Abstract: A device for removing substances from blood or other body fluids, comprising a bundle of semi-permeable hollow fibers, the ends of which are embedded and held in a pottant, and a tubular casing surrounding the hollow fiber bundle. At the ends of the casing, inlet and outlet pipes are connected with partitioned fluid chambers formed in the casing. Different aspects are provided to improve the flow properties, for increasing the filtering efficiency, such as varying packing density along the length of the fiber bundle, a distributor disc within a cap, apertures on the circumference of the pottant and an ordered structure of the rippling of the hollow fibers.