Artery. Light micrograph of a cross section through a small artery (arteriole) supplying the intestines. This blood vessel has red blood cells in its central lumen (at upper centre). Its inner wall (tunica intima) is thin and appears folded, comprising an endothelial lining and a thin elastic layer which stretches. A thicker tunica media layer (pink) makes up much of the wall of the arteriole, and is almost entirely composed of smooth muscle cells. These layers enable the arteriole to expand and contract, regulating the arterial blood pressure. Surrounding the artery are fat cells of adipose tissue (spongy appearance). Magnification: x50 at 35mm size.