Please compare and contrast the structure of human blood vessels.

There are three blood vessel types in humans: arteries, veins and capillaries.Arteries pump blood away from the heart (easy point to remember - "A" in arteries = AWAY from the heart) while veins pump blood towards the heart. Capillaries are involved in material exchange between tissues. Blood pressure is high in arteries and low in both veins and capillaries. To support and maintain these pressures, arterial lumen diameters are narrow, venous lumen diameters are wide while the lumen diameter of capillaries is very narrow, wherein they are only one cell wide. To further support the pressures exerted in each blood vessel type, wall thicknesses vary between structures. Walls are thick in arteries, thin in veins and very thin in capillaries, wherein they are only one cell thick. There are three wall layers in arteries and veins, called the Tunica adventitia, Tunica media and Tunica intima, while there is only one wall layer, the Tunica intima, in capillaries. There are large amounts of muscle and elastic fibres in arteries to support and maintain pressure, small amounts in veins and no muscle or elastic fibres in capillaries. Only veins, of the three blood vessel types, have valves.