Vena cava. Light micrograph of a transverse section through the vena cava, the body's main vein, which carries blood from the body to the heart. At centre is the lumen, which holds the blood. The lumen is lined by a layer of smooth epithelium (pink). The first layer of the vein's wall is the tunica intima (beige). This consists of a sub- endothelial layer of fine elastic and collagen fibres. 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.
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