Editing Vein

{{short description|Bloods vessels that carry blood towards the heart}}
{{Other uses}}
{{human centric|date=November 2024}}
{{Infobox anatomy
| Name        = Vein
| Latin       = vena 
| Image       = Vein (retouched).svg
| Caption     = Structure of a vein, which consists of three main layers: an outer layer of [[connective tissue]], a middle layer of [[smooth muscle]], and an inner layer lined with [[endothelium]]. 
| Width       = 200px 
| Precursor   = 
| System      = [[Circulatory system]]
| Artery      = 
| Vein        = 
| Nerve       = 
| Lymph       = 
}}

'''Veins''' ({{IPAc-en|v|eɪ|n}}) are [[blood vessel]]s in the [[circulatory system]] of humans and most other animals that carry [[blood]] towards the [[heart]]. Most veins carry [[deoxygenated blood]] from the tissues back to the heart; exceptions are those of the [[pulmonary circulation|pulmonary]] and [[fetal circulation]]s which carry [[oxygenated blood]] to the heart. In the [[systemic circulation]], [[Artery|arteries]] carry oxygenated blood away from the heart, and veins return deoxygenated blood to the heart, in the deep veins.<ref name="Moore3">{{cite journal |vauthors=Moore HM, Gohel M, Davies AH |title=Number and location of venous valves within the popliteal and femoral veins: a review of the literature |journal=J Anat |volume=219 |issue=4 |pages=439–43 |date=October 2011 |pmid=21740424 |pmc=3196749 |doi=10.1111/j.1469-7580.2011.01409.x |url=}}</ref>

There are three sizes of veins: large, medium, and small. Smaller veins are called [[venule]]s, and the smallest the post-capillary venules are microscopic that make up the veins of the [[microcirculation]].<ref name="Guven">{{cite journal |vauthors=Guven G, Hilty MP, Ince C |title=Microcirculation: Physiology, Pathophysiology, and Clinical Application |journal=Blood Purif |volume=49 |issue=1–2 |pages=143–150 |date=2020 |pmid=31851980 |pmc=7114900 |doi=10.1159/000503775 |url=}}</ref> Veins are often closer to the skin than arteries.

Veins have less [[smooth muscle]] and [[connective tissue]] and wider [[Lumen (anatomy)|internal diameters]] than arteries. Because of their thinner walls and wider lumens they are able to expand and hold more blood. This greater [[venous capacitance|capacity]] gives them the term of ''capacitance vessels''. At any time, nearly 70% of the total volume of blood in the human body is in the veins.<ref name="SEER1">{{cite web |title=Classification & Structure of Blood Vessels {{!}} SEER Training |url=https://training.seer.cancer.gov/anatomy/cardiovascular/blood/classification.html |website=training.seer.cancer.gov |access-date=29 January 2023}}</ref> In medium and large sized veins the flow of blood is maintained by one-way (unidirectional) venous valves to prevent [[regurgitation (circulation)|backflow]].<ref name="SEER1"/><ref name="Moore3"/> In the lower limbs this is also aided by [[skeletal muscle pump|muscle pumps]], also known as venous pumps that exert pressure on intramuscular veins when they [[muscle contraction|contract]] and drive blood back to the heart.{{sfn|GRAYS|2016|page=131}}

==Structure==
[[File:Figure 40 03 07.jpg|thumb|left|upright=1.4|Layers of vein wall shown in comparison to arterial wall]]
There are three sizes of vein, large, medium, and small. Smaller veins are called [[venule]]s. The smallest veins are the post-capillary venules. Veins have a similar three-layered structure to arteries. The layers known as [[tunica (biology)|tunicae]] have a concentric arrangement that forms the wall of the vessel. The outer layer, is a thick layer of [[connective tissue]] called the [[tunica externa]] or ''adventitia''; this layer is absent in the post-capillary venules.{{sfn|GRAYS|2016|page=131}} The middle layer, consists of bands of [[smooth muscle]] and is known as the [[tunica media]]. The inner layer, is a thin lining of [[endothelium]] known as the [[tunica intima]]. The tunica media in the veins is much thinner than that in the arteries as the veins are not subject to the high [[Systole|systolic]] pressures that the arteries are. There are valves present in many veins that maintain unidirectional flow.

Unlike arteries, the precise location of veins varies among individuals.<ref name="Maton">{{cite book |last=Maton |first=Anthea |author2=Jean Hopkins |author3=Charles William McLaughlin |author4=Alexandra Senckowski |author5=Susan Johnson |author6=Maryanna Quon Warner |author7=David LaHart |author8=Jill D. Wright |title=Human Biology and Health |publisher=Prentice Hall |year=1993 |location=Englewood Cliffs, New Jersey |isbn=978-0-13-981176-0 |url-access=registration |url=https://archive.org/details/humanbiologyheal00scho}}</ref>{{page needed|date=November 2024}}

Veins close to the surface of the skin appear blue for a variety of reasons. The factors that contribute to this alteration of [[visual perception|color perception]] are related to the light-scattering properties of the skin and the processing of visual input by the [[visual cortex]], rather than the actual colour of the venous blood which is dark red.<ref>{{cite journal |vauthors=Kienle A, Lilge L, Vitkin IA, Patterson MS, Wilson BC, Hibst R, Steiner R |title=Why do veins appear blue? A new look at an old question |journal=Applied Optics |volume=35 |issue=7 |pages=1151 |date=March 1996 |pmid=21085227 |doi=10.1364/AO.35.001151 |url=http://www.imt.liu.se/edu/courses/TBMT36/pdf/blue.pdf |archive-url=https://web.archive.org/web/20120210145120/https://www.imt.liu.se/edu/courses/TBMT36/pdf/blue.pdf |url-status=dead |bibcode=1996ApOpt..35.1151K |archive-date=10 February 2012}}</ref>
{{Clear}}

