Abstract: A valve has a flow restrictor positioned outside of a lumen of a tube, which transitions between an open configuration and a closed configuration. The valve further contains a closing mechanism that automatically positions the flow restrictor to the closed configuration when a pressure in a fluid within the lumen is below a low threshold pressure, and an opening mechanism that automatically positions the first flow restrictor to the open configuration when the pressure in the fluid within the lumen is above a high threshold pressure that is higher than the low threshold pressure. The flow restrictor is preferably positioned outside the tube, although it may be positioned inside the wall of the tube, or even within the lumen of the tube. There may also be a second, downstream flow restrictor. The opening and closing mechanisms advantageously comprise a magnet or solenoid. The tube may comprise a catheter, and especially a urinary catheter.