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86-442: Arteries contrast with veins , which carry deoxygenated blood back towards the heart; or in the pulmonary and fetal circulations carry oxygenated blood to the lungs and fetus respectively. The anatomy of arteries can be separated into gross anatomy , at the macroscopic level , and microanatomy , which must be studied with a microscope . The arterial system of the human body is divided into systemic arteries , carrying blood from

172-424: 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

258-486: 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

344-410: A pulse , which can be felt in different areas of the body, such as the radial pulse . Arterioles have the greatest collective influence on both local blood flow and on overall blood pressure. They are the primary "adjustable nozzles" in the blood system, across which the greatest pressure drop occurs. The combination of heart output ( cardiac output ) and systemic vascular resistance , which refers to

430-411: A diameter less than that of red blood cells ; a red blood cell is typically 7 micrometers outside diameter, capillaries typically 5 micrometers inside diameter. The red blood cells must distort in order to pass through the capillaries. These small diameters of the capillaries provide a relatively large surface area for the exchange of gasses and nutrients. Systemic arterial pressures are generated by

516-431: 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

602-417: 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

688-446: 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

774-456: 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 . Arteriole An arteriole

860-425: 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

946-414: Is a small-diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillaries . Arterioles have muscular walls (usually only one to two layers of smooth muscle cells ) and are the primary site of vascular resistance . The greatest change in blood pressure and velocity of blood flow occurs at the transition of arterioles to capillaries. This function

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1032-463: Is absent in the post-capillary venules. 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 systolic pressures that the arteries are. There are valves present in many veins that maintain unidirectional flow. Unlike arteries,

1118-429: 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

1204-399: 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

1290-503: Is extremely important because it prevents the thin, one-layer capillaries from exploding upon pressure. The arterioles achieve this decrease in pressure, as they are the site with the highest resistance (a large contributor to total peripheral resistance) which translates to a large decrease in the pressure. In a healthy vascular system, the endothelium lines all blood-contacting surfaces, including arteries, arterioles, veins, venules, capillaries, and heart chambers. This healthy condition

1376-448: 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

1462-402: 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

1548-434: Is primarily influenced by activity of the sympathetic vasomotor nerves innervating the arterioles. Enhanced sympathetic activation prompts vasoconstriction, reducing the lumen diameter. A reduced lumen diameter consequently elevates the blood pressure within the arterioles. Conversely, decreased sympathetic activity within the vasomotor nerves causes vasodilation of the vessels thereby decreasing blood pressure. The aorta

1634-414: Is promoted by the ample production of nitric oxide by the endothelium, which requires a biochemical reaction regulated by a complex balance of polyphenols , various nitric oxide synthase enzymes and L- arginine . In addition, there is direct electrical and chemical communication via gap junctions between the endothelial cells and the vascular smooth muscle. Blood pressure in the arteries supplying

1720-486: Is the tunica media , which is made up of smooth muscle cells, elastic tissue (also called connective tissue proper ) and collagen fibres. The innermost layer, which is in direct contact with the flow of blood, is the tunica intima . The elastic tissue allows the artery to bend and fit through places in the body. This layer is mainly made up of endothelial cells (and a supporting layer of elastin rich collagen in elastic arteries). The hollow internal cavity in which

1806-475: Is the effect when an artery is cut due to the higher arterial pressures. Blood is spurted out at a rapid, intermittent rate, that coincides with the heartbeat. The amount of blood loss can be copious, can occur very rapidly, and be life-threatening. Over time, factors such as elevated arterial blood sugar (particularly as seen in diabetes mellitus ), lipoprotein , cholesterol , high blood pressure , stress and smoking , are all implicated in damaging both

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1892-495: 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

1978-443: Is the root systemic artery (i.e., main artery). In humans, it receives blood directly from the left ventricle of the heart via the aortic valve . As the aorta branches and these arteries branch, in turn, they become successively smaller in diameter, down to the arterioles . The arterioles supply capillaries , which in turn empty into venules . The first branches off of the aorta are the coronary arteries , which supply blood to

2064-400: Is the system of veins in the systemic and pulmonary 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

2150-434: Is the term specifically used for the hardening of arteriole walls. This can be due to decreased elastic production from fibrinogen, associated with ageing , or hypertension or pathological conditions such as atherosclerosis . Arteritis of the arterioles occurs when the arteriole walls become inflamed as a result of either an immune response to infection or an autoimmune response. The muscular contraction of arterioles

2236-465: Is unique because the blood in it is not "oxygenated", as it has not yet passed through the lungs. The other unique artery is the umbilical artery , which carries deoxygenated blood from a fetus to its mother. Arteries have a blood pressure higher than other parts of the circulatory system. The pressure in arteries varies during the cardiac cycle . It is highest when the heart contracts and lowest when heart relaxes . The variation in pressure produces

