Arteriole. Light micrograph of a longitudinal section through a healthy arteriole. There are red blood cells (red) in its central cavity (upper right to lower left). The innermost layer of the arteriole is the tunica intima (extremely thin). The middle layer, the tunica media (pink/red), is almost entirely made up of smooth muscle cells. The outer tunica adventitia (blue) merges with the surrounding connective tissue. Arterioles link arteries to capillaries, the blood vessels through which oxygen, nutrients and waste products are exchanged with the tissues. Magnification unknown.