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Vein
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In the circulatory system, veins (from the Latin vena) are blood vessels that carry blood toward the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated blood. They differ from arteries in structure and function; for example, arteries are more muscular than veins and they carry blood away from the heart.
s generally function to return deoxygenated blood to the heart, and are essentially tubes that collapse when their lumens are not filled with blood.
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Encyclopedia
In the circulatory system, veins (from the Latin vena) are blood vessels that carry blood toward the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated blood. They differ from arteries in structure and function; for example, arteries are more muscular than veins and they carry blood away from the heart.
Anatomy
Veins generally function to return deoxygenated blood to the heart, and are essentially tubes that collapse when their lumens are not filled with blood. The thick outermost layer of a vein is made of connective tissue, called tunica adventitia or tunica externa. Deeper are bands of smooth muscle called tunica media, which are generally thin as veins do not function primarily in a contractile manner. The interior is lined with endothelial cells called tunica intima. Most veins have one-way flaps called venous valves that prevent blood from flowing back and pooling in the lower extremities due to the effects of gravity. These are infoldings of the tunica intima. The precise location of veins is much more variable from person to person than that of arteries.
Notable veins and vein systems
The greater saphenous vein is the most important superficial vein of the lower limb. First described by the Persian physician Avicenna, Saphenous derives its name from Safina, meaning hidden. This vein is "hidden" in its own fascial compartment in the thigh and only exits the fascia near the knee. Incompetence of this vein is an important cause of varicose veins of lower limbs.
The pulmonary veins carry relatively oxygenated blood from the lungs to the heart. The superior and inferior venae cavae carry relatively deoxygenated blood from the upper and lower systemic circulations, respectively.
A portal venous system is a series of veins or venules that directly connect two capillary beds. Examples of such systems include the hepatic portal vein and hypophyseal portal system.
Color
Vein color is determined in large part by the color of venous blood, which is usually dark red as a result of its low oxygen content. Veins appear blue because the subcutaneous fat absorbs low frequency light, permitting only the highly energetic blue wavelengths to penetrate through to the dark vein and reflect off. This results from a phenomenon called Rayleigh scattering.
Function
Veins serve to return blood from organs to the heart. In systemic circulation oxygenated blood is pumped by the left ventricle through the arteries to the muscles and organs of the body, where its nutrients and gases are exchanged at capillaries, entering the veins filled with cellular waste and carbon dioxide. The de-oxygenated blood is taken by veins to the right atrium of the heart, which transfers the blood to the right ventricle, where it is then pumped through the pulmonary arteries to the lungs. In pulmonary circulation the pulmonary veins return oxygenated blood from the lungs to the left atrium, which empties into the left ventricle, completing the cycle of blood circulation.
The return of blood to the heart is assisted by the action of the skeletal-muscle pump which helps maintain the extremely low blood pressure of the venous system. Fainting can be caused by failure of the skeletal-muscular pump. Long periods of standing can result in blood pooling in the legs, with blood pressure too low to return blood to the heart. Neurogenic and hypovolaemic shock can also cause fainting. In these cases the smooth muscles surrounding the veins become slack and the veins fill with the majority of the blood in the body, keeping blood away from the brain and causing unconsciousness.
The arteries are perceived as carrying oxygenated blood to the tissues, while veins carry deoxygenated blood back to the heart. This is true of the systemic circulation, by far the larger of the two circuits of blood in the body, which transports oxygen from the heart to the tissues of the body. However, in pulmonary circulation the arteries carry deoxygenated blood from the heart to the lungs and veins return blood from the lungs to the heart. The difference between veins and arteries is their direction of flow (out of the heart by arteries, returning to the heart for veins), not their oxygen content. In addition, deoxygenated blood that is carried from the tissues back to the heart for reoxygenation in systemic circulation still carries some oxygen, though it is considerably less than that carried by the systemic arteries or pulmonary veins.
Classification
Veins are classified in a number of ways, including superficial vs. deep, pulmonary vs. systemic, and large vs. small.
Superficial veins
- Superficial veins are those whose course is close to the surface of the body, and who have no corresponding artery.
Deep veins
- Deep veins are deeper in the body and have corresponding arteries.
Pulmonary veins
- The pulmonary veins are a set of veins that deliver oxygenated blood from the lungs to the heart.
Systemic veins
- Systemic veins drain the tissues of the body and deliver deoxygenated blood to the heart.
Clinical significance
Intravenous access
Veins are used medically as points of access to the blood stream, permitting the withdrawal of blood specimens (venipuncture) for testing purposes, and intravenous delivery of fluid, electrolytes, nutrition, and medications through injection with a syringe, or by inserting a catheter. In contrast to arterial blood which is uniform throughout the body, the blood removed from veins for testing can vary in its contents depending on the part of the body the vein drains; blood drained from a working muscle will contain significantly less oxygen and glucose than blood drained from the liver. However the more blood from different veins mixes as it returns to the heart, the more homogeneous it becomes.
If an intravenous catheter has to be inserted, for most purposes this is done into a peripheral vein near the surface of the skin in the hand or arm, or less desirably, the leg. Some highly concentrated fluids or irritating medications must flow into the large central veins, which are sometimes used when peripheral access cannot be obtained. Catheters can be threaded into the superior vena cava for these uses: if long term use is thought to be needed, a more permanent access point can be inserted surgically.
Phlebology
Phlebology is the medical discipline that involves the diagnosis and treatment of disorders of venous origin. Diagnostic techniques used include the history and physical examination, venous imaging techniques and laboratory evaluation related to venous thromboembolism. The American Medical Association has added phlebology to their list of self-designated practice specialties.
The American College of Phlebology (ACP) is a professional organization of physicians and health care professionals from a variety of backgrounds. ACP meetings are conducted to facilitate learning and sharing of knowledge regarding venous disease. The equivalent body for countries in the Pacific is the Australasian College of Phlebology, active in Australia and New Zealand.
Venous diseases
Venous insufficiency
Venous insufficiency is the most common disorder of the venous system, and is usually manifested as spider veins or varicose veins. A variety of treatments are used depending on the patient's particular type and pattern of veins and on the physician's preferences. Treatment can include radiofrequency ablation, vein stripping, ambulatory phlebectomy, foam sclerotherapy, lasers, or compression.
Postphlebitic syndrome is venous insufficiency that develops following deep vein thrombosis.
Deep vein thrombosis
Deep vein thrombosis is a condition where a blood clot forms in a deep vein, which can lead to pulmonary embolism and chronic venous insufficiency.
Thrombophlebitis
Thrombophlebitis is an inflammatory condition of the veins related to blood clots.
See also
External links
- Information from the Australasian College of Phlebology Website
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Scientific publications
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