Arteriole. Cut-away illustration of an arteriole carrying red blood cells (red discs), white blood cells (blue/purple) and lipids (blue spheres). Capillaries, which supply blood to the tissues, branch off at upper right. The arteriole has four layers: the outermost connective tissue (tunica adventitia, orange), smooth muscle (tunica media, pink), elastic membrane (elastic lamina, white) and endothelium (tunica intima, mauve). At top right are vasa vasorum, small blood vessels which supply nutrients to the arteriole. Also on the outside of the arteriole are nerve cells (yellow). Two macrophages (white blood cells, at far right) protect the body by digesting foreign particles.