Abstract: A pump for conveying a medium, especially a gas-liquid mixture, with a housing, an annular surface extending inside the housing, an eccentric arranged off center in relation to the annular surface and at least one displacer arranged between the annular surface and the eccentric. The displacer is held in position in relation to the annular surface and the annular surface can rotate in relation to the eccentric. A hollow shaft delivers mixture to the eccentric which has connections to the displacer for delivering mixture to and receiving it from the displacer.