An implantable valve system comprises a flexible tube and a flexible valve structure. The flexible tube and flexible valve structure can be made of a synthetic polymer material. The flexible valve structure is disposed within the flexible tube. The flexible valve structure can comprises a first leaflet, a second leaflet, and a valve orifice between the first leaflet and the second leaflet. Additional leaflets are possible. Each of the first leaflet and the second leaflet comprises an upper portion forming the valve orifice and a lower portion attached to the flexible tube. The valve orifice has a maximum circumference less than an internal circumference of the flexible tube.