2322-407: 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,

2408-407: 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

2494-423: The arterial blood pressure is due to the pulsatile nature of the cardiac output and determined by the interaction of the stroke volume versus the volume and elasticity of the major arteries. The decreased velocity of flow in the capillaries increases the blood pressure, due to Bernoulli's principle . This induces gas and nutrients to move from the blood to the cells, due to the lower osmotic pressure outside

2580-431: The arterioles , and then to the capillaries , where nutrients and gasses are exchanged. After traveling from the aorta , blood travels through peripheral arteries into smaller arteries called arterioles , and eventually to capillaries . Arterioles help in regulating blood pressure by the variable contraction of the smooth muscle of their walls, and deliver blood to the capillaries . This smooth muscle contraction

2666-499: 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

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2752-411: The capillary vessels that join arteries and veins, and there was no notion of circulation. Diogenes of Apollonia developed the theory of pneuma , originally meaning just air but soon identified with the soul itself, and thought to co-exist with the blood in the blood vessels. The arteries were thought to be responsible for the transport of air to the tissues and to be connected to the trachea . This

2838-406: 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 and fetal circulations which carry oxygenated blood to the heart. In the systemic circulation , arteries carry oxygenated blood away from the heart, and veins return deoxygenated blood to

2924-561: The endothelium and walls of the arteries, resulting in atherosclerosis . Atherosclerosis is a disease marked by the hardening of arteries. This is caused by an atheroma or plaque in the artery wall and is a build-up of cell debris, that contain lipids , (cholesterol and fatty acids ), calcium and a variable amount of fibrous connective tissue . Accidental intra-arterial injection either iatrogenically or through recreational drug use can cause symptoms such as intense pain, paresthesia and necrosis . It usually causes permanent damage to

3010-400: 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

3096-439: 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

3182-414: 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

3268-418: The tunica adventitia , is composed of collagen fibers and elastic tissue —with the largest arteries containing vasa vasorum , small blood vessels that supply the walls of large blood vessels. Most of the layers have a clear boundary between them, however the tunica externa has a boundary that is ill-defined. Normally its boundary is considered when it meets or touches the connective tissue. Inside this layer

3354-489: 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. 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

3440-556: 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,

3526-1011: The adrenal gland medulla) are vasoconstrictive acting on alpha 1-adrenergic receptors. However, the arterioles of skeletal muscle, cardiac muscle, and pulmonary circulation vasodilate in response to these hormones when they act on beta-adrenergic receptors. Generally, stretch and high oxygen tension increase tone, and carbon dioxide and low pH promote vasodilation. Pulmonary arterioles are a noteworthy exception as they vasodilate in response to high oxygen. Brain arterioles are particularly sensitive to pH with reduced pH promoting vasodilation. A number of hormones influence arteriole tone such as angiotensin II (vasoconstrictive), endothelin (vasoconstrictive), bradykinin (vasodilation), atrial natriuretic peptide (vasodilation), and prostacyclin (vasodilation). Arteriole diameters decrease with age and with exposure to air pollution . Any pathology which constricts blood flow, such as stenosis , will increase total peripheral resistance and lead to hypertension . Arteriolosclerosis

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3612-426: 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

3698-449: The blood flows is called the lumen . Arterial formation begins and ends when endothelial cells begin to express arterial specific genes, such as ephrin B2 . Arteries form part of the circulatory system . They carry blood that is oxygenated after it has been pumped from the heart . Coronary arteries also aid the heart in pumping blood by sending oxygenated blood to the heart, allowing

3784-458: 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

3870-417: The body is a result of the work needed to pump the cardiac output (the flow of blood pumped by the heart) through the vascular resistance , sometimes termed total peripheral resistance . An increase in the tunica media to luminal diameter ratio has been observed in hypertensive arterioles ( arteriolosclerosis ) as the vascular wall thickens and/or luminal diameter decreases. The up and down fluctuation of

3956-705: The capillary. The opposite process occurs when the blood leaves the capillaries and enters the venules , where the blood pressure drops due to an increase in flow rate. Arterioles receive autonomic nervous system innervation and respond to various circulating hormones in order to regulate their diameter. Retinal vessels lack a functional sympathetic innervation. Arteriole diameter varies according to autoregulation of organs or tissues to maintain sufficient blood flow despite changes in pressure via metabolic or myogenic factors which include stretch, carbon dioxide, and oxygen among other factors. Generally, norepinephrine and epinephrine (hormones produced by sympathetic nerves and

