A blood pump (1) comprises a pump casing (2) having a blood flow inlet (5) and a blood flow outlet (6) connected by a passage (7), and an impeller (3) arranged in said pump casing (2) so as to be rotatable about an axis of rotation (9). The impeller (2) is provided with blades (4) sized and shaped for conveying blood along the passage (7) from the blood flow inlet (5) to the blood flow outlet (6), the impeller (3) being rotatably supported in the pump casing (2) by at least one bearing (10, 20). A surface of the impeller (3) faces a surface of the pump casing (2) spaced from said surface of the impeller (3) by a clearance (31), the clearance (31) being in fluid connection with the passage (7) at a clearance transition point (36). In order to wash out the clearance, at least one wash out channel (30) extends through the impeller (3) and is in fluid connection with the passage (7) via a first opening (34) and with the clearance (31) via a second opening (35). The first opening (34) of the wash out channel (30) is arranged in an area of the impeller (3) that—during operation of the blood pump (1)—is under a higher pressure than the clearance transition point (36) so as to cause a blood flow from the first opening (34) through the wash out channel (30) and the clearance (31) to the clearance transition point (36).
