<P> The blood vessels are the part of the circulatory system, and microcirculation, that transports blood throughout the human body . There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and the tissues; and the veins, which carry blood from the capillaries back toward the heart . The word vascular, meaning relating to the blood vessels, is derived from the Latin vas, meaning vessel . A few structures (such as cartilage and the lens of the eye) do not contain blood vessels and are labeled . </P> <P> The arteries and veins have three layers, but the middle layer is thicker in the arteries than it is in the veins: </P> <Ul> <Li> Tunica intima (the thinnest layer): a single layer of simple squamous endothelial cells glued by a polysaccharide intercellular matrix, surrounded by a thin layer of subendothelial connective tissue interlaced with a number of circularly arranged elastic bands called the internal elastic lamina . </Li> <Li> Tunica media (the thickest layer in arteries): circularly arranged elastic fiber, connective tissue, polysaccharide substances, the second and third layer are separated by another thick elastic band called external elastic lamina . The tunica media may (especially in arteries) be rich in vascular smooth muscle, which controls the caliber of the vessel . Veins don't have the external elastic lamina, but only an internal one . </Li> <Li> Tunica adventitia: (the thickest layer in veins) entirely made of connective tissue . It also contains nerves that supply the vessel as well as nutrient capillaries (vasa vasorum) in the larger blood vessels . </Li> </Ul> <Li> Tunica intima (the thinnest layer): a single layer of simple squamous endothelial cells glued by a polysaccharide intercellular matrix, surrounded by a thin layer of subendothelial connective tissue interlaced with a number of circularly arranged elastic bands called the internal elastic lamina . </Li>

What is the muscular layer of a blood vessel