A reservoir bag for containing a dialysate fluid during a dialysis treatment, wherein the reservoir bag is defined at least by a first wall and a second wall and wherein the reservoir bag comprises: a membrane extending from the first wall of the reservoir bag to the second wall of the reservoir bag, the second wall opposing the first wall, wherein the membrane is attached to the first wall and the second wall so that an internal volume of the reservoir bag is divided into a first sub-reservoir (504, 612) and a second sub-reservoir (506, 614), wherein the first sub-reservoir and second sub-reservoir are fluidically isolated from each other, wherein the membrane is flexible such that, upon application of fluid pressure, the membrane moves causing the first sub-reservoir to have a changeable volume and the second sub-reservoir to have a changeable volume, and wherein a maximum volume of the first sub-reservoir is equal to 80% to 100% of an entire amount of the internal volume of the reservoir bag and a maximum volume of the second sub-reservoir is equal to 80% to 100% of the entire amount of the internal volume of the reservoir bag; a first inlet (512, 604) positioned on a surface of the reservoir bag for enabling fluid entry into the first sub-reservoir; a first outlet (514, 608) positioned on a surface of the reservoir bag for enabling fluid exit out of the first sub-reservoir; a second inlet (516, 606) positioned on a surface of the reservoir bag for enabling fluid entry intothe second sub-reservoir; a second outlet (518, 610) positioned on a surface of the reservoir bag for enabling fluid exit out of the second sub-reservoir; and characterized by a third inlet positioned on a surface of the reservoir bag for enabling fluid entry into, and exit out of, the first sub-reservoir. a membrane extending from the first wall of the reservoir bag to the second wall of the reservoir bag, the second wall opposing the first wall, wherein the membrane is attached to the first wall and the second wall so that an internal volume of the reservoir bag is divided into a first sub-reservoir (504, 612) and a second sub-reservoir (506, 614), wherein the first sub-reservoir and second sub-reservoir are fluidically isolated from each other, wherein the membrane is flexible such that, upon application of fluid pressure, the membrane moves causing the first sub-reservoir to have a changeable volume and the second sub-reservoir to have a changeable volume, and wherein a maximum volume of the first sub-reservoir is equal to 80% to 100% of an entire amount of the internal volume of the reservoir bag and a maximum volume of the second sub-reservoir is equal to 80% to 100% of the entire amount of the internal volume of the reservoir bag; a first inlet (512, 604) positioned on a surface of the reservoir bag for enabling fluid entry into the first sub-reservoir; a first outlet (514, 608) positioned on a surface of the reservoir bag for enabling fluid exit out of the first sub-reservoir; a second inlet (516, 606) positioned on a surface of the reservoir bag for enabling fluid entry intothe second sub-reservoir; a second outlet (518, 610) positioned on a surface of the reservoir bag for enabling fluid exit out of the second sub-reservoir; and characterized by a third inlet positioned on a surface of the reservoir bag for enabling fluid entry into, and exit out of, the first sub-reservoir.