STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY
Arteriole. Coloured scanning electron micrograph (SEM) of a cross-section through a small human artery known as an arteriole. This blood vessel reveals red blood cells travelling through its central lumen. Its inner wall (tunica intima) is thin and appears folded, comprising an endothelial lining and a thin elastic layer which stretches. A thicker tunica media layer (pink) makes up much of the wall of the arteriole, and is almost entirely composed of smooth muscle cells. An outer tunica adventitia layer merges with surrounding connec- tive tissue (blue). These layers enable the arteriole to expand and contract, regulating the arterial blood pressure. Magnification: unknown.
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