Light micrograph of a section through the wall of the aorta, the body's main artery, which carries blood from the heart around the body. At top is the lumen, which holds the blood. The lumen is lined by a layer of smooth epithelium. The first layer of the aorta wall is the tunica intima (light pink). This consists of a sub-endothelial layer of fine elastic and collagen fibres and some smooth muscle cells. The next layer, the tunica media (bottom), is the largest layer of the aorta wall. It consists of smooth muscle fibres with some elastic tissue. The elasticity of the aorta wall allows it to cope with the pulses of blood it receives from the heart. Magnification: x200 when printed at 15cm wide.