A blood filtration device comprising: a generally cylindrical housing having an interior wall; an interior member mounted interior of the housing and having an external surface; one of the interior wall of the housing and the external surface of the interior member including a porous membrane spaced apart from the facing wall of the housing or surface of the interior member so as to define an annular gap therebetween, which membrane has a surface area sufficient to separate plasma at a flow rate resulting in the residence time of red blood cells within the annular gap being insufficient to cause undue hemolysis; the housing and interior member being relatively rotatable; an inlet for directing whole blood comprising plasma and red cells into the annular gap; a first outlet for directing plasma passing through the membrane to a collection container; and a second outlet for directing from the annular gap the remaining blood components; wherein - the interior member has a diameter in the range of 3.8 - 5.4 cm, and a filtration length in the range of 12.3-19.0 cm, and characterized in that the blood filtration device further comprises a ridge extending from one of the interior wall of the housing or the external wall of the interior member so as to reduce the annular gap therebetween, the ridge defining separate first and second flow regions (92, 94) for fluid introduced into the annular gap, wherein the first region is a non-perfused region of flow and is located at an area beyond the membrane, and wherein the second region is a high perfusion region and is arranged so as to contact to membrane. a generally cylindrical housing having an interior wall; an interior member mounted interior of the housing and having an external surface; one of the interior wall of the housing and the external surface of the interior member including a porous membrane spaced apart from the facing wall of the housing or surface of the interior member so as to define an annular gap therebetween, which membrane has a surface area sufficient to separate plasma at a flow rate resulting in the residence time of red blood cells within the annular gap being insufficient to cause undue hemolysis; the housing and interior member being relatively rotatable; an inlet for directing whole blood comprising plasma and red cells into the annular gap; a first outlet for directing plasma passing through the membrane to a collection container; and a second outlet for directing from the annular gap the remaining blood components; wherein - the interior member has a diameter in the range of 3.8 - 5.4 cm, and a filtration length in the range of 12.3-19.0 cm, and - the interior member has a diameter in the range of 3.8 - 5.4 cm, and a filtration length in the range of 12.3-19.0 cm, and characterized in that the blood filtration device further comprises a ridge extending from one of the interior wall of the housing or the external wall of the interior member so as to reduce the annular gap therebetween, the ridge defining separate first and second flow regions (92, 94) for fluid introduced into the annular gap, wherein the first region is a non-perfused region of flow and is located at an area beyond the membrane, and wherein the second region is a high perfusion region and is arranged so as to contact to membrane.