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59-480: A varix ( pl. : varices ) is an abnormally dilated blood vessel with a tortuous course. Varices usually occur in the venous system , but may also occur in arterial or lymphatic vessels. Examples of varices include: This article about a medical condition affecting the circulatory system is a stub . You can help Misplaced Pages by expanding it . Venous system There are three sizes of veins: large, medium, and small. Smaller veins are called venules , and

118-424: A basal lamina . Post-capillary venules are too small to have a smooth muscle layer and are instead supported by pericytes that wrap around them. Post-capillary venules become muscular venules when they reach a diameter of 50 μm, and can reach a diameter of 1 mm. These larger venules feed into small veins. The small veins merge to feed as tributaries into medium-sized veins. The medium veins feed into

177-486: A 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. 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

236-448: A membrane , a middle layer of muscle and elastic tissue and an outer layer of fibrous connective tissue . The middle layer is poorly developed so that venules have thinner walls than arterioles . They are porous so that fluid and blood cells can move easily from the bloodstream through their walls. Short portal venules between the posterior pituitary and the anterior pituitary lobes provide an avenue for rapid hormonal exchange via

295-431: 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 . This close proximity of the artery to the veins helps in venous return due to the pulsations in the artery. It also allows for the promotion of heat transfer from

354-417: A vasodilator. The 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 . By day 17 vessels begin to form in the yolk sac, arising from

413-446: A vein without having passed from a capillary bed. Abnormal connections can be present known as arteriovenous malformations . These are usually congenital and the connections are made from a tangle of capillaries. A cerebral arteriovenous malformation is one that is located in the brain . An irregular connection between an artery and a vein is known as arteriovenous fistula . A small specialised arteriovenous anastomosis known as

472-449: 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 malignant tumor can lead to superior vena cava syndrome . Venule A venule

531-425: Is a major independent risk factor for venous disorders. 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 . There are a number of vascular surgeries and endovascular surgeries carried out by vascular surgeons to treat many venous diseases. Venous insufficiency is the most common disorder of

590-486: Is a very small vein in the microcirculation that allows blood to return from the capillary beds to drain into the venous system via increasingly larger veins. Post-capillary venules are the smallest of the veins with a diameter of between 10 and 30 micrometres (μm). When the post-capillary venules increase in diameter to 50μm they can incorporate smooth muscle and are known as muscular venules . Veins contain approximately 70% of total blood volume, while about 25%

649-429: 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

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708-399: 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. Valves in the perforating veins close when a calf muscle contracts, to prevent backflow from the deep veins to the superficial. There are more valves in the lower leg, due to increased gravitational pull, with the number decreasing as the veins travel to

767-531: Is contained in the venules. Many venules unite to form a vein. Post-capillary venules have a single layer of endothelium surrounded by a basal lamina . Their size is between 10 and 30 micrometers and are too small to contain smooth muscle. They are instead supported by pericytes that wrap around them. When the post-capillary venules increase in diameter to 50μm they can incorporate smooth muscle and are known as muscular venules . They have an inner endothelium composed of squamous endothelial cells that act as

826-445: Is located just below the openings of the tributaries to prevent reflux form these into the GSV. 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. Their action is supported by the action of skeletal muscle pumps that contract and compress the veins. A skeletal muscle

885-402: Is much thinner than that in arteries. Vascular smooth muscle cells control the size of the vein lumens, and thereby help to regulate blood pressure . The inner tunica intima is a lining of endothelium comprising a single layer of extremely flattened epithelial cells, supported by delicate connective tissue. This subendothelium is a thin but variable connective tissue. The tunica intima has

944-495: 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). DVT usually occurs in the veins of the legs, although it can also occur in the deep veins of the arms. 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

1003-407: 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,

1062-407: The anterior tibial veins there are between 8 and 11 valves. 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

1121-499: The azygous vein , and ultimately the right atrium. Venous blood from the bronchi inside the lungs drains into the pulmonary 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. In the cerebral circulation supplying the cerebrum the venous drainage can be separated into two subdivisions: superficial and deep. The superficial system

1180-400: The hepatic portal system , and the hypophyseal portal system . An anastomosis is a joining of two structures such as blood vessels. In the circulation these are called 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

1239-439: 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

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1298-414: The splanchnic mesoderm of the yolk sac wall. The capillaries are formed during vasculogenesis , and they lengthen and interconnect to form an extensive primitive vascular network. 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. In

1357-419: The superior and inferior vena cava , which empty the oxygen-depleted blood into the right atrium of the heart. 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 of capacitance vessels . This characteristic also allows for the accommodation of pressure changes in the system. The whole of

1416-425: 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 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. Veins close to

1475-553: The action of the 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 and hypovolaemic shock can also cause fainting. In these cases,

1534-426: 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

1593-443: The blood. Specifically within and between the pituitary lobes is anatomical evidence for confluent interlobe venules providing blood from the anterior to the neural lobe that would facilitate moment-to-moment sharing of information between lobes of the pituitary gland. In contrast to regular venules, high endothelial venules are a special type of venule where the endothelium is made up of simple cuboidal cells. Lymphocytes exit

1652-458: The body and have corresponding arteries. Perforator veins drain from the superficial to the deep veins. These are usually referred to in the lower limbs and feet. Superficial veins include the very small spider veins of between 0.5 and 1 mm diameter, and reticular or feeder veins . There are a number of venous plexuses where veins are grouped or sometimes combined in networks at certain body sites. The Batson venous plexus , runs through

