Structure of Blood Vessels

All blood vessels are hollow tubes, but the three types - arteries, veins and capillaries - are built very differently because they each face a different job. Except for the thinnest capillaries, a vessel wall is built of three concentric layers. The innermost layer, the tunica intima (or tunica interna), is essentially the vascular endothelium lining the lumen. The middle layer, the tunica media, is made of smooth muscle embedded in an extracellular matrix containing the elastic protein elastin; contraction of this smooth muscle narrows the vessel (vasoconstriction) while its relaxation widens the vessel (vasodilation), which is how the body actively controls blood flow to different regions. The outermost layer, the tunica externa (or tunica adventitia), is made of tough collagen fibres and anchors the vessel in place (see Figure 7.6). Arteries carry blood away from the heart, usually lie deep within the body, and have thick, non-collapsible, valve-less walls with a narrow lumen built to withstand high pressure; every artery carries oxygenated blood except the pulmonary artery. The largest artery, the aorta (about 2.5 cm across and 2 mm thick), branches into progressively smaller arteries and then, in the tissues, into thin, narrow arterioles; a small ring of muscle called a sphincter sits at the arteriole-capillary junction to regulate how much blood enters a given capillary bed. Pressure falls sharply along this route, from about 85 mm Hg as blood enters an arteriole to about 35 mm Hg as it enters a capillary. Arteries are not always dead ends into arterioles - they can also form direct cross-connections called anastomoses, which give blood an alternate route if the original vessel becomes blocked (for example, around a joint, so bending the joint cannot cut off blood flow). Capillaries are the thinnest vessels of all, made of just a single layer of squamous epithelium with no tunica media or elastin, and it is here - across walls guarded by semilunar valves - that the actual exchange of materials between blood and tissue happens; the blood inside is a mix of oxygenated and deoxygenated blood, flowing slowly despite the capillaries collectively holding a large blood volume. Veins have thinner, wider, collapsible wal …