Connecting unit for producing a fluid connection between a patient-side coupling unit and a medical apparatus, wherein the medical apparatus is a respiratory apparatus or an anaesthesia apparatus, wherein the connecting unit comprises - a hollow patient-side connecting piece for attaching a patient-side fluid-conducting unit, which is connected or connectable to the patient-side coupling unit, - a hollow apparatus-side connecting piece for attaching an apparatus-side fluid-conducting unit (10, 17, 21.1, 21.2, 25), which is connected or connectable to the medical apparatus, - an attachment piece and - a hollow central piece with two ends, wherein the attachment piece comprises - a rectilinear tube segment and - a planar element with an arched surface and a feedthrough opening (Ö), wherein the feedthrough opening (Ö) - is arranged in the arched surface of the planar element and - takes up at most half of the arched surface of the planar element, wherein the central piece comprises a curved tube segment with a wall, wherein the one end of the central piece is connected fluid-tightly to the patient-side connecting piece and the other end of the central piece is connected fluid-tightly to the apparatus-side connecting piece, wherein the connecting unit provides a curved tube which comprises the two hollow connecting pieces (1, 2) and the curved tube segment, wherein the attachment piece is inserted or insertable into a receiving opening of the central piece in such a manner that, when the attachment piece is inserted, the arched surface of the planar element forms part of the wall of the curved tube segment, and wherein the rectilinear tube segment of the attachment piece and the central piece together provide a rectilinear tube which - is interrupted by the planar element of the inserted attachment piece, - leads through the feedthrough opening (Ö) and - continues in the interior of the patient-side connecting piece. - a hollow patient-side connecting piece for attaching a patient-side fluid-conducting unit, which is connected or connectable to the patient-side coupling unit, - a hollow apparatus-side connecting piece for attaching an apparatus-side fluid-conducting unit (10, 17, 21.1, 21.2, 25), which is connected or connectable to the medical apparatus, - an attachment piece and - a hollow central piece with two ends, wherein the attachment piece comprises - a rectilinear tube segment and - a planar element with an arched surface and a feedthrough opening (Ö), wherein the feedthrough opening (Ö) - is arranged in the arched surface of the planar element and - takes up at most half of the arched surface of the planar element, wherein the central piece comprises a curved tube segment with a wall, wherein the one end of the central piece is connected fluid-tightly to the patient-side connecting piece and the other end of the central piece is connected fluid-tightly to the apparatus-side connecting piece, wherein the connecting unit provides a curved tube which comprises the two hollow connecting pieces (1, 2) and the curved tube segment, wherein the attachment piece is inserted or insertable into a receiving opening of the central piece in such a manner that, when the attachment piece is inserted, the arched surface of the planar element forms part of the wall of the curved tube segment, and wherein the rectilinear tube segment of the attachment piece and the central piece together provide a rectilinear tube which wherein the central piece comprises a curved tube segment with a wall, wherein the one end of the central piece is connected fluid-tightly to the patient-side connecting piece and the other end of the central piece is connected fluid-tightly to the apparatus-side connecting piece, wherein the connecting unit provides a curved tube which comprises the two hollow connecting pieces (1, 2) and the curved tube segment, wherein the attachment piece is inserted or insertable into a receiving opening of the central piece in such a manner that, when the attachment piece is inserted, the arched surface of the planar element forms part of the wall of the curved tube segment, and wherein the rectilinear tube segment of the attachment piece and the central piece together provide a rectilinear tube which - is interrupted by the planar element of the inserted attachment piece, - leads through the feedthrough opening (Ö) and - continues in the interior of the patient-side connecting piece.