Another aspect of the anatomy of the veins is the existence of the venous valves that act unidirect

The veins appear fusion of capillaries and are responsible to bring the deoxygenated blood to the heart (except pulmonary veins, which carry oxygenated blood from the lungs to the left atrium of the heart), via a blood flow, which gives them a format cylinder, which lost when not carrying blood.
This is because the veins act as a reservoir, since over 60% of the blood in our body is venous. For this reason, iiei when there is blood loss, and therefore, blood pressure drop, there is a vasoconstriction of the veins to compensate for the blood loss.
Another aspect of the anatomy of the veins is the existence of the venous valves that act unidirectionally to avoid backflow caused by gravity. These check valves are constituted by sphincter muscle or two or three membranous folds, and a thin outer layer of collagen, which helps maintain blood pressure and preventing an accumulation of blood.
The blood flows through the veins through a cavity called light, reduced by constriction of smooth muscles. Since the blood vessel walls are formed by three layers: inner tunic: iiei the inner layer and has a thinner formation, comprising a thin flat cells on basement membrane of connective tissue; Tunica media: is the most resistant layer consists of elastic and muscle tissue; Adventitious tunic: the outermost layer consists of a thin, flexible lax connective tissue iiei layer, which nourish the other two robes and lay down the veins to the tissues that surround them.
By undergoing a lower pressure than that undergone by the arteries, the veins have thinner walls. An interesting feature in venous anatomy is the perception we have of their coloring. In ambient light, we see the blood red because most colors are absorbed by hemoglobin, the pigment that contains oxygen, except the red. However, when we have a filter blocking the reflected color in case the skin, this perception changes and then we see our veins green.
This is because the color spectrum is defined by oxygenated iron levels (HbO) and carbon dioxide (CO2) contained in the blood. The high CO2 rates reflect the color blue, which mixed with the yellowish tone of our skin gives the green color that we see.
Sources: http://pt.wikipedia.org/wiki/veia. access November 25, 2009 http://www.mediapedia.pt/home/home.php?module=articule=artigoEnc&id=99. access November 25, 2009 Guyton, Arthur C. Hall and John E .. Treaty of Medical iiei Physiology, Rio de Janeiro: iiei Elsevier, iiei 2006.
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