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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 arteriesto the muscles and organs of the body, where it nutrients and gasses 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. Portal Veins The Portal venous system is a special case where the hepatic portal vein transports blood rich in products of digestion from the intestines to the liver. The pituitary gland also has a portal system, in which veins empty from the hypothalamus to the anterior pituitary gland. These portal systems are unique because the veins link two organs, whereas most veins empty directly from an organ or muscle into the venous system, then directly to the heart. 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. 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. Medical interest
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. See also | ||||||||||||
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