Vein

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For other uses, see Vein (disambiguation).

Cross section of a vein showing a valve which prevents backflow

In the circulatory system, a vein is a blood vessel that carries blood toward the heart.

1 Anatomy
2 Function
3 Medical interest
4 Common diseases
- 4.1 Deep vein thrombosis
- 4.2 Phlebology
5 Notable veins and vein systems
6 Color
7 Types of veins
- 7.1 List of important named veins
- 7.2 Names of important venule systems
8 See also
9 External Links
- 9.1 Scientific publications

[edit] Anatomy

Veins are essentially hollow tubes that collapse when not filled with blood. The hollow internal cavity in which the blood flows is called the lumen. Veins are surrounded by helical bands of smooth muscles which help maintain blood flow to the right atrium. Most veins have one-way flaps called venous valves to prevent gravity from causing blood to backflow and pool in the lower extremities. They also have a thick collagen outer layer, which helps maintain blood pressure and stop blood pooling. The total capacity of the veins is more than sufficient to hold the entire blood volume of the body; this capacity is reduced through the constriction of smooth muscles, minimizing the cross-sectional area (and hence volume) of the individual veins and therefore total venous system.

The precise location of veins is much more variable from person to person than that of arteries.

[edit] 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 to the pulmonary arteries and eventually [[lung]s]. 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 blood, absorbing a large portion of the total blood volume, keeping blood away from the brain and causing unconsciousness.

[edit] Medical interest

Venous valves prevent reverse blood flow.

Veins are used medically as points of access to the blood stream, permitting the withdrawal of blood specimens (venipuncture) for testing purposes, and enabling the infusion of fluid, electrolytes, nutrition, and medications. The latter is called intravenous delivery. It can be done by an injection with a syringe, or by inserting a catheter (a flexible tube). 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. In example, 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 (a 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.

[edit] Common diseases

The most common vein disorder is venous insufficiency, usually manifested by 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 radio-frequency ablation, vein stripping, ambulatory phlebectomy, foam sclerotherapy, sclerotherapy, lasers or compression.

[edit] Deep vein thrombosis

Main article: 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.

[edit] 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.

[edit] Notable veins and vein systems

The pulmonary veins carry relatively oxygenated blood from the lungs to the heart. The superior and inferior vena 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.

[edit] Color

The blood carried by veins is dark red due to its high percentage of CO₂ as it returns to the heart (in contrast to the high levels of O₂ in arterial blood, which is bright red). Veins appear blue because the subcutaneous fat in the skin absorbs lower-frequency light, permitting only the highly energetic blue wavelengths to penetrate and reflect off human skin.