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Arteriole. Coloured scanning electron micrograph (SEM) of a cross-section through a small human artery known as an arteriole. Red blood cells are seen in the central space (lumen) and connective tissue surrounds the arteriole. The inner wall (tunica intima) of the lumen is thin and folded. It comprises an endothelial lining and a thin elastic layer that stretches. These layers enable the arteriole to expand and contract, regulating arterial blood pressure. Magnification: x480 when printed 10cm wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/STEVE GSCHMEISSNER)