Vein

# Overview
In the circulatory system, a vein is a blood vessel that carries blood toward the heart. The majority of veins in the body carry low-oxygen blood from the tissues back to the heart; the exceptions being the pulmonary and umbilical veins which both carry oxygenated blood.

# Anatomy
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, outer-most layer of a vein is comprised of collagen, wrapped in bands of smooth muscle while the interior is lined with endothelial cells.  Most veins have one-way flaps called venous valves that prevent blood from backflowing and pooling in the lower extremities due to the effects of gravity. The precise location of veins is much more variable from person to person than that of arteries.

## Venous tone
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 venous tone of the smooth muscles, minimizing the cross-sectional area (and hence volume) of the individual veins and therefore total venous system.  The helical bands of smooth muscles which wrap around veins help maintain blood flow to the right atrium.  In cases of vasovagal syncope, the smooth muscles relax and the veins of the extremities below the heart fill up with blood, failing to return sufficient volume to maintain cardiac output and blood flow to the brain.

# 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 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 blood, absorbing a large portion of the total blood volume, keeping blood away from the brain and causing unconsciousness.
Often the generalization is made that arteries carry oxygenated blood to the tissues, the tissues consume the oxygen, and the remaining deoxygenated blood is carried back to the heart for reoxygenation. This is an oversimplification: all veins carry oxygenated blood, although the blood carried by the veins is usually considerably less oxygenated than the blood carried by most arteries.

# Medical interest
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.

# 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.

# 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.

# 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.

# Notable veins and vein systems
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
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 through to the dark vein and reflect off. This physical effect can also be seen in the iris of blue eyes (pigmentless iris in the front, dark retina in the back) and is called Rayleigh scattering.

# Types of veins
Veins can be classified into:
- portal vein vs. non-portal (most common)
- superficial veins vs. deep veins
- pulmonary veins vs. systemic veins
List of important named veins
- Jugular veins
- Pulmonary veins
- Portal vein
- Superior vena cava
- Inferior vena cava
- Iliac vein
- Femoral vein
- Popliteal vein
- Great saphenous vein
- Small saphenous vein
Names of important venule systems
- Portal venous system
- Systemic venous system