of mated diameter the section of the channel thus created and its length u2014 the number of turns times the diameter limits the rate of flow possible for a fluid of defined viscosity and a defined pressure differential acting on the fluid such a screw constrictor is shown on fig2 d and fig4 a the channel in the preferred embodiment has a section equivalent to that of a tube of diameter 0 25 mm and 0 30 mm and a length 25 and 50 mm the preferred flow rate is from approximately 2 to 10 cc per minute and preferably 3 to 6 cc per minute to a volume of 1 to 2 cc preferably 1 5 cc the capillary restriction flow rate for blood can be derived from the hagen poiseuille law k is a constant q flow rate r capillary radius l capillary length u03b4p pressure drop u03bc blood viscosity considering that it is desirable to limit the pressure differential and maintain a practical flow rate it is possible to select alternate tube diameters and corresponding tube length for either internal or external flow rate control device or baffle disc located on the inlet side of the filter assembly fitted with one or numerous pinholes if one chooses an equivalent.