A blood pump comprising: a pump housing , wherein the pump housing provides an inlet and outlet ; an impeller chamber defined by the pump housing , wherein an impeller is housed in the impeller chamber ; wherein the impeller comprises an array of arc shaped segments joined by a central ring ; a motor housing defining a motor chamber, wherein a motor is housed in the motor chamber; wherein the motor provides a shaft with a hub mounted to the shaft ; wherein the hub comprises a first permanent magnet ; a diaphragm provided by the pump housing , the diaphragm separating the motor chamber and the impeller chamber ; wherein the diaphragm defines a cavity providing a region for the hub to rotate within; wherein the diaphragm also provides a cylindrical bearing surface for impeller to rotate around with hydrodynamic radial support; wherein the impeller is radially supported by a first hydrodynamic bearing between the bearing surface and an impeller internal surface , wherein the first hydrodynamic bearing is configured to provide said radial hydrodynamic support, the bearing surface being received within the impeller internal surface , the impeller further comprising a second permanent magnet in the central ring and providing a magnetic coupling between the impeller and the hub , the first permanent magnet and the second permanent magnet being arranged to attract to each other to form said magnetic coupling, wherein the magnetic coupling causes the impeller to rotate when the motor rotates; characterized in that a maximum height of the array of arc shaped segments overlaps with the impeller internal surface , the maximum height being in a direction of an axis of impeller rotation, wherein the topmost extent of the array of arc shaped segments is axially closer to the inlet than is the topmost extent of the impeller internal surface . a pump housing , wherein the pump housing provides an inlet and outlet ; an impeller chamber defined by the pump housing , wherein an impeller is housed in the impeller chamber ; wherein the impeller comprises an array of arc shaped segments joined by a central ring ; a motor housing defining a motor chamber, wherein a motor is housed in the motor chamber; wherein the motor provides a shaft with a hub mounted to the shaft ; wherein the hub comprises a first permanent magnet ; a diaphragm provided by the pump housing , the diaphragm separating the motor chamber and the impeller chamber ; wherein the diaphragm defines a cavity providing a region for the hub to rotate within; wherein the diaphragm also provides a cylindrical bearing surface for impeller to rotate around with hydrodynamic radial support; wherein the impeller is radially supported by a first hydrodynamic bearing between the bearing surface and an impeller internal surface , wherein the first hydrodynamic bearing is configured to provide said radial hydrodynamic support, the bearing surface being received within the impeller internal surface , the impeller further comprising a second permanent magnet in the central ring and providing a magnetic coupling between the impeller and the hub , the first permanent magnet and the second permanent magnet being arranged to attract to each other to form said magnetic coupling, wherein the magnetic coupling causes the impeller to rotate when the motor rotates; characterized in that a maximum height of the array of arc shaped segments overlaps with the impeller internal surface , the maximum height being in a direction of an axis of impeller rotation, wherein the topmost extent of the array of arc shaped segments is axially closer to the inlet than is the topmost extent of the impeller internal surface . <EOS>