Filter device for separating blood fractions

A filter apparatus for separating blood fractions into a specifically heavier fraction and a specifically lighter fraction comprising:

• a cylindrical vessel for containing said blood fractions, said cylindrical vessel including a cavity therein;
• a piston means adapted for slidable air-tight engagement within the cavity of said cylindrical vessel, said piston means including a filter section permeable for the specifically lighter blood fractions and a hollow riser tube connected to said filter section and adapted to pass liquids filtered by said filter section, the top of said riser tube including a flared upper opening at the top thereof, said piston means including a plurality of projections on the outside thereof;
• and,
• a cap extending coextensive with said riser tube, said cap including a closed upper end and an open lower end, said cap including a conical closure plug attached to the inside of said cap and located near the closed upper end thereof, said plug being adapted for seating reception in the upper end of said riser tube, said cap including recesses located at the lower end thereof which are complimentary to the projections on said piston means, said projections being provided in such a manner that in a first position they engage with the recesses in said cap thereby allowing said plug to come in sealing relationship with the flared opening of said riser tube and in a second position they are out of engagement with said recesses thereby allowing liquid communication between the inside of the riser tube and the inside of the cap.