4042-410: The collective resistance of all of the body's arterioles , are the principal determinants of arterial blood pressure at any given moment. Arteries have the highest pressure and have narrow lumen diameter. Systemic arteries are the arteries (including the peripheral arteries ), of the systemic circulation , which is the part of the cardiovascular system that carries oxygenated blood away from

4128-406: 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

4214-406: 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

4300-422: 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

4386-497: 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

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4472-425: 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

4558-401: The forceful contractions of the heart's left ventricle . High blood pressure is a factor in causing arterial damage. Healthy resting arterial pressures are relatively low, mean systemic pressures typically being under 100  mmHg (1.9  psi ; 13  kPa ) above surrounding atmospheric pressure (about 760 mmHg, 14.7 psi, 101 kPa at sea level). To withstand and adapt to

4644-494: The heart muscle itself. These are followed by the branches of the aortic arch, namely the brachiocephalic artery , the left common carotid , and the left subclavian arteries. The capillaries are the smallest of the blood vessels and are part of the microcirculation . The microvessels have a width of a single cell in diameter to aid in the fast and easy diffusion of gasses, sugars and nutrients to surrounding tissues. Capillaries have no smooth muscle surrounding them and have

4730-476: The heart to the whole body, and pulmonary arteries , carrying deoxygenated blood from the heart to the lungs . Large arteries (such as the aorta) are composed of many different types of cells, namely endothelial, smooth muscle, fibroblast, and immune cells. As with veins, the arterial wall consists of three layers called tunics, namely the tunica intima , tunica media , and tunica externa , from innermost to outermost. The externa , alternatively known as

4816-519: The heart, in the deep veins. There are three sizes of veins: large, medium, and small. Smaller veins are called venules , and 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

4902-482: The heart, to the body , and returns deoxygenated blood back to the heart. Systemic arteries can be subdivided into two types—muscular and elastic—according to the relative compositions of elastic and muscle tissue in their tunica media as well as their size and the makeup of the internal and external elastic lamina. The larger arteries (>10  mm diameter) are generally elastic and the smaller ones (0.1–10 mm) tend to be muscular. Systemic arteries deliver blood to

4988-423: The heart. There are three sizes of vein, large, medium, and small. Smaller veins are called venules . The smallest veins are 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

5074-502: 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

5160-422: 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

5246-419: 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

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5332-440: 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

5418-470: The limb; often amputation is necessary. Among the Ancient Greeks before Hippocrates , all blood vessels were called Φλέβες, phlebes . The word arteria then referred to the windpipe . Herophilos was the first to describe anatomical differences between the two types of blood vessel. While Empedocles believed that the blood moved to and fro through the blood vessels, there was no concept of

5504-487: 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

5590-416: 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

5676-464: 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

5762-413: 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

5848-569: The modern concept of the circulatory system and the roles of arteries and veins in the 17th century. Alexis Carrel at the beginning of the 20th century first described the technique for vascular suturing and anastomosis and successfully performed many organ transplantations in animals; he thus actually opened the way to modern vascular surgery that was previously limited to vessels' permanent ligation. ocular group: central retinal Vein Veins ( / v eɪ n / ) are blood vessels in

5934-427: 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

6020-426: The muscles to function. Arteries carry oxygenated blood away from the heart to the tissues, except for pulmonary arteries , which carry blood to the lungs for oxygenation (usually veins carry deoxygenated blood to the heart but the pulmonary veins carry oxygenated blood as well). There are two types of unique arteries. The pulmonary artery carries blood from the heart to the lungs , where it receives oxygen. It

6106-493: 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

6192-413: The precise location of veins varies among individuals. Veins close to 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

6278-438: The pressures within, arteries are surrounded by varying thicknesses of smooth muscle which have extensive elastic and inelastic connective tissues . The pulse pressure, being the difference between systolic and diastolic pressure, is determined primarily by the amount of blood ejected by each heart beat, stroke volume , versus the volume and elasticity of the major arteries. A blood squirt , also known as an arterial gush,

6364-411: 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

6450-434: 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

6536-448: 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

6622-446: 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

6708-421: 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

6794-414: 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

6880-447: The small veins and venules. All of the systemic veins are tributaries of the largest veins, 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

6966-457: 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

7052-409: 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

7138-426: The term of capacitance vessels . At any time, nearly 70% of 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

7224-415: 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

7310-445: 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

7396-411: Was as a result of finding the arteries of cadavers devoid of blood. In medieval times, it was supposed that arteries carried a fluid, called "spiritual blood" or "vital spirits", considered to be different from the contents of the veins . This theory went back to Galen . In the late medieval period, the trachea , and ligaments were also called "arteries". William Harvey described and popularized

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