A device for use in combination to an apparatus for the extracorporeal treatment of blood and comprising: a circuit (4) connectable to the patient by means of first (1) and second (5) connection means intended, respectively, to the collection of blood to be treated and to the return of the treated blood a blood pump (2) arranged and acting on said circuit (4) so as to determine a flow towards the said second means of connection to the patient (5) an oxygenator (3), disposed and acting on said circuit (4) provided with a first input (30) for the blood to be treated and a first output (31 ) for the treated blood disposed downstream of said first inlet (30) the device (10) is characterized in that it comprises a first chamber (11) disposed on said circuit (4) upstream of said oxygenator (3), a second chamber (12) disposed downstream of said oxygenator (3), and a mobile wall (13) interposed between the said chambers (11, 12) and defining a wall or portion of the same chambers and provided with moving means (14 18) adapted to move said wall (13) for generating an alternating flow of blood which crosses the said active element (3) and which is added to the flow generated by said pump (2).
