Abstract: The invention relates to an impeller (1) of a centrifugal pump apparatus, in particular of a blood pump apparatus, comprising a discoidal body (1) rotatable around an axis (2) of rotation, the body (1) comprising an upper surface (3), a lower surface (4), a central channel (5) extending in the direction of the axis (2) of rotation between the upper and lower surface (3, 4) for guiding fluid, in particular blood through the body (1) in axial direction when the body (1) is rotated in a pump chamber of a pump and several blades (6) supported by the upper surface (3) for pumping fluid, in particular blood, when the body (1) is rotated in a pump chamber of a pump, several spiral grooves (7) in the lower surface (4), each groove (7) having a bottom (7a) and two sidewalls (7b, 7c) and being open in an axial direction, merging into a circumferential surface (8) of the body (1) and extending from this circumferential surface (8) to the central channel (5) at least for providing a pumping action of fluid, in particular