The structure of venules and veins is essentially similar structure to arterioles and arteries. In contrast to arteries, the tunica media is considerably thinner and the tunica adventitia may form the thickest wall component. Post capillary venules have a particularly important function in inflammatory responses, contributing to fluid leakage and leukocyte diapedesis. This function is not readily apparent in routine histologic sections. All veins and venules contain semilunar valves that prevent retrograde flow.
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