Arteriole. Light micrograph of a section through an arteriole, a small artery. The lumen (centre) of the arteriole is filled with red blood cells. The lumen is lined by a single layer of epithelial cells, which are surrounded by a ring of smooth muscle (pink). Outside of this is elastic, collagen connective tissue and capillaries. Magnification: x250 when printed at 10 centimetres wide. Human tissue.