Abstract: An apparatus for maintaining a fluid supply, such as human blood going to a hospital patient, at optimum temperatures comprises an electrically heated inner mandrel maintained at a thermostatically controlled temperature. A substantially flat plastic bag having sealed linear forming labyrinth liquid flow passageways is wrapped around the mandrel and supply and exhaust tubes are connected to the ends of the passageways. An outer housing having a pair of hinged wings is provided for holding the bag in wrapped-around contact with the mandrel. The linear sealed areas are interrupted by spaced gaps adapted to inhibit the accumulation of air bubbles along the sealed linear areas.