Renal artery. Light micrograph of a section through a normal human renal artery, which serves the kidneys. This is a muscular artery with a thick wall that enables it to carry blood under pressure. This wall is composed of three layers. The innermost tunica intima surrounds the lumen (tube, white). It possesses a fine elastic sheet (convoluted) known as the internal elastic lamina. The middle layer, the thick tunica media, is mainly composed of smooth muscle fibres (blue). The external tunica adventitia (black) is composed mainly of connective tissue. Magnification unknown.