A blood treatment cassette having a cassette body comprising a hard part and a film that at least partially covers the hard part. The hard part comprises a valve base that cooperates with a section of the film adjacent the valve base to form a valve that is configured to be moved from a first open position in which the valve base and the section of the film do not touch each other to a second position in which the valve base and the section of the film touch each other when a force is applied to the section of the film. The blood treatment cassette comprises a spacer supported by the hard part, and the spacer is configured to apply, in the first position of the valve, force to the section of the film.