==Venous system==
{{See also|List of veins of the human body}}
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| caption1          = Main veins of the [[systemic circulation]]
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The venous system is the system of veins in the [[Systemic circulation|systemic]] and [[pulmonary circulation]]s that return blood to the heart. In the systemic circulation the return is of deoxygenated blood from the organs and tissues of the body, and in the pulmonary circulation the pulmonary veins return oxygenated blood from the lungs to the heart. Almost 70% of the blood in the body is in the veins, and almost 75% of this blood is in the small veins and venules.<ref name="bmcmed">{{cite journal |last1=Zivadinov |first1=Robert |last2=Chung |first2=Chih-Ping |title=Potential involvement of the extracranial venous system in central nervous system disorders and aging |journal=BMC Medicine |pages=260 |doi=10.1186/1741-7015-11-260 |date=17 December 2013 |volume=11 |pmid=24344742 |pmc=3866257 |doi-access=free}}</ref> All of the systemic veins are tributaries of the largest veins, the [[Superior vena cava|superior]] and [[inferior vena cava]], which empty the oxygen-depleted blood into the [[Right heart|right atrium]] of the heart.<ref name="Moore1"/> The thin walls of the veins, and their greater internal diameters ([[Lumen (anatomy)|lumens]]) enable them to hold a greater volume of blood, and this greater capacitance gives them the term of ''capacitance vessels''.{{sfn|GRAYS|2016|page=131}} This characteristic also allows for the accommodation of pressure changes in the system. The whole of the venous system, bar the post-capillary venules is a large volume, low pressure system.{{sfn|GRAYS|2016|page=134}} The venous system is often asymmetric, and whilst the main veins hold a relatively constant position, unlike arteries, the precise location of veins varies among individuals.<ref name="Maton"/><ref name="bmcmed"/>
[[File:Diagram of the human heart (cropped).svg|thumb|Positions of [[venae cavae]] and vessels of the [[pulmonary circulation]]]]

Veins vary in size from the smallest post-capillary [[venule]]s, and more muscular venules, to small veins, medium veins, and large veins. The thickness of the walls of the veins varies as to their location – in the legs the vein walls are much thicker than those in the arms.{{sfn|GRAYS|2016|page=130}} In the circulatory system, blood first enters the venous system from [[capillary bed]]s where arterial blood changes to venous blood.

Large arteries such as the [[thoracic aorta]], [[subclavian artery|subclavian]], [[femoral artery|femoral]] and [[popliteal artery|popliteal arteries]] lie close to a single vein that drains the same region. Other arteries are often accompanied by a pair of veins held in a connective tissue sheath. The accompanying veins are known as [[venae comitantes]], or ''satellite veins'', and they run on either side of the artery. When an associated nerve is also enclosed, the sheath is known as a [[neurovascular bundle]].{{sfn|GRAYS|2016|page=127}} This close proximity of the artery to the veins helps in [[venous return]] due to the pulsations in the artery.{{sfn|GRAYS|2016|page=41}} It also allows for the promotion of heat transfer from the larger arteries to the veins in a [[Countercurrent exchange|counterflow exchange]] that helps to preserve normal body heat.{{sfn|GRAYS|2016|page=127}}

;Venules
[[File:2134 Thoracic Upper Limb Veins.jpg|thumb|Deep and superficial veins of the arm and near thorax]]
The first entry of venous blood is from the convergence of two or more [[capillaries]] into a microscopic, [[Venule|post-capillary venule]].<ref name="Saladin2">{{cite book |last1=Saladin |first1=Kenneth S. |title=Human anatomy |date=2011 |publisher=McGraw-Hill |location=New York |isbn=9780071222075 |pages=570–571 |edition=3rd}}</ref> Post-capillary venules have a diameter of between 10 and 30 [[micrometre]]s (μm), and are part of the [[microcirculation]]. Their endothelium is made up of flattened oval or polygon shaped cells surrounded by a [[basal lamina]]. Post-capillary venules are too small to have a smooth muscle layer and are instead supported by [[pericyte]]s that wrap around them.{{sfn|GRAYS|2016|page=135}} Post-capillary venules become [[Venule|muscular venules]] when they reach a diameter of 50&nbsp;μm,{{sfn|GRAYS|2016|page=130}} and can reach a diameter of 1&nbsp;mm.<ref name="Saladin2"/> These larger venules feed into small veins.
;Small, medium, and large veins
The small veins merge to feed as tributaries into medium-sized veins. The medium veins feed into the large veins which include the [[internal jugular vein|internal jugular]], and [[renal vein]]s, and the [[venae cavae]] that carry the blood directly into the heart.<ref name="Saladin2"/> The venae cavae enter the right [[Atrium (heart)|atrium]] of the heart from above and below. From above, the [[superior vena cava]] carries blood from the arms, head, and chest to the right atrium of the heart, and from below, the [[inferior vena cava]] carries blood from the legs and [[abdomen]] to the right atrium. The inferior vena cava is the larger of the two. The inferior vena cava is [[retroperitoneal space|retroperitoneal]] and runs to the right and roughly parallel to the [[abdominal aorta]] along the [[Vertebral column|spine]].

