An apparatus for extracorporeal blood treatment comprising: at least one treatment unit (2) having at least a first chamber (3) and at least a second chamber (4) separated from each other by a semipermeable membrane (5)； at least one blood collection path (6) connected to an inlet opening of a first chamber and predisposed to draw blood from a patient； at least one blood return path (7) connected to an exit orifice of the first chamber and predisposed to return treated blood to the patient； at least one expansion chamber (11, 12) placed at least in one of the blood collection path (6) and the blood return path (7), the expansion chamber being in use to contain a predetermined amount of gas in an upper part and a predetermined amount of blood at a predetermined level in a lower part thereof, forming the blood collection path (6), the blood return path (7), the first chamber (3 ) and the at least one expansion chamber (11, 12) part of an extracorporeal blood circuit (8)； at least one blood pump (9) operating in the extracorporeal blood circuit (8) to move blood in the circuit； at least one pressure sensor (13, 14) associated with the expansion chamber (11, 12) and configured to allow pressure values to be determined inside the expansion chamber (11, 12)； at least one fluid evacuation path (10) connected to an outlet port of the second chamber； a control unit (21) connected to at least one pressure sensor (13, 14), with the pump (9), and configured to: - move the blood pump (9) to generate a variable blood flow (Q (t)) comprising a constant flow component (Qb) equal to a desired blood flow value and a variable flow component (Qvar (t)) that oscillates around the constant component (Qb) and that has a substantially average value null, generating the variable blood flow component (Qvar (t)), at least in the expansion chamber (11, 12), a pressure progression that is variable over time (P (t)) comprising a pressure component (Pvar (t)) that oscillates around an average val
