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Cell Biology of the Vascular System The human vascular system consists of many different cells in differing vessels, each specialised for appropriate functions. The vessels include arteries, arterioles, capillaries, venules and veins; blood flows through them in that order, and each has a different function. The basic structure of a blood vessel has three tissue layers, tunica intima, tunica media and tunica adventitia. The tunica intima is the innermost layer and comprises a thin endothelial layer on a basal lamina, as well as loose connective tissue. The tunica media is primarily smooth muscle in connective tissue, and also has a blood supply by means of vasa vasorum, which penetrate through the tunica adventitia. The adventitial layer is made of connective tissue and often is continuous with the connective tissue surrounding the cells supplied by the blood vessel. However, the thicknesses, compositions and properties of each tunica vary by vessel. Some of the cells in the vascular system are also specialised, for example pericytes and endothelial cells. The endothelium has many properties which allow it to fulfil important requirements of the body's blood supply, such as angiogenesis, regulating leukocyte migration, and control of thrombus formation and blood flow. These layers and cells can of course develop pathologic conditions, for example atherosclerosis, and connective tissue disorders can lead to dysfunctional tunicae. The tunicae in different parts of the vascular system have different properties in accordance with different functions and blood pressures of those vessels. The tunica media in arteries has many fenestrated sheets of elastin surrounded by collagenous tissue and a little smooth muscle. Elastic fibres are needed to withstand the high blood pressure generated by the heart, especially in the ascending aorta where the elastic fibres provide elastic recoil after systole, to aid the pumping of a full stroke volume. Indeed, elastic arteries are so thick that they require specialised blood vessels, vasa vasorum, to supply their tunica media as diffusion from the lumen would not be effective. Muscular arteries contain only two layers of elastic fibres, the internal elastic lamina between tunica intima and tunica media, and the external elastic lamina between tunica media and tunica adventitia; instead they have a thick layer of muscle in their tunica media. This is because the blood has flowed further from from the heart and therefore the arteries do not have to deal with such high pressures or large variations in pressure, and the muscles are able to exert a vasoconstrictor effect on the vessel, controlling blood flow to certain organs and parts of the body. Arterioles have even less elastic tissue and some muscle; vasoconstriction here controls blood flow to specific capillary beds. Both muscular arteries and arterioles are able to have an effect on overall blood pressure as their muscle tone increases peripheral flow resistance. Some veins also have thick muscular tunica media layers, as below the heart they must deal with high blood pressure caused by gravity, but in general they have thinner walls than arteries as the blood is under less pressure and less pulsatile; the tunica adventitia is often thickest here, for structural support. In veins below the heart, the tunica intima displays specialisation in that it folds to create valves, preventing backflow of blood. In contrast, capillaries have no muscular layers, and their tunica intima consists of a single endothelium layer. This is because capillary walls must be thin enough to allow diffusion of oxygen, carbon dioxide and nutrients from the blood to surrounding tissues. Specialised cells called pericytes whose processes are wrapped around capillaries (and sometimes venules) can form a tunica media for these vessels. The presence of contractile machinery, e.g. actin, tropomyosin and myosin, implies that these cells have a contractile function and so possibly a role in vasoconstriction 1. Under certain circumstances pericytes appear to control development and act as stem cells. In capillaries and venules, the tunica adventitia is represented only by a thin layer of collagen. Smooth muscle cells in the tunica media are responsible for vasodilation and vasoconstriction and so are vital to the maintenance of blood pressure and homeostasis. The
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