;Deep, superficial, and perforator veins
The three main compartments of the venous system are the [[deep vein]]s, the [[superficial vein]]s, and the [[perforator vein]]s.<ref name="Baliyan">{{cite journal |vauthors=Baliyan V, Tajmir S, Hedgire SS, Ganguli S, Prabhakar AM |title=Lower extremity venous reflux |journal=Cardiovasc Diagn Ther |volume=6 |issue=6 |pages=533–543 |date=December 2016 |pmid=28123974 |pmc=5220199 |doi=10.21037/cdt.2016.11.14 |url= |doi-access=free}}</ref> Superficial veins are those closer to the surface of the body, and have no corresponding arteries. Deep veins are deeper in the body and have corresponding arteries. Perforator veins drain from the superficial to the deep veins.<ref name="Dorlands">{{cite book |last1=Albert |first1=consultants Daniel |title=Dorland's illustrated medical dictionary. |date=2012 |publisher=Saunders/Elsevier |location=Philadelphia, PA |isbn=978-1-4160-6257-8 |page=2042 |edition=32nd}}</ref> These are usually referred to in the lower limbs and feet.<ref>{{Cite journal |last=Sureka |first=Binit |date=September 15, 2015 |title=Portal vein variations in 1000 patients: surgical and radiological importance |journal=British Journal of Radiology |volume=88 |issue=1055 |pages=1055 |doi=10.1259/bjr.20150326 |pmid=26283261 |pmc=4743455}}</ref> Superficial veins include the very small [[spider veins]] of between 0.5 and 1&nbsp;mm diameter, and [[reticular|reticular or feeder veins]].<ref name="IJA">{{cite journal |vauthors=Radhakrishnan N, George D, Jayakrishnan R, Sumi S, Kartha CC |title=Vein Size and Disease Severity in Chronic Venous Diseases |journal=Int J Angiol |volume=27 |issue=4 |pages=185–189 |date=December 2018 |pmid=30410288 |pmc=6221802 |doi=10.1055/s-0038-1639355 |url=}}</ref>

;Venous plexuses
There are a number of [[venous plexus]]es where veins are grouped or sometimes combined in networks at certain body sites. The [[Batson venous plexus]], runs through the inner vertebral column connecting the thoracic and pelvic veins. These veins are noted for being valveless, believed to be the reason for [[metastasis]] of certain cancers.

A subcutaneous venous plexus is continuous, and a high rate of flow is supplied by small [[arteriovenous anastomoses]]. The high rate of flow ensures heat transfer to the vein wall.<ref name="Hall"/>

===Venous valves===
[[File:Venous valve 00013.gif|thumb|Video of a valve in the [[popliteal vein]] opening to allow blood to flow through and closing to prevent backflow]]
[[File:Venous valve.svg|thumb|Venous valve stopping backflow]]

Blood flows back to the heart in the systemic deep veins, with the flow of blood maintained by one-way valves in the deep veins, superficial veins, and in the perforator veins.<ref name="Phlebo">{{cite web |last1=Publishing |first1=Licorn |title=The venous valves of the lower limbs |url=https://www.phlebolymphology.org/the-venous-valves-of-the-lower-limbs/ |website=Servier - Phlebolymphology |access-date=11 March 2023 |date=9 April 2013}}</ref> The venous valves serve to prevent [[Regurgitation (circulation)|regurgitation]] (backflow) due to the low pressure of veins, and the pull of gravity.<ref name="Moore3"/> They also serve to prevent the over-widening of  the vein.<ref name="Phlebo"/><ref name="openstax"/>

A venous valve is bicuspid (having two leaflets) and is formed by an infolding of part of the tunica intima on either side of the lumen of the veins. The leaflets are strengthened with collagen, and elastic fibres, and covered with endothelium.{{sfn|GRAYS|2016|page=130}} The endothelial cells  on the surfaces of the leaflets facing the vein wall, are arranged transversely. On the leaflet surfaces that open to let the blood flow, the cells are arranged longitudinally in the direction of the flow. The leaflets are attached to the venous wall at their convex edges. Their margins are concave and are directed with the flow lying against the wall.{{sfn|GRAYS|2016|page=131}} As the valve forms, the vein wall where the leaflets attach, becomes dilated on each side. These widenings form the pockets, hollow cup-shaped regions, on the cardial side, known as the valvular sinuses.<ref name="Bazigou">{{cite journal |vauthors=Bazigou E, Makinen T |title=Flow control in our vessels: vascular valves make sure there is no way back |journal=Cell Mol Life Sci |volume=70 |issue=6 |pages=1055–66 |date=March 2013 |pmid=22922986 |pmc=3578722 |doi=10.1007/s00018-012-1110-6 |url=}}</ref> The endothelial cells in the sinuses are able to stretch twice as much as those in areas without valves.<ref name="Bazigou"/> When the blood tries to reverse its direction (due to low venous pressure and the pull of gravity), the sinuses fill first closing the leaflets and keeping them together.{{sfn|GRAYS|2016|page=131}}<ref name="Moore1"/> Approximately 95% of the venous valves are in the small veins of less than 300 micrometres.<ref name="Rajeeva">{{cite journal |vauthors=Rajeeva Pandian NK, Jain A |title=In silico analyses of blood flow and oxygen transport in human micro-veins and valves |journal=Clin Hemorheol Microcirc |volume=81 |issue=1 |pages=81–96 |date=2022 |pmid=35034895 |pmc=9307074 |doi=10.3233/CH-211345 |url=}}</ref>

The deep veins of the lower limb include the [[common femoral vein]], [[femoral vein]], and the [[deep femoral vein]]; the [[popliteal vein]], the tibial, and [[fibular veins]]. In the common femoral vein one valve is located above the [[saphenofemoral junction]] called the ''suprasaphenic valve''. There are sometimes two valves in the same tract. In the femoral vein there are often three valves, the most constantly found valve is just below the joining of the deep femoral vein. The deep femoral vein and its perforators have valves. In the popliteal veins there are between one and three valves; in each [[posterior tibial vein]] there are between 8 and 19 valves, and in  the [[anterior tibial vein]]s there are between 8 and 11 valves.<ref name="Phlebo"/>

In the superficial veins there are between one and seven valves along the thigh portion of the [[great saphenous vein]] (GSV); two to six below the knee and one to four in the marginal veins of the foot. There is a valve  at the termination of the GSV known as the ''terminal valve'' to prevent reflux from the femoral vein  A ''preterminal valve'' is located just below the openings of the tributaries to prevent reflux form these into the GSV.<ref name="Phlebo"/> Incompetence of the GSV is a common cause of varicose veins.

