Breathing tube arrangement for a lung function diagnostics device, comprising a breathing tube defining a guiding path for breathing air to be analyzed by a lung function diagnostics device, the breathing tube having a proximal end, a distal end and an analysis zone located between the proximal end and the distal end, wherein the proximal end is designed to face a subject whose lung function is to be analyzed and to receive exhaled breathing air from the subject, wherein the analysis zone serves for allowing an analysis of breathing air flowing through the breathing tube, and wherein the distal end serves for releasing analyzed breathing air to an environment of the breathing tube, wherein the breathing tube arrangement comprises a filter element having a filter material, a breathing air releasing region arranged distally of the filter material, and a diffusor zone arranged proximally of the filter material, wherein the diffusor zone comprises a proximal end having a first proximal cross-sectional area and a distal end having a first distal cross-sectional area that is bigger than the first proximal cross-sectional area, wherein the proximal end of the diffusor zone is connected to the distal end of the breathing tube such that breathing air flowing from the breathing tube through the distal end of the breathing tube towards an environment of the breathing tube has to pass the filter element, and wherein the breathing air releasing region has a second distal cross-sectional area, wherein the second distal cross-sectional area is bigger than the first proximal cross-sectional area and at least as big as the first distal cross-sectional area, wherein a ratio between the first proximal cross-sectional area and the second distal cross-sectional area lies in a range from 1:2 to 1:20, characterized in that the diffusor zone has sidewalls (51, 52, 55, 56) that diverge in a direction from the proximal end of the diffusor zone towards the distal end of the diffusor zone at least in a section between the proximal end of the diffusor zone and the distal end of the diffusor zone at an angle lying in a range from 5° to 30°. wherein the breathing tube arrangement comprises a filter element having a filter material, a breathing air releasing region arranged distally of the filter material, and a diffusor zone arranged proximally of the filter material, wherein the diffusor zone comprises a proximal end having a first proximal cross-sectional area and a distal end having a first distal cross-sectional area that is bigger than the first proximal cross-sectional area, wherein the proximal end of the diffusor zone is connected to the distal end of the breathing tube such that breathing air flowing from the breathing tube through the distal end of the breathing tube towards an environment of the breathing tube has to pass the filter element, and wherein the breathing air releasing region has a second distal cross-sectional area, wherein the second distal cross-sectional area is bigger than the first proximal cross-sectional area and at least as big as the first distal cross-sectional area, wherein a ratio between the first proximal cross-sectional area and the second distal cross-sectional area lies in a range from 1:2 to 1:20, characterized in that the diffusor zone has sidewalls (51, 52, 55, 56) that diverge in a direction from the proximal end of the diffusor zone towards the distal end of the diffusor zone at least in a section between the proximal end of the diffusor zone and the distal end of the diffusor zone at an angle lying in a range from 5° to 30°.