1711-406: The coronary circulation, the cerebral circulation, the bronchial circulation, and the renal circulation. In the coronary circulation , the blood supply to the heart, is drained by 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

1770-406: The deep structures of the brain, which join behind the midbrain to form the 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 . A portal venous system is a series of veins or venules that directly connect two capillary beds . The two systems in verebrates are

1829-422: The flow lying against the wall. 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. The endothelial cells in the sinuses are able to stretch twice as much as those in areas without valves. When the blood tries to reverse its direction (due to low venous pressure and

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1888-492: 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. 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 veins carry blood from

1947-425: The flow of blood maintained by one-way valves in the deep veins, superficial veins, and in the perforator veins. The venous valves serve to prevent regurgitation (backflow) due to the low pressure of veins, and the pull of gravity. They also serve to prevent the over-widening of the vein. A venous valve is bicuspid (having two leaflets) and is formed by an infolding of part of the tunica intima on either side of

2006-403: 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. All of the systemic veins are tributaries of the largest veins,

2065-502: The hip. There are no valves in the veins of the thorax or abdomen. There is a valve at the junction of the inferior vena cava (one of the great vessels ) 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. There are some separate parallel systemic circulatory routes that supply specific regions, and organs. They include

2124-422: 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. Blood flows back to the heart in the systemic deep veins, with

2183-419: The large veins which include the internal jugular , and renal veins , and the venae cavae that carry the blood directly into the heart. The venae cavae enter the right 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

2242-440: The larger arteries to the veins in a counterflow exchange that helps to preserve normal body heat. The first entry of venous blood is from the convergence of two or more capillaries into a microscopic, post-capillary venule . Post-capillary venules have a diameter of between 10 and 30 micrometres (μm), and are part of the microcirculation . Their endothelium is made up of flattened oval or polygon shaped cells surrounded by

2301-487: The limbs and hands and feet . 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. The middle tunica media is mainly of vascular smooth muscle cells , elastic fibers and collagen . This layer

2360-416: The lower limbs is varicose veins. 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 ) or compression from tumors or tuberculosis lesions. When the pressure increases in

2419-464: The lumen of the veins. The leaflets are strengthened with collagen, and elastic fibres, and covered with endothelium. 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

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2478-413: 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. In the systemic circulation, veins serve to return oxygen-depleted blood from organs, and tissues to the right heart . From here it passes to

2537-427: 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 nitric oxide synthase (eNOS). Other endothelial secretions are endothelin , and thromboxane (vasoconstrictors), and prostacyclin

2596-493: The portal vein, a collateral circulation develops, causing visible veins such as esophageal varices . Phlebitis is the inflammation of a vein. It is usually accompanied by a 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 . Some disorders as syndromes result from compression of

2655-406: The post-capillary venules. Veins have a similar three-layered structure to arteries. The layers known as 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. The middle layer, consists of bands of smooth muscle and is known as

2714-411: The pull of gravity), the sinuses fill first closing the leaflets and keeping them together. Approximately 95% of the venous valves are in the small veins of less than 300 micrometres. 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

2773-434: 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

2832-448: The right atrium. The inferior vena cava is the larger of the two. The inferior vena cava is retroperitoneal and runs to the right and roughly parallel to the abdominal aorta along the spine . The three main compartments of the venous system are the deep veins , the superficial veins , and the perforator veins . Superficial veins are those closer to the surface of the body, and have no corresponding arteries. Deep veins are deeper in

2891-446: 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 veins . In the neck, the jugular veins 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

2950-421: 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. The main risk factor for SVT in

3009-414: 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). In the bronchial circulation that supplies blood to the lung tissues, bronchial veins drain venous blood from the large main bronchi into

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3068-450: The smallest the post-capillary venules are microscopic that make up the veins of the microcirculation . Veins are often closer to the skin than arteries. Veins have less smooth muscle and connective tissue and wider internal diameters than arteries. Because of their thinner walls and wider lumens they are able to expand and hold more blood. This greater capacity gives them the term of capacitance vessels . At any time, nearly 70% of

3127-457: 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 a thrombus or insufficiency of the valves, or both of these. Other conditions may be due to inflammation , or compression. Ageing

3186-429: The surface of the skin appear blue for a variety of reasons. The factors that contribute to this alteration of 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. The venous system is the system of veins in the systemic and pulmonary circulations that return blood to

3245-409: 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 regulate blood pressure . The post-capillary venules are exchange vessels whose ultra-thin walls allow the ready diffusion of molecules from the capillaries. The return of blood to the heart is assisted by

3304-494: The total volume of blood in the human body is in the veins. In medium and large sized veins the flow of blood is maintained by one-way (unidirectional) venous valves to prevent backflow . In the lower limbs this is also aided by muscle pumps , also known as venous pumps that exert pressure on intramuscular veins when they contract and drive blood back to the heart. There are three sizes of vein, large, medium, and small. Smaller veins are called venules . The smallest veins are

3363-415: The venous system, and is usually manifested as either 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 . Venous thrombosis

3422-423: The venous system, bar the post-capillary venules is a large volume, low pressure system. 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. Veins vary in size from the smallest post-capillary venules , and more muscular venules, to small veins, medium veins, and large veins. The thickness of

3481-445: The walls of the veins varies as to their location – in the legs the vein walls are much thicker than those in the arms. In the circulatory system, blood first enters the venous system from capillary beds where arterial blood changes to venous blood. Large arteries such as the thoracic aorta , subclavian , femoral and popliteal arteries lie close to a single vein that drains the same region. Other arteries are often accompanied by

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