The valves also divide the column of blood into segments which helps move the blood unidirectionally to the heart.<ref name="Meissner">{{cite journal |vauthors=Meissner MH |title=Lower extremity venous anatomy |journal=Semin Intervent Radiol |volume=22 |issue=3 |pages=147–56 |date=September 2005 |pmid=21326687 |pmc=3036282 |doi=10.1055/s-2005-921948 |url=}}</ref> Their action is supported by the action of [[skeletal muscle pump]]s that contract and compress the veins. A skeletal muscle is confined in its fascia and contraction of the muscle which makes it wider results In compression on the vein that pushes the blood forward.<ref name="Moore1"/> Valves in the perforating veins close when a calf muscle contracts, to prevent backflow from the deep veins to the superficial.<ref name="Cleveland">{{cite web |title=Veins: Anatomy and Function |url=https://my.clevelandclinic.org/health/body/23360-veins |website=Cleveland Clinic |access-date=16 March 2023 |language=en}}</ref> There are more valves in the lower leg, due to increased gravitational pull, with the number decreasing as the veins travel to the hip. There are no valves in the veins of the thorax or abdomen.{{sfn|GRAYS|2016|page=131}}

There is a valve at the junction of the inferior vena cava (one of the [[great vessel]]s) and the right atrium known as the [[valve of inferior vena cava]] also known as the ''eustachian valve''. This valve is an embryological remnant and is insignificant in the adult. However, when persistent it can cause problems.<ref name="Cai">{{cite journal |vauthors=Cai Q, Ahmad M |title=Eustachian valve, interatrial shunt, and paradoxical embolism |journal=Echocardiography |volume=37 |issue=6 |pages=939–944 |date=June 2020 |pmid=32426851 |doi=10.1111/echo.14682 |url=}}</ref>

===Circulatory routes===
[[File:2101 Blood Flow Through the Heart.jpg|thumb|left|upright=1.4|Diagram showing venous blood flow from [[capillary bed]]s in some specific locations including the [[lung]]s, [[liver]], and [[kidney]]s]]
There are some separate parallel systemic circulatory routes that supply specific regions, and organs.<ref name="Moore1"/> They include the coronary circulation, the cerebral circulation, the bronchial circulation, and the renal circulation.

;Coronary circulation
In the [[coronary circulation]], the blood supply to the heart, is drained by [[Coronary circulation#Cardiac veins|cardiac veins]] (or coronary veins) that remove the deoxygenated blood from the [[heart muscle]]. These include the [[great cardiac vein]], the [[middle cardiac vein]], the [[small cardiac vein]], the [[smallest cardiac veins]], and the [[anterior cardiac veins]]. Cardiac veins carry blood with a poor level of oxygen, from the heart muscle to the [[right atrium]]. Most of the blood of the cardiac veins returns through the [[coronary sinus]]. The anatomy of the veins of the heart is very variable, but generally it is formed by the following veins: heart veins that go into the coronary sinus: the great cardiac vein, the middle cardiac vein, the small cardiac vein, the posterior vein of the [[left ventricle]], and the [[oblique vein of the left atrium]] (oblique vein of Marshall). Heart veins that go directly to the right atrium: the anterior cardiac veins, and the smallest cardiac veins (Thebesian veins).<ref>{{cite encyclopedia |url=https://radiopaedia.org/articles/coronary-veins |title=Coronary veins |encyclopedia=Radiopaedia.org |first1=Matt |last1=Adams |first2=Matt A. |last2=Morgan |date=11 November 2014 |display-authors=etal}}</ref>

;Bronchial circulation
In the [[bronchial circulation]] that supplies blood to the lung tissues, [[bronchial veins]] drain [[venous blood]] from the large [[bronchus|main bronchi]] into the [[azygous vein]], and ultimately the right atrium. Venous blood from the bronchi inside the lungs drains into the [[pulmonary vein]]s and empties into the left atrium; since this blood never went through a capillary bed it was never oxygenated and so provides a small amount of shunted deoxygenated blood into the systemic circulation.<ref name="Weinberger">{{cite book |last1=Weinberger |first1=Steven E. |title=Principles of pulmonary medicine |date=2019 |location=Philadelphia, PA |isbn=9780323523714 |page=178 |edition=Seventh}}</ref>

;Cerebral circulation
In the [[cerebral circulation]] supplying the [[cerebrum]] the venous drainage can be separated into two subdivisions:  superficial and deep.
The superficial system is composed of [[dural venous sinuses]], which have walls composed of dura mater as opposed to a traditional vein. The dural sinuses are therefore located on the surface of the cerebrum. The most prominent of these sinuses is the [[superior sagittal sinus]] which flows in the sagittal plane under the midline of the cerebral vault, posteriorly and inferiorly to the [[confluence of sinuses]], where the superficial drainage joins with the sinus that primarily drains the deep venous system. From here, two [[transverse sinuses]] bifurcate and travel laterally and inferiorly in an S-shaped curve that forms the [[sigmoid sinuses]] which go on to form the two [[jugular vein]]s. In the neck, the [[jugular vein]]s parallel the upward course of the [[carotid arteries]] and drain blood into the [[superior vena cava]].

The deep venous drainage is primarily composed of traditional veins inside the deep structures of the brain, which join behind the midbrain to form the [[Internal cerebral veins|vein of Galen]]. This vein merges with the [[inferior sagittal sinus]] to form the [[straight sinus]] which then joins the superficial venous system mentioned above at the [[confluence of sinuses]].

;Portal venous systems
A [[portal venous system]] is a series of veins or venules that directly connect two [[capillary bed]]s. The two systems in verebrates are the [[hepatic portal system]], and the [[hypophyseal portal system]].

