Structure of a vein, which consists of three main layers: an outer layer ofconnective tissue, a middle layer ofsmooth muscle, and an inner layer lined withendothelium.
There are three sizes of veins: large, medium, and small. Smaller veins are calledvenules, and the smallest the post-capillary venules are microscopic that make up the veins of themicrocirculation.[2] Veins are often closer to the skin than arteries.
Veins have lesssmooth muscle andconnective tissue and widerinternal diameters than arteries. Because of their thinner walls and wider lumens they are able to expand and hold more blood. This greatercapacity gives them the term ofcapacitance vessels. At any time, nearly 70% of the total volume of blood in the human body is in the veins.[3] In medium and large sized veins the flow of blood is maintained by one-way (unidirectional) venous valves to preventbackflow.[3][1] In the lower limbs this is also aided bymuscle pumps, also known as venous pumps that exert pressure on intramuscular veins when theycontract and drive blood back to the heart.[4]
Layers of vein wall shown in comparison to arterial wall
There are three sizes of vein, large, medium, and small. Smaller veins are calledvenules. The smallest veins are the post-capillary venules. Veins have a similar three-layered structure to arteries. The layers known astunicae have a concentric arrangement that forms the wall of the vessel. The outer layer, is a thick layer ofconnective tissue called thetunica externa oradventitia; this layer is absent in the post-capillary venules.[4] The middle layer, consists of bands ofsmooth muscle and is known as thetunica media. The inner layer, is a thin lining ofendothelium known as thetunica intima. The tunica media in the veins is much thinner than that in the arteries as the veins are not subject to the highsystolic 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.[5][page needed]
Veins close to the surface of the skin appear blue for a variety of reasons. The factors that contribute to this alteration ofcolor perception are related to the light-scattering properties of the skin and the processing of visual input by thevisual cortex, rather than the actual colour of the venous blood which is dark red.[6]
The venous system is the system of veins in thesystemic andpulmonary circulations 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.[7] All of the systemic veins are tributaries of the largest veins, thesuperior andinferior vena cava, which empty the oxygen-depleted blood into theright atrium of the heart.[8] The thin walls of the veins, and their greater internal diameters (lumens) enable them to hold a greater volume of blood, and this greater capacitance gives them the term ofcapacitance vessels.[4] 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.[9] 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.[5][7]
Veins vary in size from the smallest post-capillaryvenules, 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.[10] In the circulatory system, blood first enters the venous system fromcapillary beds where arterial blood changes to venous blood.
Large arteries such as thethoracic aorta,subclavian,femoral andpopliteal 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 asvenae comitantes, orsatellite veins, and they run on either side of the artery. When an associated nerve is also enclosed, the sheath is known as aneurovascular bundle.[11] This close proximity of the artery to the veins helps invenous return due to the pulsations in the artery.[12] It also allows for the promotion of heat transfer from the larger arteries to the veins in acounterflow exchange that helps to preserve normal body heat.[11]
Venules
Deep and superficial veins of the arm and near thorax
The first entry of venous blood is from the convergence of two or morecapillaries into a microscopic,post-capillary venule.[13] Post-capillary venules have a diameter of between 10 and 30micrometres (μm), and are part of themicrocirculation. Their endothelium is made up of flattened oval or polygon shaped cells surrounded by abasal lamina. Post-capillary venules are too small to have a smooth muscle layer and are instead supported bypericytes that wrap around them.[14] Post-capillary venules becomemuscular venules when they reach a diameter of 50 μm,[10] and can reach a diameter of 1 mm.[13] 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 theinternal jugular, andrenal veins, and thevenae cavae that carry the blood directly into the heart.[13] The venae cavae enter the rightatrium of the heart from above and below. From above, thesuperior vena cava carries blood from the arms, head, and chest to the right atrium of the heart, and from below, theinferior vena cava carries blood from the legs andabdomen to the right atrium. The inferior vena cava is the larger of the two. The inferior vena cava isretroperitoneal and runs to the right and roughly parallel to theabdominal aorta along thespine.
Deep, superficial, and perforator veins
The three main compartments of the venous system are thedeep veins, thesuperficial veins, and theperforator veins.[15] 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.[16] These are usually referred to in the lower limbs and feet.[17] Superficial veins include the very smallspider veins of between 0.5 and 1 mm diameter, andreticular or feeder veins.[18]
Venous plexuses
There are a number ofvenous plexuses where veins are grouped or sometimes combined in networks at certain body sites. TheBatson 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 formetastasis of certain cancers.
