Arteriole. Coloured scanning electron micrograph (SEM) of a section through a small artery, known as an arteriole. In the centre is the lumen, through which the blood passes. The inner lining, the tunica intima, consists of a thin layer of endothelial tissue and a layer of elastic tissue. Surrounding this is the thicker tunica media, which consists of smooth muscles cells that expand and contract, regulating the flow of blood through the vessel. The outer layer is the tunica adventitia, which contains collagen and merges with the surrounding connective tissue. Magnification: x900 when printed at 10 centimetres wide.
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