An apparatus for treating urinary retention of a patient by discharging urine from the urinary bladder. The apparatus comprises an expandable member, adapted to be implanted inside the urinary bladder of a patient, for discharging urine from the urinary bladder as a result of its expansion in volume, an implantable control device for controlling the volume of the expandable member, and an external energy transmission device for wireless transmission of energy from the outside of the patient's body to the inside of the patient's body configured to operate the expandable member and other energy consuming implantable parts of the apparatus.
