Veins carry blood toward the heart

Veins carry blood toward the heart - muscle fibers are...

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Veins  carry blood toward the heart. The three kinds of veins are listed  here in the sequence they occur regarding the flow of blood back to the heart:  Postcapillary venules, the smallest veins, form when capillaries  merge as they exit a capillary bed. Much like capillaries, they are very  porous, but with scattered smooth muscle fibers in the tunica media. Venules form when postcapillary venules join. Although the walls of  larger venules contain all three layers, they are still porous enough to allow  white blood cells to pass. Veins have walls with all three layers, but the tunica intima and tunica  media are much thinner than in similarly sized arteries. Few elastic or 
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Unformatted text preview: muscle fibers are present. The wall consists primarily of a well-developed tunica adventitia. Many veins, especially those in the limbs, have valves, formed from folds of the tunica intima, that prevent the backflow of blood. If these valves fail to close properly, varicose veins may occur. Many regions of the body receive blood supplies from two or more arteries. The points where these arteries merge are called arterial anastomoses. Arterial anastomoses allow tissues to receive blood even after one of the arteries supplying blood has been blocked....
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