A subcutaneous venous plexus is continuous, and a high rate of flow is supplied by smallarteriovenous anastomoses. The high rate of flow ensures heat transfer to the vein wall.[19]
Video of a valve in thepopliteal vein opening to allow blood to flow through and closing to prevent backflowVenous 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.[20] The venous valves serve to preventregurgitation (backflow) due to the low pressure of veins, and the pull of gravity.[1] They also serve to prevent the over-widening of the vein.[20][21]
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.[10] 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.[4] 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.[22] The endothelial cells in the sinuses are able to stretch twice as much as those in areas without valves.[22] 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.[4][8] Approximately 95% of the venous valves are in the small veins of less than 300 micrometres.[23]
The deep veins of the lower limb include thecommon femoral vein,femoral vein, and thedeep femoral vein; thepopliteal vein, the tibial, andfibular veins. In the common femoral vein one valve is located above thesaphenofemoral junction called thesuprasaphenic 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 eachposterior tibial vein there are between 8 and 19 valves, and in theanterior tibial veins there are between 8 and 11 valves.[20]
In the superficial veins there are between one and seven valves along the thigh portion of thegreat 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 theterminal valve to prevent reflux from the femoral vein Apreterminal valve is located just below the openings of the tributaries to prevent reflux form these into the GSV.[20] 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.[24] Their action is supported by the action ofskeletal muscle pumps 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.[8] Valves in the perforating veins close when a calf muscle contracts, to prevent backflow from the deep veins to the superficial.[25] 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.[4]
There is a valve at the junction of the inferior vena cava (one of thegreat vessels) and the right atrium known as thevalve of inferior vena cava also known as theeustachian valve. This valve is an embryological remnant and is insignificant in the adult. However, when persistent it can cause problems.[26]
There are some separate parallel systemic circulatory routes that supply specific regions, and organs.[8] They include the coronary circulation, the cerebral circulation, the bronchial circulation, and the renal circulation.
Coronary circulation
In thecoronary circulation, the blood supply to the heart, is drained bycardiac veins (or coronary veins) that remove the deoxygenated blood from theheart muscle. These include thegreat cardiac vein, themiddle cardiac vein, thesmall cardiac vein, thesmallest cardiac veins, and theanterior cardiac veins. Cardiac veins carry blood with a poor level of oxygen, from the heart muscle to theright atrium. Most of the blood of the cardiac veins returns through thecoronary 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 theleft ventricle, and theoblique 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).[27]
Bronchial circulation
In thebronchial circulation that supplies blood to the lung tissues,bronchial veins drainvenous blood from the largemain bronchi into theazygous vein, and ultimately the right atrium. Venous blood from the bronchi inside the lungs drains into thepulmonary veins 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.[28]
Cerebral circulation
In thecerebral circulation supplying thecerebrum the venous drainage can be separated into two subdivisions: superficial and deep.The superficial system is composed ofdural 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 thesuperior sagittal sinus which flows in the sagittal plane under the midline of the cerebral vault, posteriorly and inferiorly to theconfluence of sinuses, where the superficial drainage joins with the sinus that primarily drains the deep venous system. From here, twotransverse sinuses bifurcate and travel laterally and inferiorly in an S-shaped curve that forms thesigmoid sinuses which go on to form the twojugular veins. In the neck, thejugular veins parallel the upward course of thecarotid arteries and drain blood into thesuperior 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 thevein of Galen. This vein merges with theinferior sagittal sinus to form thestraight sinus which then joins the superficial venous system mentioned above at theconfluence of sinuses.
Ananastomosis is a joining of two structures such as blood vessels. In the circulation these are calledcirculatory anastomoses, one of which is the join between an artery with a vein known as anarteriovenous 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.[19][14]
A small specialised arteriovenous anastomosis known as aglomus 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.[14]
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 ametarteriole 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.[21][4]
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 thetunica adventitia is a sheath of thick connective tissue. This layer is absent in the post-capillary venules.[8]
The inner tunica intima is a lining of endothelium comprising a single layer of extremely flattened epithelial cells, supported by delicate connective tissue.[8] This subendothelium is a thin but variable connective tissue.[4] 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 producenitric oxide a soluble gas, to the cells of the adjacent smooth muscle layer. This constant synthesis is carried out by the enzymeendothelial nitric oxide synthase (eNOS).[32] Other endothelial secretions areendothelin, andthromboxane (vasoconstrictors), andprostacyclin a vasodilator.[9]
Thedevelopment of the embryo is completely reliant on thevitelline circulation, the bidirectional flow of blood between theyolk 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 theplacenta.[33] By day 17 vessels begin to form in the yolk sac, arising from thesplanchnic mesoderm of the yolk sac wall.[34] The capillaries are formed duringvasculogenesis, and they lengthen and interconnect to form an extensive primitive vascular network.[35] Blood is supplied from the primitive aorta, and drained byvitelline veins from the yolk sac to the embryo. By the end of the third week the yolk sac,connecting stalk, andchorionic villi are entirely vascularised.[35]
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.[36]
In the systemic circulation, veins serve to return oxygen-depleted blood from organs, and tissues to theright heart. From here it passes to the pulmonary arteries for the pulmonary circulation to return oxygen-rich blood to theleft 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 ofcapacitance 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 cells control the size of the vein lumens, and thereby help to regulateblood pressure.[31]
The post-capillary venules areexchange vessels whose ultra-thin walls allow the ready diffusion of molecules from the capillaries.[10]
The return of blood to the heart is assisted by the action of themuscle 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 abaroreflex such that angiotensin II and norepinephrine stimulate vasoconstriction and heart rate increases to return blood flow.Neurogenic andhypovolaemic 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.
