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A vented drainage system including a drainage tube having a lumen for passage of fluid and an opening in the wall of the tube, a vent for the tube communicating between the atmosphere and the tube opening, and an air permeable filter element positioned in the vent to filter air passing through the vent to the tube lumen. The drainage system has a section of flexible tubing positioned in the drainage tube with one end thereof on the upstream side of the opening in the wall of the drainage tube and the other end on the downstream side of the opening. The tubing has an outside diameter approximately equal to the inside diameter of the drainage tube to provide interfacial contact between the outside surface of the tubing and the inside surface of the tube to prevent passage of fluid between the interfacial surfaces thereof. The tubing is flexible to become disengaged from contact with the inner surface of the tube to provide a passageway for air to flow from the vent into the lumen when the pressure in the tube is less than atmospheric.