A device for treating blood in an extracorporeal circuit comprising a venous blood reservoir having an inlet and an outlet, and a hemoconcentrator having a first inlet connector for the blood to be treated, a second outlet connector for the outflow of concentrated blood, and a third outlet connector for the outflow of the ultrafiltered liquid, wherein the hemoconcentrator is disposed within the venous blood reservoir and the hemoconcentrator and the venous blood reservoir are integrated into a single monolithic assembly, and wherein the hemoconcentrator comprises an enclosure containing structure suitable to concentrate blood.