Abstract: A device is provided for transferring mass and/or exchanging energy between two media, particularly between blood and a gas/gas mixture. The device includes a chamber (2), through which the first medium flows and inside which at least one mass-permeable and/or energy-permeable hollow fiber (5), preferably a plurality of mass-permeable and/or energy-permeable hollow fibers (5), is/ are arranged. The second medium can flow through the at least one hollow fiber (5), while the first medium can flow therearound. At least one pumping element (5), which allows the first medium to be displaced from the chamber (2) and/or to be sucked into the chamber (2), is disposed in the chamber (2) of the device.