Q.Sketch and label T.S. of vein.

Blood travels through three structurally distinct types of vessel, each built for a different job. Except for capillaries, every vessel wall has three concentric layers: an inner tunica intima (the vascular endothelium), a middle tunica media (smooth muscle plus the elastic protein elastin, whose contraction and relaxation cause vasoconstriction and vasodilation), and an outer tunica externa (tough collagen fibres). Arteries carry blood away from the heart under high pressure, so they have thick, non-collapsible, valve-less walls and a narrow lumen; every artery carries oxygenated blood except the pulmonary artery. Pressure falls steeply as arteries branch into arterioles and then into capillaries - from about 85 mm Hg entering an arteriole to about 35 mm Hg entering a capillary - and a small sphincter at each arteriole-capillary junction regulates local blood supply. Arteries can also cross-connect directly through anastomoses, giving blood an alternate route if one vessel is blocked. Capillaries are made of a single layer of squamous epithelium with no muscular or elastic layer, which is exactly what allows the actual exchange of gases, nutrients and wastes between blood and tissue fluid across their walls; because their combined cross-sectional area is far larger than that of the arterioles feeding them, blood velocity is at its slowest here, despite the total blood volume in capillaries being relatively high. Veins carry blood back toward the heart at low pressure through thinner, wider, more collapsible walls with a t …