In a blood transfer chamber, a filter (4) is arranged at an outlet located on a bottom of the chamber. The filter is truncoconical with a vertically-disposed axis. The filter has a plurality of openings (13, 14) for fluid passage, a size of which openings increases gradually in an upwards direction going from bottom to top of the chamber. The filter is used in a venous air-blood separation chamber of a dialysis set. The invention enables a reduction of a risk of hemolysis during treatment as well as a reduction of the risk of formation of air bubble retention in the filter during a priming stage.