the theoretical principles of the determination of the transmembrane pressure and the device according to the invention for determining the transmembrane pressure and the method according to the invention according to which the device for determining the transmembrane pressure works are described in detail below four pressure sensors are in principle required for the exact determination of mean trans membrane pressure tmp after the measurement of the pressure at blood side inlet p b out the pressure at blood side outlet p b out the pressure at dialysate side inlet p d in and the pressure at dialysate side outlet p d out transmembrane pressure p tm tmp can be calculated according to the following equation p tm transmembrane pressure tmp p b in pressure at the blood side inlet of the dialyzer p b out pressure at the blood side outlet of the dialyzer venous pressure p ven p d in pressure at the dialysate side inlet of the dialyzer p d out pressure at the dialysate side outlet of the dialyzer in the present example embodiment however the pressure is measured not by means of four pressure sensors at the aforementioned measuring points but only by means of three pressure sensors 20 21 22 which measure pressure p b out at blood side outlet 3 b of blood chamber 3 of dialyzer 1 p d in at dialysate side inlet 4 a and p d out at dialysate side outlet 4 b of dialyzing fluid chamber 4 of dialyzer 1 the differences between the determination of the transmembrane pressure on the basis of a measurement at three measuring points and.