Arteriole. Light micrograph of a section through a small human artery known as an arteriole. This blood vessel reveals a few red blood cells travelling through its central lumen. Its inner wall (tunica intima) appears folded, comprising an endothelial lining and a thin elastic layer which stretches. A thicker tunica media layer (blue and pink) makes up much of the wall of the arteriole, and is almost entirely composed of smooth muscle cells with some elastic tissue. An outer tunica adventitia layer (blue) merges with surrounding connective tissue. These layers enable the arteriole to expand and contract, regulating the arterial blood pressure.