Describe the differences in the structure of an artery, vein and capillary?Explain the functional use of their structures??

Both veins and arteries have a three layered structure - tunica intima (endothelium and the inner layer),tunica media (muscle layer) and tunica externa ( mainly contains collagen and connective tissues).Arteries are involved in transporting oxygenated blood from the heart into the systemic circulation (all of the body excluding the lungs) as a result the blood they're carrying has a very high pressure in order to reach different parts of the body.The arteries have consequently adapted by having a very thick,muscular tunica media to combat the high pressure. The veins carry deoxygenated blood from the body to the heart but the blood is at a lower pressure. The veins aren't as rounded as arteries and often have valves.The valves are there to prevent backflow of blood when you're transporting blood against gravity, e.g from the feet to the heart. Blood is squeezed up the veins by skeletal muscle interactions. Unlike the veins and the arteries,the capillaries don't have a three layer structure only has the tunica intima (the endothelium).The endothelium is one cell thick to allow a short diffusion pathway for gas exchange.In the capillary bed, it also has a greater cross sectional area to increase the surface area to volume ratio for diffusion.