Though veins and arteries are similar, they have different functions and different structures to better accommodate those functions. Veins and arteries work together to transport blood throughout the body. Arteries carry oxygenated blood from the heart to the rest of the body. Once the oxygen is deposited, veins carry the deoxygenated blood back to the heart, where it will be sent to the lungs to become re-oxygenated.
Because the blood that passes through arteries is under a considerable amount of pressure, these structures have thick walls that are able to withstand the pressure without bursting. Veins, on the other hand, have wider inner diameters that allow more blood to pass through (though at a slower rate), as well as valves that keep the blood from flowing backward.