Blood Vessels Blood Vessels • The vascular network through which blood flows to all parts of the body comprises of arteries, arterioles, capillaries, veins and venules. • Arteries and arterioles • Arteries are high pressure vessels which carry blood from the heart to the tissues. • The largest artery in the body is the aorta which is the main artery leaving the heart. • The aorta constantly subdivides and gets smaller. • The constant subdivision decreases the diameter of the vessel arteries, which now become arterioles. Structure of Arteries • Arteries are composed of three layers of tissue: • 1 an outer fibrous layer — the tunica adventitia or tunica externa • 2 a thick middle layer — the tunica media • 3 a thin lining of cells to the inside — the endothelium or tunica intima. • The tunica media is comprised of smooth muscle and elastic tissue, which enables the arteries and arterioles to alter their diameter. Structure of Arteries • Arteries tend to have more elastic tissue, while arterioles have greater amounts of smooth muscle; this allows the vessels to increase the diameter through vasodilation or decrease the diameter through vasoconstriction. • It is through vasoconstriction and vasodilation that the vessels can regulate blood pressure and ensure the tissues are receiving sufficient blood — particularly during exercise. Arteries and arterioles have three basic functions: • to act as conduits carrying and controlling blood flow to the tissues • to cushion and smooth out the pulsate flow of blood from the heart • to help control blood pressure. Veins and venules • Veins are low pressure vessels which return blood to the heart. The structure is similar to arteries, although they possess less smooth muscle and elastic tissue. • Venules are the smallest veins and transport blood away from the capillary bed intothe veins. • Veins gradually increase in thickness the nearer to the heart they get, until they reach the largest vein in the body, the venae cavae, which enters the right atrium of the heart. Veins and Venules • The thinner walls of the veins often distend and allow blood to pooi in them. This is also allowed to happen as the veins contain pocket valves which close intermittently to prevent back flow of blood. • This explains why up to 70% of total blood volume is found in the venous system at any one time. Capillaries • Capillaries are the functional units or the vascular system. • Composed of a single layer of endothelial cells, they are just thin enough to allow red blood cells to squeeze through their wall. • The capillary network is very well developed as they are so small; large quantities are able to cover the muscle, which ensures efficient exchange of gases. Capillaries • If the cross-sectional area of all the capillaries in a muscle cell were to be added together, the total area would be much greater than that of the aorta. • Distribution of blood through the capillary network is regulated by special structures known as pre-capillary sphincters, the structure of which will be dealt with later in this chapter.