;Anastomoses
An [[anastomosis]] is a joining of two structures such as blood vessels. In the circulation these are called [[circulatory anastomosis|circulatory anastomoses]], one of which is the join between an artery with a vein known as an  [[arteriovenous anastomosis]]. This connection which is highly muscular, enables venous blood to travel directly from an artery into a vein without having passed from a capillary bed.<ref name="Hall">{{cite book |last1=Hall |first1=John E. |title=Guyton and Hall textbook of medical physiology |date=2011 |location=Philadelphia, Pa. |isbn=9781416045748 |page=868 |edition=Twelfth}}</ref>{{sfn|GRAYS|2016|page=135}}

Abnormal connections can be present known as [[arteriovenous malformation]]s. These are usually congenital and the connections are made from a tangle of capillaries.<ref name="gosh">{{cite web |title=AVM |url=https://media.gosh.nhs.uk/documents/AVM_F1102_A5_col_FINAL_Feb13.pdf |access-date=5 February 2023}}</ref>  A [[cerebral arteriovenous malformation]] is one that is located in the [[cerebrum|brain]]. An irregular connection between an artery and a vein is known as [[arteriovenous fistula]].

A small specialised arteriovenous anastomosis known as a [[glomus body|glomus body or organ]] serves to transfer heat in the fingers and toes. The small connection is surrounded by a capsule of thickened connective tissue. In the hands and feet there are a great number of glomera.{{sfn|GRAYS|2016|page=135}}

;Vascular shunt
A vascular shunt can also bypass the capillary bed and provide a route for blood supply directly to a collecting venule. This is achieved by a [[metarteriole]] that supplies around a hundred capillaries. At their junctions are precapillary sphincters that tightly regulate the flow of blood into the capillary bed. When all of the sphincters are closed blood can flow from a metarteriole into a thoroughfare channel and into a collecting venule bypassing the capillary bed.<ref name="openstax">{{cite web |title=20.1 Structure and Function of Blood Vessels - Anatomy and Physiology 2e {{!}} OpenStax |url=https://openstax.org/books/anatomy-and-physiology-2e/pages/20-1-structure-and-function-of-blood-vessels |website=openstax.org |date=20 April 2022 |access-date=17 March 2023 |language=en}}</ref>{{sfn|GRAYS|2016|page=131}}

;Other
A [[communicating vein]] directly connects two parts of the same system such as the [[Giacomini vein]] that connects the (superficial) [[small saphenous vein]] with the (superficial) [[great saphenous vein]]. [[Peripheral vein]]s carry blood from the limbs and [[hand]]s and [[foot|feet]].

==Microanatomy==
[[File:2106 Large Medium Vein Venule.jpg|thumb|upright=0.6|Diagram of different sized veins with differing component proportions]]

The three layers of the vein wall are the outer tunica externa, the middle tunica media and the inner tunica intima. There are also numerous valves present in many of the veins.

The outer tunica externa, also known as the ''tunica adventitia'' is a sheath of thick connective tissue. This layer is absent in the post-capillary venules.<ref name="Moore1"/>

The middle tunica media is mainly of vascular [[smooth muscle cells]], [[elastic fiber]]s and [[collagen]]. This layer is much thinner than that in arteries.{{sfn|GRAYS|2016|page=132–134}} [[Vascular smooth muscle|Vascular smooth muscle cells]] control the size of the vein lumens, and thereby help to regulate [[blood pressure]].<ref name="Brozovich">{{cite journal |vauthors=Brozovich FV, Nicholson CJ, Degen CV, Gao YZ, Aggarwal M, Morgan KG |title=Mechanisms of Vascular Smooth Muscle Contraction and the Basis for Pharmacologic Treatment of Smooth Muscle Disorders |journal=Pharmacol Rev |volume=68 |issue=2 |pages=476–532 |date=April 2016 |pmid=27037223 |pmc=4819215 |doi=10.1124/pr.115.010652 |url=}}</ref>

The inner tunica intima is a lining of endothelium comprising a single layer of extremely flattened epithelial cells, supported by delicate connective tissue.<ref name="Moore1">{{cite book |last1=Moore |first1=Keith L. |title=Clinically oriented anatomy |date=2018 |location=Philadelphia |isbn=9781496347213 |pages=38–41 |edition=Eighth}}</ref> This subendothelium is a thin but variable connective tissue.{{sfn|GRAYS|2016|page=131}} The tunica intima has the most variation in blood vessels, in terms of their wall thickness and relative size of their lumen. The endothelial cells continuously produce [[nitric oxide]] a soluble gas, to the cells of the adjacent smooth muscle layer. This constant synthesis is carried out by the enzyme [[endothelial NOS|endothelial nitric oxide synthase]] (eNOS).<ref name="Tousoulis">{{cite journal |vauthors=Tousoulis D, Kampoli AM, Tentolouris C, Papageorgiou N, Stefanadis C |title=The role of nitric oxide on endothelial function |journal=Curr Vasc Pharmacol |volume=10 |issue=1 |pages=4–18 |date=January 2012 |pmid=22112350 |doi=10.2174/157016112798829760 |url=}}</ref> Other endothelial secretions are [[endothelin]], and [[thromboxane]] (vasoconstrictors), and [[prostacyclin]] a vasodilator.{{sfn|GRAYS|2016|page=134}}

==Development==
The [[human embryonic development|development of the embryo]] is completely reliant on the [[vitelline circulation]], the bidirectional flow of blood between  the [[yolk sac]] and the embryo. The yolk sac is the first extraembryonic structure to appear. This circulation is critical in allowing the exchange of nutrients, prior to the full development of the [[placenta]].<ref name="Donovan">{{cite journal |last1=Donovan |first1=Mary F. |last2=Arbor |first2=Tafline C. |last3=Bordoni |first3=Bruno |title=Embryology, Yolk Sac |url=https://www.ncbi.nlm.nih.gov/books/NBK555965/ |website=StatPearls |publisher=StatPearls Publishing |access-date=22 March 2023 |date=2023 |pmid=32310425}}</ref> By day 17 vessels begin to form in the yolk sac, arising from the [[splanchnic mesoderm]] of the  yolk sac wall.<ref name="Larsen's2">{{cite book |last1=Schoenwolf |first1=Gary C. |title=Larsen's human embryology |date=2015 |location=Philadelphia, PA |isbn=9781455706846 |page=304 |edition=Fifth}}</ref> The capillaries are formed during [[vasculogenesis]], and they lengthen and interconnect to form an extensive primitive vascular network.<ref name="Larsen's">{{cite book |last1=Schoenwolf |first1=Gary C. |title=Larsen's human embryology |date=2015 |location=Philadelphia, PA |isbn=9781455706846 |page=306 |edition=Fifth}}</ref> Blood is supplied from the primitive aorta, and drained by [[vitelline veins]] from the yolk sac to the embryo. By the end of the third week the yolk sac, [[connecting stalk]], and [[chorionic villi]] are entirely vascularised.<ref name="Larsen's"/>

