Vein
From Wikipedia, the free encyclopedia
- For other uses, see Vein (disambiguation).
In the circulatory system, a vein is a blood vessel that carries blood toward the heart. The vessels that carry blood away from the heart are known as arteries. The study of veins and diseases of the veins is known as phlebology and is a poorly defined discipline that is being developed through a variety of interdisciplinary scientists rather than having a specific set of specialists.
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[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 it 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 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.hi
[edit] Anatomy
Most veins have one-way valves called venous valves to prevent backflow caused by gravity. They also have a thick collagen outer layer, which helps maintain blood pressure and stop blood pooling. 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. 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.
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 ascend to the heart. Neurogenic and Hypovolaemic shock can also cause fainting. In these cases the smooth muscles surrounding the veins become slack and fill with blood, absorbing a large portion of the total blood volume, keeping blood away from the brain and causing unconsciousness.
[edit] Common diseases
Common diseases of the veins include varicose veins (with are treated with lasers and ultrasound) and deep vein thrombosis.
[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 numbered vein is one of considerable importance that it is numerically classified by today's medical society, the most prominent of these being the 15 vein.[citation needed]
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] 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).
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.
The precise location of veins is much more variable from person to person than that of arteries.
[edit] Color
In the light, blood appears red because most colors are absorbed except for red, which bounces back from the blood. Every colour but red is absorbed by the oxygen-carrying pigment hemoglobin (Hb). If a filter that blocks the reflected color is positioned between the blood and the eyes of the person watching, the perceived color changes. In the case of humans, the skin serves as a filter for the color red, and the remaining color ends up being green. The exact color spectra is determined by the relative levels of oxygenated iron (HbO) and CO2 in the blood. High oxygen reflects red and high CO2 reflects blue, which mixed with the yellowish color of the fat and/or skin ends up showing as green.
[edit] See also
[edit] External Links
- Merck Manual article on veins.
- Phlebology - international journal of venous diseases
- American College of Phlebology
| Cardiovascular system |
|---|
| Blood | Heart → Aorta → Arteries → Arterioles → Capillaries → Venules → Veins → Vena cava → Heart → Pulmonary arteries → Lungs → Pulmonary veins → Heart |
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