An apparatus includes a base, including two motors and a mechanical coupling mechanism, and a cartridge shaped to define two stators and respective pairs of ports in fluidic communication with the stators. The cartridge is removably insertable into the base and includes two rotors disposed, respectively, within the stators. The mechanical coupling mechanism is configured to mechanically couple the rotors to the motors, respectively, such that, following insertion of the cartridge into the base, the motors rotate the rotors, thereby pumping fluid through the pairs of ports. Other embodiments are also described.