Arteries and veins serve a common purpose; to carry blood around the body from organ to organ. However their structure is modified from one another to aid in the specifics of how and where they pump that blood. Arteries carry blood away from the heart, supplying oxygenated blood to organs that require it. Veins on the other hand carry blood from the tissues of the body back to the heart. These different transport functions require slightly different structures in order to assist in their roles. Both arteries and veins consist of three layers:The innermost layer, the tunica intima, is made up of a thin layer of endothelial cells.The middle layer, the tunica media, consists of smooth muscle cells and elastic tissue.The outermost layer, the tunica adventitia, consists of connective tissue made up of collagen fibres.The internal area in which the blood itself flows is called the Lumen. Arteries carry blood away from the heart, and this blood is ejected from the heart at very high pressures. This means that the walls of arteries are very thick, in order to be able to withstand this pressure as the heart beats. The tunica media of an artery is very thick.In arteries close to the heart, the abundant elastic fibres in this part of the artery are vital, as they allow the vessel to expand as blood is pumped into them from the ventricle, and then recoil as each beat of the heart causes the surge of blood to pass. This constant expansion and retraction of the arteries pushes the blood along, ensuring a constant flow in one direction and maintaining the pressure gradient. As we view arteries further away from the heart, we see an increase in the amount of smooth muscle tissue in the tunica media and a reduction in the amount of elastic fibre. This is because blood pressure caused by blood being ejected from the heart is reduced, and so elasticity in these arteries is of less importance. Instead, in these vessels we see Vasoconstriction. The smooth muscles in the tunica media constrict, reducing the size of the lumen and so increasing pressure. The diameter of the lumen in arteries is relatively small, in order to maintain the high pressure needed to keep blood flowing through the body. When compared to arteries, veins have a much more noticeably wide lumen. This is to increase the ease with which the low-pressure blood moves through them, and to increase the quantity that can be carried. The walls of veins are much thinner than those of arteries, containing very few elastic fibres and smooth muscle cells. This is because they receive blood under a significantly lower pressure than arteries, and so have no need to be able to expand. This lack of pressure means that veins have a different way of keeping blood flowing through them. Their thin walls allow them to be positioned close to muscles, and the lack of muscle fibres mean they can press flat against them. Veins located along or within muscles compress as the muscles around them contract, and decompress as the muscles relax. This acts in a similar way to a pump, driving blood through the veins back towards the heart. This action works against gravity, and so veins contain valves. These valves are folds in the endothelium, which allow blood to flow one directionally through them, but prevent backflow. As pressure increases, the valve opens and blood flows through, but as it decreases the blood is prevented from flowing into the previous section of the vein. This constant contracting and relaxing of the veins is vital for venous return; the act of returning deoxygenated blood to the heart.