A modular assembly system for a portable hemodialysis unit, comprising: a dialysis unit including a dialysis unit housing that contains components for performing hemodialysis, including a dialyzer mount with fluidic connections for a dialyzer, a blood pump mounting section with pneumatic connections for operation of one or more pneumatically driven blood pumps to circulate blood through a dialyzer when mounted in the dialysis unit housing, one or more pneumatically driven dialysate pumps to circulate a dialysate through the dialyzer, a dialysate heater, and one or more electronic controllers for controlling the one or more blood and dialysate pumps; and a pneumatic power unit within a pneumatic power unit housing configured to provide pneumatic power to the dialysis unit via mating pneumatic ports between the pneumatic power unit housing and the dialysis unit housing; wherein the pneumatic power unit housing is reversibly connectable to the dialysis unit housing via a mechanical and pneumatic connection to the dialysis unit.