In the middle of the fourth week the heart begins to beat and the circulation of blood begins. The primitive outflow tract is of three pairs of aortic arches. The inflow tract is formed of six paired veins, the vitelline veins, [[umbilical veins]], and the cardinal veins.<ref name="Larsen's3">{{cite book |last1=Schoenwolf |first1=Gary C. |title=Larsen's human embryology |date=2015 |location=Philadelphia, PA |isbn=9781455706846 |page=279 |edition=Fifth}}</ref>

== Function ==
{{Main|Circulatory system}}
In the systemic circulation, veins serve to return oxygen-depleted blood from organs, and tissues to the [[right heart]]. From here it passes to the pulmonary arteries for the pulmonary circulation to return oxygen-rich blood to the [[left heart]]  in the pulmonary veins, to be pumped back into the systemic circulation to complete the cycle. Veins have thinner walls than arteries, and a wider diameter that allow them to expand and hold a greater volume of blood. This gives them a functional role of ''capacitance'' that makes possible the accommodation of different pressures in the system. The venous system apart from the post-capillary venules is a high volume, low pressure system. [[Vascular smooth muscle|Vascular smooth muscle cells]] control the size of the vein lumens, and thereby help to regulate [[blood pressure]].<ref name="Brozovich"/>

The post-capillary venules are ''exchange'' vessels whose ultra-thin walls allow the ready diffusion of molecules from the capillaries.{{sfn|GRAYS|2016|page=130}}

The return of blood to the heart is assisted by the action of the [[skeletal-muscle pump|muscle pump]], and by the thoracic pump action of breathing during respiration. Standing or sitting for a prolonged period of time can cause low venous return from venous pooling (vascular) shock. [[Fainting]] can occur but usually baroreceptors within the aortic sinuses initiate a [[baroreflex]] such that angiotensin II and norepinephrine stimulate vasoconstriction and heart rate increases to return blood flow. [[Neurogenic shock|Neurogenic]] and [[shock (circulatory)#Hypovolaemic|hypovolaemic shock]] can also cause fainting. In these cases, the smooth muscles surrounding the veins become slack and the veins fill with the majority of the blood in the body, keeping blood away from the brain and causing unconsciousness. Jet pilots wear pressurized suits to help maintain their venous return and blood pressure.

== Clinical significance ==
{{Further|Vascular surgery#Management of venous diseases}}
Most venous diseases involve obstruction such as a [[thrombus]] or insufficiency of the valves, or both of these.<ref name="Cochrane2021">{{cite journal |last1=Goel |first1=RR |last2=Hardy |first2=SC |last3=Brown |first3=T |title=Surgery for deep venous insufficiency. |journal=The Cochrane Database of Systematic Reviews |date=30 September 2021 |volume=2021 |issue=9 |pages=CD001097 |doi=10.1002/14651858.CD001097.pub4 |pmid=34591328 |pmc=8483065}}</ref><ref name="Phlebo"/> Other conditions may be due to [[inflammation]], or compression. [[Ageing]] is a major independent risk factor for venous disorders.<ref>{{cite journal |vauthors=Molnár AÁ, Nádasy GL, Dörnyei G, Patai BB, Delfavero J, Fülöp GÁ, Kirkpatrick AC, Ungvári Z, Merkely B |title=The aging venous system: from varicosities to vascular cognitive impairment |journal=Geroscience |volume=43 |issue=6 |pages=2761–2784 |date=December 2021 |pmid=34762274 |pmc=8602591 |doi=10.1007/s11357-021-00475-2 |url=}}</ref> The medical speciality involved with the diagnosis and treatment of venous disorders is known as [[phlebology]] (also ''venology''), and the specialist concerned is a [[phlebologist]].<ref name="Collins">{{cite web |title=Phlebology |url=https://www.collinsdictionary.com/dictionary/english/phlebology |access-date=9 January 2023}}</ref> There are a number of [[vascular surgery|vascular surgeries]] and endovascular surgeries carried out by vascular surgeons to treat many venous diseases.

=== Venous insufficiency ===
{{main article|Chronic venous insufficiency}}
Venous insufficiency is the most common disorder of the venous system, and is usually manifested as either [[Telangiectasia|spider veins]] or [[varicose veins]]. Several treatments are available including [[endovenous thermal ablation]] (using radiofrequency or laser energy), [[vein stripping]], [[ambulatory phlebectomy]], foam [[sclerotherapy]], [[laser]], or compression.

[[Postphlebitic syndrome]] is venous insufficiency that develops following [[deep vein thrombosis]].<ref name="pmid16822286">{{cite journal |author=Kahn SR |title=The post-thrombotic syndrome: progress and pitfalls |journal=British Journal of Haematology |volume=134 |issue=4 |pages=357–65 |date=August 2006 |pmid=16822286 |doi=10.1111/j.1365-2141.2006.06200.x |s2cid=19715556 |doi-access=free}}</ref>

=== Venous thrombosis ===
{{Further|Deep vein thrombosis}}
[[Venous thrombosis]] is the formation of a [[thrombus]] (blood clot) in a vein. This most commonly affects a [[deep vein]] known as [[deep vein thrombosis]] (DVT), but can also affect a [[superficial vein]] known as [[superficial vein thrombosis]] (SVT).

