JOHN BAVOSI / SCIENCE PHOTO LIBRARY JOHN BAVOSI / SCIENCE PHOTO LIBRARY
Arteriole and capillaries. Illustration of a peripheral arteriole carrying red blood cells (foreground) and capillaries (right), which supply blood to the tissues. The long branched bodies (white) are pericytes. Pericytes are contractile connective tissue cells. Their function is not certain, but they are thought to squeeze the capillaries to increase blood flow. The arteriole wall is encircled by smooth muscle cells which are innervated by sympathetic nerves (white strands). These nerves stimulate the smooth muscle to contract, thereby maintaining blood pressure. Hormones and local factors also affect the diameter, and therefore the blood flow, in these vessels.
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