its direction changed in ways other than by reversing the flow for example as illustrated in fig3 a a planar element 210 opposite a filter or membrane 220 moves relative to the filter or membrane 220 generating a couette flow of blood 117 this effect could be generated in a filter bank of planar filters 250 260 by stopping the flow and straining the blood by moving every other layer relative to those between them alternatingly in opposite directions referring now to fig4 an example of a way to provide closely spaced planar layers of filter or membrane 230 235 is shown the adjacent layers of filter or membrane 230 235 are spaced apart by bumps 240 to create a passages 245 between them the narrow passages 245 are also susceptible to pressure drop referring now to fig5 an extracorporeal blood circuit draws blood from a patient 340 via a pump 325 runs it through a filter 300 and returns it to the patient 340 in the example embodiment the circuit includes a four way valve 320 that switches the blood circuit ends of the filter 342 and 343 such that blood can be run through the filter 300 in either direction selectively depending on the configuration of the four way valve 320 the pump 325 can run in a single direction and blood is drawn from the patient 340 without changing the draw return roles of the accesses referring now to fig6 in many applications replacement.