Arteries and Veins

Both arteries and veins are tubes made of four layers--a protective fibrous covering, a middle layer of smooth muscle and elastic fibers and an inner layer of connective tissue lined with a smooth layer of cells. Arteries have a thicker muscle layer to withstand the greater blood pressure. Veins are wider and adapt to the changing volume of blood. Blood flows slowly through veins because it has little pressure behind it. Blood flow in the veins relies on the rhythmic contraction of surrounding muscles and the pumping action of nearby arteries to return it to the heart. One way semilunar valves, which occur at intervals inside the veins, prevent any possible backflow of blood.