A system is provided for pumping fluid, with the system including a fluid pump and a cassette. The pump includes a motor and a piston that is movable toward and away from a flexible diaphragm of the cassette. A linkage connects the motor and the piston to move the piston toward and away from the diaphragm. At least a portion of the piston or the diaphragm is magnetized, with the other having at least a portion that is magnetized or formed of a ferromagnetic material, thereby magnetically coupling the piston and the diaphragm such that movement of the piston moves the diaphragm into and out of a cassette cavity aligned with the diaphragm and piston. Movement of the diaphragm into and out of the cavity changes the effective volume of the cavity, which has the effect of drawing fluid into or forcing fluid out of the cavity.
