An occluder for closing an opening in a circuit system can be transferred from a compact manifestation which extends along a longitudinal axis (7) into an expanded manifestation. The occluder has a first expansion unit and a second expansion unit which bear on in each case one side of the opening in the expanded manifestation. Furthermore, the occluder has a first coupling part (1) and a second coupling part (2) which can be brought into engagement with one another, with the result that they fix the occluder in its expanded manifestation. The first coupling part (1) has a region (12) which extends between the first and second expansion units in the expanded manifestation. According to the invention, this region (12) is configured such that it can expand along the longitudinal axis (7). It is preferably configured such that it can expand counter to a restoring force, that is to say resiliently or elastically. This occluder therefore permits automatic setting of the spacing between two expansion and/or closure units according to the structural conditions at the location of the opening to be closed.