This characteristic also allows for the accommodation of pressure changes in the system. The thin walls of the veins, and their greater internal diameters ( lumens) enable them to hold a greater volume of blood, and this greater capacitance gives them the term of capacitance vessels. All of the systemic veins are tributaries of the largest veins, the superior and inferior vena cava, which empty the oxygen-depleted blood into the right atrium of the heart. Almost 70% of the blood in the body is in the veins, and almost 75% of this blood is in the small veins and venules. In the systemic circulation the return is of deoxygenated blood from the organs and tissues of the body, and in the pulmonary circulation the pulmonary veins return oxygenated blood from the lungs to the heart. The venous system is the system of veins in the systemic and pulmonary circulations that return blood to the heart. The factors that contribute to this alteration of color perception are related to the light-scattering properties of the skin and the processing of visual input by the visual cortex, rather than the actual colour of the venous blood which is dark red. Veins close to the surface of the skin appear blue for a variety of reasons. Unlike arteries, the precise location of veins varies among individuals. There are valves present in many veins that maintain unidirectional flow. The tunica media in the veins is much thinner than that in the arteries as the veins are not subject to the high systolic pressures that the arteries are. The inner layer, is a thin lining of endothelium known as the tunica intima. The middle layer, consists of bands of smooth muscle and is known as the tunica media. The outer layer, is a thick layer of connective tissue called the tunica externa or adventitia this layer is absent in the post-capillary venules. The layers known as tunicas have a concentric arrangement that forms the wall of the vessel. Veins have a similar three-layered structure to arteries. The smallest veins are the post-capillary venules. There are three sizes of vein, large, medium, and small. Structure Layers of vein wall shown in comparison to arterial wall In the lower limbs this is also aided by muscle pumps, also known as venous pumps that exert pressure on intramuscular veins when they contract and drive blood back to the heart. In medium and large sized veins the flow of blood is maintained by one-way (unidirectional) venous valves to prevent backflow. At any time, nearly 70% of the total volume of blood in the human body is in the veins. This greater capacity gives them the term of capacitance vessels. Because of their thinner walls and wider lumens they are able to expand and hold more blood. Veins have less smooth muscle, and connective tissue, and wider internal diameters than arteries. Veins are often closer to the skin than arteries. Smaller veins are called venules, and the smallest the post-capillary venules are microscopic that make up the veins of the microcirculation. There are three sizes of veins, large, medium, and small. In the systemic circulation arteries carry oxygenated blood away from the heart, and veins return deoxygenated blood to the heart, in the deep veins. Most veins carry deoxygenated blood from the tissues back to the heart exceptions are those of the pulmonary and fetal circulations which carry oxygenated blood to the heart. Veins are blood vessels in the circulatory system of humans and most other animals that carry blood toward the heart.
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