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microcirculation The part of the blood circulatory system consisting of the networks of capillaries and associated vessels that supply blood to tissue cells. At its end, each artery gives rise to a microcirculatory bed of increasingly narrow vessels: arterioles, which in turn subdivide to form metarterioles, and then capillaries. The latter are typically 1 mm long and 3–10 μm in diameter, just wide enough for erythrocytes to pass through. The capillaries connect to the venous system via postcapillary venules, which in turn join the larger veins. Blood flow into the capillary bed is controlled by a muscular precapillary sphincter at the end of each arteriole, and blood can bypass the capillaries via a direct arteriovenous connection, or anastomosis.