A cartridge for use in a hemodialysis machine, the cartridge comprising a dialysate flow path including a dialyser, the dialysate flow path for delivering a flow of dialysate to the dialyser a first mixing pump comprising a chamber having a fixed volume between a concave recess and a flexible membrane, said chamber for receiving a predetermined volume of a first dialysate solution base, and a volume of water a flow balancing pump comprising: a chamber having a fixed volume between a concave recess and a flexible membrane, and inlet through which it receives dialysate a fluid flow path connecting the first mixing pump to the flow balancing pump the cartridge further comprising a first check valve having an inlet in fluid communication with said fluid flow path, said check valve configured to open if the pressure in the fluid flow path exceeds a predetermined pressure to allow excess fluid in the fluid flow path to flow through the first check valve to a drain, and to close when the pressure in the fluid flow path falls back below said predetermined pressure.
