Abstract: A fluid pump, particularly an extracorporeal blood pump, comprises a pump chamber (1) having an inlet opening (2) and an outlet opening (3), each connected to a respective non-return valve (4,5). The inlet (1) and the outlet (2) are movable in relation to each other and connected to a drive means (6) for periodic displacement of the inlet and the outlet alternately towards and away from each other, the interior volume of the pump chamber (1) being variable in correspondence with the relative distance between the inlet and the outlet. The pump chamber (1) is on its outside influenced by the surrounding pressure and has a wall of a flexible material constructed in such a manner that the pump chamber will collapse and reduce its volume, if the interior pressure in the pump falls below the surrounding pressure by an amount greater than the required opening pressure difference for the nomn-return valve connected to the inlet.