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vein
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.Contents [show]
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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.[1]
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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.
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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.
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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.
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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.
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Clinical significance
Venous valves prevent reverse blood flow.
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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.
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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.
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Venous diseases
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Venous insufficiency
Main article: 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.[2]
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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.
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Thrombophlebitis
Main article: Thrombophlebitis
Thrombophlebitis is an inflammatory condition of the veins related to blood clots.