Venules are small blood vessels that collect spent blood from capillary beds and transport it to the larger veins for transport back to the heart. Apart from their small size and narrow interior lumens, venules are structurally similar to veins, and several venules often merge together to form a vein.
Venule walls consist of three layers. The inner layer is a membrane built from endothelial cells that, in certain specialized venules, permit the passage of fluid and white blood cells through the vessel wall. The middle layer of a venule wall is an extremely thin sheet of smooth muscle and elastic tissue that helps the vessel maintain its shape. This middle layer is much thinner in venules than it is in other blood vessels. The outer layer of the venule is composed of a tough, fibrous sheath of connective tissue that binds the entire structure together.
Venules do more than simply transport blood from capillary beds to the veins. At sites where an infection has developed, venules release white blood cells to fight the foreign cells. By slowly releasing fluid through their semipermeable membranes, venules also play a role in maintaining the balance of the extracellular interstitial fluid. This fluid is then drained away by the lymphatic system.