A pump (1) for pumping highly viscous fluids is presented that includes a casing (3), an inlet (7), an outlet (8) and a closed impeller (5) rotatably arranged in the casing between the inlet and the outlet and that has a side room (6) between a shroud (4) of the impeller and the casing (3). In addition, the pump (1) includes a sealing element (7a, 7b, 8a, 8b) between the impeller (5) and the casing (3) each at an inlet side and at an outlet side of the impeller for restricting back flow through the side room (6) and for allowing the fluid contained in the side room to heat up, and an injection port (9) leading into the side room (6) for injecting a fluid into the side room for diminishing disk friction between the shroud (4) of the impeller and the casing (3).