Capillaries and venules form an interface with a surface area > 6000 m2 for the exchange of water and solutes between the circulation and tissues. Arterioles feed capillary networks via smaller metarterioles and the capillaries drain into venules. Metarterioles possess smooth muscle contractile elements in their walls and give rise to capillaries through smooth muscle pre-capillary sphincters (Figure CR.20). While arterioles are supplied by the autonomic system, the innervation
of metarterioles and pre-capillary sphincters is uncertain, and these vessels may only be responsive to local or humoral agents. Capillary blood flow varies according to the activity of the tissue. At rest the majority of capillary beds is collapsed and arteriovenous anastomoses allowing direct communication between arterioles and venules, shunt blood away from the tissues. These anastomotic channels are widespread in skin and play a prominent role in thermoregulation.
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