Elastic vein wall. Light micrograph of a transverse section through the wall of an elastic vein. Veins carry blood from the body to the heart. The white area is the lumen, which holds the blood. The lumen is lined by a layer of smooth epithelium. The first layer of the vein's wall is the tunica intima. This consists of a sub-endothelial layer of fine elastic fibres (dark convoluted) and collagen. The next layer, the tunica media (pink), consists of smooth muscle fibres with some elastic tissue. Contraction of these muscle fibres, along with the action of valves, prevent the backflow of blood due to gravity. The external tunica adventitia is mainly composed of connective tissue.