Arteriole. Light micrograph of an arteriole in cross-section. 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 (purple folded line) is very thin, comprising only the vessel lining, some connective tissue and a thin elastic layer. The middle tunica media (pink and purple) is composed of concentric layers of smooth muscle cells (not clearly seen). The outermost tunica adventitia merges with the surrounding tissues (blue). Trichrome stained. Magnification: x160 at 5x7cm size. x80 at 35mm size