Transmission electron micrograph of the cross section through an arteriole. Arterioles regulate the flow of blood through the capillaries by constriction and dilation of their walls, which are composed of three layers. The innermost tunica intima is very thin, comprising only the vessel lining, some connective tissue and a thin elastic layer. The middle tunica media is composed of concentric layers of smooth muscle cells. The outermost tunica adventitia merges with the surrounding tissues.