Dialysis systems are disclosed comprising new fluid flow circuits. Systems may include blood and dialysate flow paths, where the dialysate flow path includes balancing, mixing, and/or directing circuits. Dialysate preparation may be decoupled from patient dialysis. Circuits may be defined within one or more cassettes. The fluid circuit fluid flow paths may be isolated from electrical components. A gas supply in fluid communication with the dialysate flow path and/or the dialyzer able to urge dialysate through the dialyzer and urge blood back to the patient may be included for certain emergency situations Fluid handling devices, such as pumps, valves, and mixers that can be actuated using a control fluid, may be included. Control fluid may be delivered by an external pump or other device, which may be detachable and/or generally rigid, optionally with a diaphragm dividing the device into first and second compartments.