Arteriole. Coloured scanning electron micrograph (SEM) of a cross-section through a small human artery known as an arteriole. Red blood cells and some fibrin fibres are seen in the central lumen. The inner wall (tunica intima) of the lumen is thin and folded. It comprises an endothelial lining and a thin elastic layer that stretches. Around this is the thicker tunica media layer (pink), which is almost entirely formed of smooth muscle cells. The outer tunica adventitia layer merges with surrounding connective tissue. These layers enable the arteriole to expand and contract, regulating arterial blood pressure. Magnification: x1430 when printed 10cm wide.

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