A blood mixer comprises a support structure supporting the components of the mixer (10), a tubing fastening and guiding arrangement, configured to secure a blood collecting tubing in the mixer (10), a tray (18) adapted to house a blood collecting bag (16) having a blood tube (14) attached thereto, and configured to measure the weight of the collected blood in the blood collecting bag (16) and a pump (20) configured to draw blood from the donor (12) to a blood collecting bag (16) through said tubing (14). The pump (20) is configured to exert a kneading action on the blood supply tubing (14) by means of at least two kneading elements arranged to alternatingly displace the contents of the tube in the direction towards the bag (16). There is also provided a cooling arrangement (19) configured to cool the blood in the blood collecting bag (16).
