The present invention defines a blood pump comprising a cartridge, the cartridge comprising a first recess therein, said first recess having a surface, and a flexible diaphragm closing said first recess, the first recess and the flexible diaphragm defining a first pump chamber, said first pump chamber having an inlet and an outlet wherein the flexible diaphragm of the first pump chamber is movable between a first position, separated in use from the surface of the first recess, wherein in said first position said first pump chamber has a maximum volume, and a second position, substantially adjacent to the surface of the first recess, wherein in said second position said first pump chamber has a minimum volume, a pump driver arranged to interface with the cartridge, said pump driver operable to move the flexible diaphragm of the first pump chamber in a first direction into said first recess to, in use, pump blood from the chamber and to move the flexible diaphragm of the first pump chamber in a second direction away from the first recess to, in use, draw blood into said first pump chamber, wherein, the cartridge further comprises one or more sensor cavities defined by respective recesses in the cartridge, the, or each, recess being closed by a flexible diaphragm.
