Vein

In the circulatory system, veins (from the Latin vena) are blood vessels that carry blood towards 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 to the heart. Veins differ from arteries in structure and function; for example, arteries are more muscular than veins, veins are often closer to the skin and contain valves to help keep blood flowing toward the heart, while arteries carry blood away from the heart.

Anatomy
In general, veins 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, in general, 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. Veins often display a lot of anatomical variation compared with arteries within a species and between species.

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, this vein derives its name from the word safina, meaning "hidden". This vein is "hidden" in its own fascial compartment in the thigh and exits the fascia only 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

The Thebesian veins within the myocardium of the heart are valveless veins that drain directly into the chambers of the heart. The coronary veins all empty into the coronary sinus which empties into the right atrium.

The Dural sinuses within the dura mater surrounding the brain receive blood from the brain and also are a point of entry of cerebrospinal fluid from arachnoid villi absorption. Blood eventually enters the internal jugular vein.

Color
Vein color is determined in large part by the color of venous blood, which is usually dark red (and not blue as is commonly believed) 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. A recent study found the color of blood vessels is determined by the following factors: the scattering and absorption characteristics of skin at different wavelengths, the oxygenation state of blood, which affects its absorption properties, the diameter and the depth of the vessels, and the visual perception process.

Function
Veins serve to return blood from organs to the heart. Veins are also called "capacitance vessels" because most of the blood volume (60%) is contained within veins. 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, the blood then enter veinules, then 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, 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 sinues initiate a baroreflex such 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, 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.

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.

Although most veins take blood back to the heart, there is an exception. Portal veins carry blood between capillary beds. For example, the hepatic portal vein takes blood from the capillary beds in the digestive tract and transports it to the capillary beds in the liver. The blood is then drained in the gastrointestinal tract and spleen, where it is taken up by the hepatic veins, and blood is taken back into the heart. Since this is an important function in mammals, damage to the hepatic portal vein can be dangerous. Blood clotting in the hepatic portal vein can cause portal hypertension, which results in a decrease of blood fluid to the liver.

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 have no corresponding arteries.


 * 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.

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. A medical specialist in Phlebology is termed a Phlebologist. A related image is called a phlebography

The American College of Phlebology (ACP) is a professional organization of physicians and healthcare 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.

The American Venous Forum (AVF) is the leading academic international consortium of venous and lymphatic specialists dedicated to improving patient care. The highest-quality research in clinical and basic sciences is reported at the AVF annual meetings, along with the reports on new developments in diagnosis and treatment of venous diseases. The main body of the AVF consists of vascular surgeons and other physicians who specialized in management of not only simple varicose veins but the whole spectrum of venous and lymphatic diseases from congenital abnormalities to deep vein thrombosis to chronic 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. Several varieties 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 in which 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.

Scientific publications

 * Phlebology: The Journal of Venous Disease
 * Phlebology: 1986-1999

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