonary circulation]] in greater, more accurate detail than his predecessors, though he believed, as they did, in the notion of vital spirit ([[pneuma]]), which he believed was formed in the left ventricle. Ibn al-Nafis stated in his ''Commentary on Anatomy in Avicenna's Canon'':<ref name="West 2008"/>

<blockquote>...the blood from the right chamber of the heart must arrive at the left chamber but there is no direct pathway between them. The thick septum of the heart is not perforated and does not have visible pores as some people thought or invisible pores as Galen thought. The blood from the right chamber must flow through the vena arteriosa (pulmonary artery) to the lungs, spread through its substances, be mingled there with air, pass through the arteria venosa ([[pulmonary vein]]) to reach the left chamber of the heart and there form the vital spirit...</blockquote>

In addition, Ibn al-Nafis had an insight into what later became a larger theory of the [[capillary]] circulation. He stated that "there must be small communications or pores (''manafidh'' in Arabic) between the pulmonary artery and vein," a prediction that preceded the discovery of the capillary system by more than 400 years.<ref name="West 2008">{{Cite journal |last1=West |first1=J.B. |title=Ibn al-Nafis, the pulmonary circulation, and the Islamic Golden Age |doi=10.1152/japplphysiol.91171.2008 |journal=Journal of Applied Physiology |volume=105 |issue=6 |pages=1877–1880 |year=2008 |pmid=18845773 |pmc =2612469}}</ref> Ibn al-Nafis' theory was confined to blood transit in the lungs and did not extend to the entire body.

[[Michael Servetus]] was the first European to describe the function of pulmonary circulation, although his achievement was not widely recognized at the time, for a few reasons. He firstly described it in the "Manuscript of Paris"<ref>Gonzalez Etxeberria, Patxi (2011) ''Amor a la verdad, el&nbsp;– vida y obra de Miguel servet'' [''The love for truth. Life and work of Michael Servetus'']. Navarro y Navarro, Zaragoza, collaboration with the Government of Navarra, Department of Institutional Relations and Education of the Government of Navarra. {{ISBN |84-235-3266-6}} pp. 215–228 & 62nd illustration (XLVII)</ref><ref>[https://www.michaelservetusresearch.com/ENGLISH/works.html Michael Servetus Research] {{Webarchive |url=https://web.archive.org/web/20121113223851/https://www.michaelservetusresearch.com/ENGLISH/works.html |date=2012-11-13 }} Study with graphical proof on the Manuscript of Paris and many other manuscripts and new works by Servetus</ref> (near 1546), but this work was never published. And later he published this description, but in a theological treatise, ''Christianismi Restitutio'', not in a book on medicine. Only three copies of the book survived but these remained hidden for decades, the rest were burned shortly after its publication in 1553 because of persecution of Servetus by religious authorities.{{cn|date=January 2025}}

A better known discovery of pulmonary circulation was by [[Vesalius]]'s successor at [[University of Padua |Padua]], [[Realdo Colombo]], in 1559.{{cn|date=January 2025}}

[[File:William Harvey ( 1578-1657) Venenbild.jpg |thumb |Image of veins from [[William Harvey]]'s ''[[Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus]]'', 1628]]

Finally, the English physician [[William Harvey]], a pupil of [[Hieronymus Fabricius]] (who had earlier described the valves of the veins without recognizing their function), performed a sequence of experiments and published his ''[[Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus]]'' in 1628, which "demonstrated that there had to be a direct connection between the venous and arterial systems throughout the body, and not just the lungs. Most importantly, he argued that the beat of the heart produced a continuous circulation of blood through minute connections at the extremities of the body. This is a conceptual leap that was quite different from Ibn al-Nafis' refinement of the anatomy and bloodflow in the heart and lungs."<ref>Pormann, Peter E. and Smith, E. Savage (2007) ''Medieval Islamic medicine'' Georgetown University, Washington, D.C., p. 48, {{ISBN |1-58901-161-9}}.</ref> This work, with its essentially correct exposition, slowly convinced the medical world. However, Harvey did not identify the capillary system connecting arteries and veins; this was discovered by [[Marcello Malpighi]] in 1661.{{cn|date=January 2025}}

==See also==
{{Portal|Medicine}}
{{col div|colwidth=30em}}
* {{annotated link|Cardiology}}
* {{annotated link|Cardiovascular drift}}
* {{annotated link|Cardiac cycle}}
* {{annotated link|Vital heat}}
* {{annotated link|Cardiac muscle}}
* {{annotated link|Major systems of the human body}}
* {{annotated link|Amato Lusitano}}
* {{annotated link|Vascular resistance}}
{{colend}}

==References==
{{Reflist}}

==External links==
{{Commons category|Cardiovascular system}}
{{Wiktionary}}

{{Library resources box
| by          = no
| onlinebooks = no
| others      = no
| about       = yes
| label       = Circulatory system}}
* [https://cnx.org/content/m46646/latest/ Circulatory Pathways] in ''Anatomy and Physiology'' by [[OpenStax]]
* [https://web.archive.org/web/20040611041411/https://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookcircSYS.html The Circulatory System]
* [https://web.archive.org/web/20121113223851/https://www.michaelservetusresearch.com/ENGLISH/works.html Michael Servetus Research] Study on the Manuscript of Paris by Servetus (1546 description of the Pulmonary Circulation)

{{Heart anatomy}}
{{Arteries and veins}}
{{Cardiovascular physiology}}
{{Development of circulatory system}}
{{Circulatory system pathology}}
{{Human systems and organs}}
{{Underwater diving|scidiv}}
{{Authority control}}

{{DEFAULTSORT:Circulatory System}}
[[Category:Circulatory system| ]]
[[Category:Exercise physiology]]
[[Category:Angiology]]