The present invention provides methods and apparatus for cleansing blood through hemodialysis by the process of diffusion across a membrane into dialysate. This dialyzer also removes solutes from the blood by a process of convection, where fluid and dissolved solutes pass through the membrane out of the blood. In one embodiment in accordance with the present invention, the MECS dialyzer uses a counter-flow between the dialysate and blood through a plurality of microchannels. The dialyzer comprises a plurality of flat semi-permeable membranes interleaved between microchannel sheets to define a plurality of flow channels. The stack of membranes and microchannel sheets are aligned and consolidated to form the MECS dialyzer. The MECS dialyzer acts as a flow manifold with ports and headers to collect the blood and dialysate and direct them to and from the microchannels.