====Deep vein thrombosis====
DVT usually occurs in the veins of the legs, although it can also occur in the deep veins of the arms.<ref name="Heil">{{cite journal |last1=Heil |first1=J |last2=Miesbach |first2=W |last3=Vogl |first3=T |last4=Bechstein |first4=WO |last5=Reinisch |first5=A |title=Deep Vein Thrombosis of the Upper Extremity. |journal=Deutsches Ärzteblatt International |date=7 April 2017 |volume=114 |issue=14 |pages=244–249 |doi=10.3238/arztebl.2017.0244 |pmid=28446351 |pmc=5415909}}</ref> Immobility, active cancer, obesity, traumatic damage and congenital disorders that make clots more likely are all risk factors for deep vein thrombosis. It can cause the affected limb to swell, and cause pain and an overlying skin rash. In the worst case, a deep vein thrombosis can extend, or a part of a clot can break off as an [[embolus]] and lodge in a [[pulmonary artery]] in the lungs, known as a [[pulmonary embolism]].

The decision to treat deep vein thrombosis depends on its size, symptoms, and their risk factors. It generally involves [[anticoagulation]] to prevents clots or to reduce the size of the clot. [[Intermittent pneumatic compression]] is a method used to improve venous circulation in cases of edema or in those at risk from a deep vein thrombosis.

====Superficial vein thrombosis====
SVT is the development of a thrombus in a superficial vein. SVT is not normally clinically significant, but the thrombus can migrate into the deep venous system where it can also give rise to a pulmonary embolism.<ref name="Cosmi 2015">{{cite journal |last1=Cosmi |first1=B. |title=Management of superficial vein thrombosis |journal=Journal of Thrombosis and Haemostasis |date=July 2015 |volume=13 |issue=7 |pages=1175–1183 |doi=10.1111/jth.12986 |pmid=25903684 |s2cid=5276848 |doi-access=free}}</ref> The main risk factor for SVT in the lower limbs is varicose veins.<ref name="Cosmi 2015"/>

=== Portal hypertension ===
The [[portal vein]] also known as the ''hepatic portal vein'' carries blood drained from most of the [[gastrointestinal tract]] to the [[liver]]. [[Portal hypertension]] is mainly caused by [[cirrhosis]] of the liver. Other causes can include an obstructing clot in a [[hepatic vein]] ([[Budd–Chiari syndrome|Budd Chiari syndrome]]) or compression from tumors or tuberculosis lesions. When the pressure increases in the portal vein, a [[collateral circulation]] develops, causing visible veins such as [[esophageal varices]].

===Phlebitis===
[[Phlebitis]] is the [[inflammation]] of a vein. It is usually accompanied by a [[thrombus|blood clot]] when it is known as [[thrombophlebitis]]. When the affected vein is a superficial vein in the leg, it is known as [[superficial thrombophlebitis]], and unlike deep vein thrombosis there is little risk of the clot breaking off as an [[embolus]].<ref name="Cochrane2018">{{cite journal |vauthors=Di Nisio M, Wichers IM, Middeldorp S |title=Treatment for superficial thrombophlebitis of the leg |journal=Cochrane Database Syst Rev |volume=2018 |issue=2 |pages=CD004982 |date=February 2018 |pmid=29478266 |pmc=6953389 |doi=10.1002/14651858.CD004982.pub6 |url=}}</ref>

===Compression===
Some disorders as syndromes result from compression of a vein. These include a venous type of [[thoracic outlet syndrome]], due to compression of a [[subclavian vein]]; [[nutcracker syndrome]] most usually due to compression of the left [[renal vein]], and [[May–Thurner syndrome]] associated with compression of the [[iliac vein]] which can lead to [[iliofemoral DVT]]. Compression of the [[superior vena cava]] most usually by a [[cancer|malignant tumor]] can lead to [[superior vena cava syndrome]].<ref name="Medscape">{{cite journal |vauthors=Nickloes TA, Lopez Rowe V, Kallab AM, Dunlap AB |title=Superior Vena Cava Syndrome |url=https://emedicine.medscape.com/article/460865-overview?pa=SJtbYBgLxsEGdxNvrE72u%2F5tqQajfphPnZDdCfeVkEb8dVnNptXVp6lf8lEy4UNsLCEJNCrbkqLWYvqLrhntWA%3D%3D#a7 |website=Medscape |publisher=WebMD LLC |access-date=13 March 2023 |date=8 March 2022}}</ref>

===Vascular anomalies===
A [[vascular anomaly]] can be either a [[vascular tumor]] or a [[birthmark]], or a [[vascular malformation]].
<ref name="Steiner">{{cite journal |last1=Steiner |first1=JE |last2=Drolet |first2=BA |title=Classification of Vascular Anomalies: An Update. |journal=Seminars in Interventional Radiology |date=September 2017 |volume=34 |issue=3 |pages=225–232 |doi=10.1055/s-0037-1604295 |pmid=28955111 |pmc=5615389}}</ref> In a tumor such as [[infantile hemangioma]] the mass is soft, and easily compressed, and their coloring is due to the dilated anomalous involved veins.<ref name="Chim">{{cite journal |last1=Chim |first1=H |last2=Drolet |first2=B |last3=Duffy |first3=K |last4=Koshima |first4=I |last5=Gosain |first5=AK |title=Vascular anomalies and lymphedema. |journal=Plastic and Reconstructive Surgery |date=August 2010 |volume=126 |issue=2 |pages=55e–69e |doi=10.1097/PRS.0b013e3181df803d |pmid=20679788 |s2cid=30865392}}</ref> They are most commonly found in the head and neck. [[Venous malformation]]s are the type of vascular malformation that involves the veins. They can often extend deeper from their surface appearance, reaching underlying muscle or bone.<ref name="Chen">{{cite journal |last1=Chen |first1=RJ |last2=Vrazas |first2=JI |last3=Penington |first3=AJ |title=Surgical Management of Intramuscular Venous Malformations. |journal=Journal of Pediatric Orthopedics |date=January 2021 |volume=41 |issue=1 |pages=e67–e73 |doi=10.1097/BPO.0000000000001667 |pmid=32815867 |s2cid=221199574}}</ref> In the neck they may extend into the [[oral mucosa|lining of the mouth cavity]] or into the [[salivary gland]]s.<ref name="Chim"/> They are the most common of the [[vascular malformations]].<ref name="Markovic">{{cite journal |last1=Markovic |first1=JN |last2=Shortell |first2=CK |title=Venous malformations. |journal=The Journal of Cardiovascular Surgery |date=October 2021 |volume=62 |issue=5 |pages=456–466 |doi=10.23736/S0021-9509.21.11911-1 |pmid=34105926 |s2cid=261366559}}</ref> A severe venous malformation can involve the lymph vessels as a ''lymphaticovenous malformation''.<ref name="Chim" />

