The invention relates to an apparatus for exchanging material between blood and a gas/gas mixture, comprising a chamber (1) through which blood can flow and in which a plurality of material-permeable fiber tubes is provided, the gas/gas mixture being flowable through the fiber tubes, blood being flowable around the fiber tubes. At least one deformable element (9) is provided in the chamber (1) in addition to the fiber tubes, through which the gas/gas mixture can flow, this deformable element being deformable and restorable, in particular compressible out of a relaxed shape and restorable to a relaxed shape by pressure fluctuations acting on the at least one element (9) externally, in particular pressure fluctuations transmitted by the blood in the chamber (1).