Apparatus and methods are described including a cartridge, which includes a first port, a second port, and a third port, and which holds tubes such that respective proximal portions of a purging-fluid tube, a waste tube, and a flushing tube pass through the first port, respective distal portions of the purging-fluid tube, the waste tube, and a pressure-sensing tube pass through the second port, and a distal portion of the flushing tube and a proximal portion of the pressure-sensing tube pass through the third port. Other applications are also described.