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Blood vessels. Artwork of an artery (top), capillary (middle) and vein (bottom). From inside to out, arteries and veins are made of: a thin layer (intima) of endothelial cells, a thick layer (media) of smooth muscle, elastic fibres and connective tissue that support the vessel's structure, and an outer protective layer (adventitia) of connective tissue and nerves. Arteries mainly carry oxygenated blood from the heart at high pressures, so they have thicker, stronger and more elastic walls. Capillaries are the thinnest type of vessel. They are composed of one layer of epithelial cells, allowing nutrients and waste products to be exchanged through them. Veins have much thinner and less elastic walls than arteries as they do not need to withstand high pressures. They rely on gravity and skeletal muscle contractions to transport blood back to the heart. Some contain one-way valves to prevent blood flowing in the wrong direction.