Blood vessel types. Artworks of the three types of blood vessels, from left: arteries, veins, and capillaries. Arteries are thick blood vessels carrying oxygenated blood around the body away from the heart-lung system. Veins are thinner blood vessels with valves (flaps) preventing backflow (arrows) of deoxygenated blood as it is returned from the body to the heart-lung system. Capillaries, the smallest blood vessels, are the width of a single red blood cell, and are where the oxygen in these cells is exchanged with the body's tissues. Capillaries also have valves. Red blood cells are shown in the artery and the capillary.