Structural differencesTo begin with, arteries have thicker and muscular. The thickness of the walls helps the arteries to withstand high pressure. The masculine walls help in vasoconstriction and vasodilation as well as in effective regulation of blood pressure. Veins have thin and non-muscular walls. They are structurally thin blood vessels to allow the surrounding muscles to press against them and aid in pushing blood towards the heart. They lack masculine walls as blood in them flows in low pressure. On the other hand, capillaries have a one cell layer thick wall which enables easy diffusion of substances in and out of the capillaries. They are the smallest blood vessels.Additionally, veins have valves which prevent backflow of blood as blood in them flows under low pressure., arteries do not have valves as blood in them flow under a high pressure hence there no chances of back flow of blood. Capillaries are also valve-less.Arteries have thick outer layer made up of collagen and elastic fibres which aid in preventing bulges and leakages, while capillaries have pores within their wall which allow for the leakage of plasma fluid which is essential in transporting body substances. On the other hand veins have a thin layers made up of few elastic fibres for easy transportation of blood.Functional differencesArteries move oxygenated blood away from the heart to other body organs while Veins carry deoxygenated blood back to the heart. On the other hand, capillaries transport blood away from the body and also allow the exchange of carbon dioxide, oxygen, nutrients and waste products in the body. They also serve as a link between veins and arteries.SimilaritiesArteries, veins and capillaries all carry blood. All blood vessels generally transport blood. Like veins, arteries comprise of three layers; and inner layer made up of epithelial cells, an outer layer of tissue and a layer of muscles between them. In distributing blood and nutrients, all the three vessels supplement each other. Capillaries connect arteries to veins and hence allowing the exchange of products between them.