Abstract: A pump comprising a pump housing (12) and a rotor (28) driven by a shaft (10), said rotor having at least two outwardly spring-biased elements (30). These lie in contact with the wall of a chamber (27) surrounding the rotor and transport, during rotation of the rotor, a gas and fluid medium from a suction port (15) to a pressure port (16) in the housing. The chamber (27) is arranged in a sliding block (18) which is displaceable inside the housing (12) in such a way that the chamber is movable between two end positions, of which one end position is concentric with, or presents a comparatively small eccentricity with respect to, the rotating shaft (10) of the rotor and which second end position presents a comparatively large eccentricity with respect to said shaft.