The invention relates to an extracorporeal blood treatment apparatus witha control device (50) for controlling the opening and closing of an arterial line (2) with simultaneous closing and opening of a venous line (3) in a single-lumen-arrangement (10), with which untreated blood is removed through the single-lumen-arrangement (10) and the arterial line (2), led through a blood treatment device (5) and treated blood is fed back through the venous line (3) and the single-lumen-arrangement (10),an arterial line container (20) which is arranged in the arterial line (2) in front of the blood treatment device (5),a venous line container (30) which is arranged in the venous line (3) following the blood treatment device (5),a sensor device (31) for detecting an amount of treated blood being above a limit value in the venous line container (30),a sensor device (21) for detecting an amount of untreated blood being below a limit value in the arterial line container (20),an arterial line closure device (22) for closing the arterial line (2) with simultaneous opening of the venous line (3) in response to a signal A which is transmitted by the sensor device (31) to the arterial line closure device (22),a venous line closure device (32) for closing the venous line (3) with simultaneous opening of the arterial line (2) in response to a signal B which is transmitted by the sensor device (21) to a venous line closure device (32) andat least one pump (40) and is characterized in that a control device (50) is provided, with which a phase volume Vact is adjustable automatically in dependence on the current pumping rate QBP of the at least one pump (40) during a phase to a predefined value Vnom (FIG. 1).
