Vein
Vein | |
---|---|
Details | |
System | Circulatory system |
Identifiers | |
Latin | vena |
MeSH | D014680 |
TA98 | A12.0.00.030 A12.3.00.001 |
TA2 | 3904 |
FMA | 50723 |
Anatomical terminology |
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.[2] Veins are often closer to the skin than arteries.
Veins have less
Structure
There are three sizes of vein, large, medium, and small. Smaller veins are called
Unlike arteries, the precise location of veins varies among individuals.[5]
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.[6]
Venous system
The venous system is the system of veins in the
Veins vary in size from the smallest post-capillary
Large arteries such as the
- Venules
The first entry of venous blood is from the convergence of two or more
- 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, and renal veins, and the venae cavae that carry the blood directly into the heart.[13] 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 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.
- Deep, superficial, and perforator veins
The three main compartments of the venous system are the
- Venous plexuses
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 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
Venous valves
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 prevent regurgitation (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 the
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.[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 of skeletal 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 the
Circulatory routes
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 the
- Bronchial circulation
In the
- 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
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 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
- Anastomoses
An
Abnormal connections can be present known as arteriovenous malformations. These are usually congenital and the connections are made from a tangle of capillaries.[29] 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 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.[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 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.[21][4]
- Other
A
.Microanatomy
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.[8]
The middle tunica media is mainly of vascular
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 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).[32] Other endothelial secretions are endothelin, and thromboxane (vasoconstrictors), and prostacyclin a vasodilator.[9]
Development
The
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,
Function
In the systemic circulation, veins serve to return oxygen-depleted blood from organs, and tissues to the
The post-capillary venules are exchange 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 the
Clinical significance
Most venous diseases involve obstruction such as a
Venous insufficiency
Venous insufficiency is the most common disorder of the venous system, and is usually manifested as either
Venous 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.[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 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
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.[42] The main risk factor for SVT in the lower limbs is varicose veins.[42]
Portal hypertension
The
Phlebitis
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.[43]
Compression
Some disorders as syndromes result from compression of a vein. These include a venous type of
Vascular anomalies
A vascular anomaly can be either a vascular tumor or a birthmark, or a vascular malformation.
Venous access
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 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
Recognition techniques
Some imaging techniques using veins have been developed for identification purposes. These
History
The
In 2nd century AD
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.[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 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."[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 by Marcello Malpighi in 1661.[56]
See also
References
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- ^ a b "Classification & Structure of Blood Vessels | SEER Training". training.seer.cancer.gov. Retrieved 29 January 2023.
- ^ a b c d e f g h GRAYS 2016, p. 131.
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- ^ a b GRAYS 2016, p. 127.
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- ^ a b "20.1 Structure and Function of Blood Vessels - Anatomy and Physiology 2e | OpenStax". openstax.org. 20 April 2022. Retrieved 17 March 2023.
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Bibliography
- Standring, Susan, ed. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice (Forty first ed.). [Philadelphia]: Churchill Livingstone. ISBN 978-0-7020-5230-9.
Further reading
- Shoja, M. M.; Tubbs, R. S.; Loukas, M.; Khalili, M.; Alakbarli, F.; Cohen-Gadol, A. A. (2009). "Vasovagal syncope in the Canon of Avicenna: The first mention of carotid artery hypersensitivity". International Journal of Cardiology. 134 (3): 297–301. PMID 19332359.
External links
- Merck Manual article on veins
- A lecture on YouTubeon the veins' and lymphatic systems of the upper limb