Arteriole and venule in cross-section
Arterioles are the smallest vessels that precede the capillary system and are usually accompanied by a venule, a small vein. The wall of the arteriole (red) is significantly stronger than the wall of the venule (blue), which is caused by the relatively thick layer of smooth muscle cells. The picture shows the endothelium that lines the lumen of both vessels, and the muscular layer of the arteriole.
