A hollow fiber membrane is produced having an outer surface, and inner surface enclosing a lumen, and a continuous network of interconnecting pores extending from the outer surface to the inner surface, wherein substantially all of the pores have an elongated dimension and a shortened dimension, the elongated dimension being oriented substantially parallel to the axis of the hollow fiber, wherein the ratio of the elongated dimension to the shortened dimension is substantially constant from the outer surface to the inner surface, and wherein the pores change shape to increase the shortened dimension in response to a high pressure differential imposed across the membrane by increasing fluid pressure within the lumen.