An extracorporeal blood treatment system, comprising: a first holder mounted on a housing, the first holder configured to hold a blood treatment device having a blood chamber, a fluid chamber and a semipermeable membrane that separates the chambers (15, 16) from each other; a second holder mounted on the housing, the second holder configured to hold an extracorporeal blood circuit coupled to the blood treatment device; an infusion pump; and an infusion valve unit having a first part and a second part releasable connectable to the first part, wherein the first part is mounted on the housing of the extracorporeal blood treatment system, and has an inlet coupled to the infusion pump; and wherein the second part has a first outlet and a second outlet for connecting to an infusion circuit which is configured to couple to the extracorporeal blood circuit,and a one-way valve configured to allow infusion liquid to flow from the inlet towards at least one of the first and second outlets (7a, 10a), and to block fluid flow towards the inlet, characterized in that the infusion pump is arranged inside the housing and the second part further comprises: • a valve seat having a platform that is attached to a body of the second part and a hollow cylinder that extends from the platform towards the first part and is open on a side opposite to the platform, on the open side the cylinder having a flange; and • a sealing element mounted to the valve seat, wherein the valve seat has a central opening and at least one fluid channel, the central opening and the fluid channel being in the valve seat's platform, wherein the sealing element has a stem extending through the central opening and a flexible structure extending radially from the stem and being configured to cover in a closed state the fluid channel; the second part has a male connecting part, and the first part has a female connecting part, and wherein the first part includes a seal having an annular groove sized to receive the hollow cylinder of the valve seat with its flange. a first holder mounted on a housing, the first holder configured to hold a blood treatment device having a blood chamber, a fluid chamber and a semipermeable membrane that separates the chambers (15, 16) from each other; a second holder mounted on the housing, the second holder configured to hold an extracorporeal blood circuit coupled to the blood treatment device; an infusion pump; and an infusion valve unit having a first part and a second part releasable connectable to the first part, wherein the first part is mounted on the housing of the extracorporeal blood treatment system, and has an inlet coupled to the infusion pump; and wherein the second part has a first outlet and a second outlet for connecting to an infusion circuit which is configured to couple to the extracorporeal blood circuit,and a one-way valve configured to allow infusion liquid to flow from the inlet towards at least one of the first and second outlets (7a, 10a), and to block fluid flow towards the inlet, • a valve seat having a platform that is attached to a body of the second part and a hollow cylinder that extends from the platform towards the first part and is open on a side opposite to the platform, on the open side the cylinder having a flange; and • a sealing element mounted to the valve seat, wherein the valve seat has a central opening and at least one fluid channel, the central opening and the fluid channel being in the valve seat's platform, wherein the sealing element has a stem extending through the central opening and a flexible structure extending radially from the stem and being configured to cover in a closed state the fluid channel;