===Venous access===
{{See also|Venipuncture}}
[[Venous access]] is any method used to access the bloodstream through the veins, either to administer [[intravenous therapy]] such as medication, or fluid, [[parenteral nutrition]], to obtain blood for analysis, or to provide an access point for blood-based treatments such as [[Kidney dialysis|dialysis]] or [[apheresis]]. Access is most commonly achieved via the placement of a [[central venous catheter]], a [[Seldinger technique]], and guidance tools such as [[ultrasound]] and [[fluoroscopy]] can also be used to assist with access location.

=== Imaging ===
[[Medical ultrasound|Ultrasound]], particularly [[Duplex ultrasonography|duplex ultrasound]], is the most usual and widely used way of viewing veins in the diagnosis of venous disease.<ref name="Lee">{{cite journal |vauthors=Lee DK, Ahn KS, Kang CH, Cho SB |title=Ultrasonography of the lower extremity veins: anatomy and basic approach |journal=Ultrasonography |volume=36 |issue=2 |pages=120–130 |date=April 2017 |pmid=28260355 |pmc=5381851 |doi=10.14366/usg.17001 |url=}}</ref><ref name="Garcia">{{cite journal |vauthors=Garcia R, Labropoulos N |title=Duplex Ultrasound for the Diagnosis of Acute and Chronic Venous Diseases |journal=Surg Clin North Am |volume=98 |issue=2 |pages=201–218 |date=April 2018 |pmid=29502767 |doi=10.1016/j.suc.2017.11.007 |url=}}</ref> [[Venography]] is an invasive procedure that uses a [[catheter]] to deliver a [[contrast agent]] in giving an [[X-ray]] of veins. An [[augmented reality]] healthcare application is a [[near-infrared vein finder]] that films subcutaneous veins, and projects their image either onto a screen or onto the person's skin.<ref name="Miyake">{{cite journal |title=Vein imaging: a new method of near infrared imaging, where a processed image is projected onto the skin for the enhancement of vein treatment |vauthors=Miyake RK, etal |s2cid=8872471 |pmid=16918565 |doi=10.1111/j.1524-4725.2006.32226.x |volume=32 |issue=8 |journal=[[Dermatol Surg]] |pages=1031–8 |year=2006}}</ref>

====Recognition techniques====
Some imaging techniques using veins have been developed for identification purposes. These [[vein matching]] technologies, include [[finger vein recognition]],<ref name="Zhang">{{cite journal |vauthors=Zhang Z, Wang M |title=A Simple and Efficient Method for Finger Vein Recognition |journal=Sensors |volume=22 |issue=6 |date=March 2022 |page=2234 |pmid=35336406 |pmc=8949429 |doi=10.3390/s22062234 |bibcode=2022Senso..22.2234Z |url= |doi-access=free}}</ref> and [[eye vein verification]].

== History ==
{{Further|Circulatory system#History}}
The [[Ancient Greek Medicine|Greek physician]] [[Herophilus]] (born 335 BC) distinguished veins from arteries, noting the thicker walls of 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 entered by very small vessels between veins and arteries. Thus he apparently postulated capillaries but with reversed flow of blood.<ref>[http://www.scienceclarified.com/Al-As/Anatomy.html Anatomy&nbsp;– History of anatomy]. Scienceclarified.com. Retrieved 2013-09-15.</ref>

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.

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

In addition, [[Ibn al-Nafis]] had an insight into what would become 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>{{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, however, was confined to blood transit in the lungs and did not extend to the entire body.

Finally, [[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 ''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 DC, p. 48, {{ISBN|1589011619}}.</ref> This work, with its essentially correct exposition, slowly convinced the medical world. However, Harvey was not able to identify the capillary system connecting arteries and veins; these were later discovered by [[Marcello Malpighi]] in 1661.<ref name="scielo">{{cite journal |last1=Romero Reverón |first1=Rafael |title=Marcello Malpighi (1628–1694), Founder of Microanatomy |journal=International Journal of Morphology |pages=399–402 |doi=10.4067/S0717-95022011000200015 |date=June 2011 |volume=29 |issue=2 |doi-access=free}}</ref>

== See also ==
* [[Cardiovascular disease]]
* [[Central venous pressure]]
* [[Compliance (physiology)]]

== References ==
{{Reflist}}

==Bibliography==
* {{cite book |<!--"GRAYS"-->editor1-last=Standring |editor1-first=Susan |title=Gray's Anatomy: The Anatomical Basis of Clinical Practice |date=2016 |publisher=Churchill Livingstone |location=[Philadelphia] |isbn=978-0-7020-5230-9 |edition=Forty first |ref={{harvid |GRAYS|2016}}}}

==Further reading==
*{{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}}

== External links ==
{{wiktionary}}
* [http://www.merck.com/mmhe/sec03/ch036/ch036a.html Merck Manual article on veins]
* A {{YouTube|m8E6hBG2XQk|lecture}} on the veins' and lymphatic systems of the upper limb

{{Human systems and organs}}
{{Vascular diseases}}
{{Arteries and veins}}
{{Human veins}}

{{Authority control}}
{{Use dmy dates|date=April 2017}}

[[Category:Veins]]
[[Category:Cardiovascular physiology]]