Apparatus and method are described including identifying a subject as suffering from a condition that causes the subject to have elevated central venous pressure. In response thereto, a device is placed inside the subject's vena cava such that an upstream end of the device is placed at a location upstream of junctions of the vena cava with all of the subject's renal veins. The device defines a passage through the device, at least a portion of the passage converging in a direction from an upstream end of the device to downstream end of the device. Other applications are also described.