Most venous diseases involve obstruction such as athrombus or insufficiency of the valves, or both of these.[37][20] Other conditions may be due toinflammation, or compression.Ageing is a major independent risk factor for venous disorders.[38] The medical speciality involved with the diagnosis and treatment of venous disorders is known asphlebology (alsovenology), and the specialist concerned is aphlebologist.[39] There are a number ofvascular surgeries and endovascular surgeries carried out by vascular surgeons to treat many venous diseases.
DVT usually occurs in the veins of the legs, although it can also occur in the deep veins of the arms.[41] 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 anembolus and lodge in apulmonary artery in the lungs, known as apulmonary embolism.
The decision to treat deep vein thrombosis depends on its size, symptoms, and their risk factors. It generally involvesanticoagulation 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.
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.[42] The main risk factor for SVT in the lower limbs is varicose veins.[42]
TheGreek physicianHerophilus (born 335 BC) distinguished veins from arteries, noting the thicker walls of arteries, but thought that thepulse was a property of arteries themselves. Greek anatomistErasistratus 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.[53]
In 2nd century ADRome, theGreek physicianGalen 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.
Image of veins fromWilliam Harvey'sExercitatio 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 thecapillary 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.[54] 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 ofHieronymus Fabricius (who had earlier described the valves of the veins without recognizing their function), performed a sequence of experiments, and publishedExercitatio 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."[55] 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 byMarcello Malpighi in 1661.[56]
^abMaton, Anthea; Jean Hopkins; Charles William McLaughlin; Alexandra Senckowski; Susan Johnson; Maryanna Quon Warner; David LaHart; Jill D. Wright (1993).Human Biology and Health. Englewood Cliffs, New Jersey: Prentice Hall.ISBN978-0-13-981176-0.
^Albert, consultants Daniel (2012).Dorland's illustrated medical dictionary (32nd ed.). Philadelphia, PA: Saunders/Elsevier. p. 2042.ISBN978-1-4160-6257-8.
^abHall, John E. (2011).Guyton and Hall textbook of medical physiology (Twelfth ed.). Philadelphia, Pa. p. 868.ISBN978-1-4160-4574-8.{{cite book}}: CS1 maint: location missing publisher (link)
^Cai Q, Ahmad M (June 2020). "Eustachian valve, interatrial shunt, and paradoxical embolism".Echocardiography.37 (6):939–944.doi:10.1111/echo.14682.PMID32426851.
^Adams, Matt; Morgan, Matt A.; et al. (11 November 2014)."Coronary veins".Radiopaedia.org.
^Weinberger, Steven E. (2019).Principles of pulmonary medicine (Seventh ed.). Philadelphia, PA. p. 178.ISBN978-0-323-52371-4.{{cite book}}: CS1 maint: location missing publisher (link)
^Tousoulis D, Kampoli AM, Tentolouris C, Papageorgiou N, Stefanadis C (January 2012). "The role of nitric oxide on endothelial function".Curr Vasc Pharmacol.10 (1):4–18.doi:10.2174/157016112798829760.PMID22112350.
^Donovan, Mary F.; Arbor, Tafline C.; Bordoni, Bruno (2023)."Embryology, Yolk Sac".StatPearls. StatPearls Publishing.PMID32310425. Retrieved22 March 2023.
^Schoenwolf, Gary C. (2015).Larsen's human embryology (Fifth ed.). Philadelphia, PA. p. 304.ISBN978-1-4557-0684-6.{{cite book}}: CS1 maint: location missing publisher (link)
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^Garcia R, Labropoulos N (April 2018). "Duplex Ultrasound for the Diagnosis of Acute and Chronic Venous Diseases".Surg Clin North Am.98 (2):201–218.doi:10.1016/j.suc.2017.11.007.PMID29502767.
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