Measuring cuvette for detecting at least one fluid component of a fluid flowing through the measuring cuvette with the involvement of electromagnetic radiation, the measuring cuvette comprising: - a duct penetrating through the measuring cuvette along a duct path, where the duct path is conceived as penetrating through the duct centrally, - a first connector formation for connecting a first fluid-conveying line, - a second connector formation for connecting a second fluid-conveying line, - a coupling and detection section between the first and the second connector formation (12, 16), which • Extends along a straight longitudinal axis between its first longitudinal end which is nearer to the first connector formation and its second longitudinal end which is nearer to the second connector formation, • Is configured at least for irradiation of electromagnetic radiation into a duct region of the measuring cuvette surrounded by the coupling and detection section, and • Is configured for detachable mechanical coupling with a radiation component, comprising a radiation source and/or a radiation sensor of the electromagnetic radiation, where the measuring cuvette exhibits a physical orientation structure which allows detachable mechanical coupling of the measuring cuvette with the radiation component in a predetermined desirable relative orientation and prevents it in an undesirable relative orientation, wherein the physical orientation structure is arranged and configured between the first and the second longitudinal end (22a, 22b) of the coupling and detection section at a distance (b) to the longitudinal axis which varies along the longitudinal axis and/or with a dimension (a) orthogonally to the longitudinal axis which varies along the longitudinal axis, characterized in that the orientation structure extends along the longitudinal axis over more than one third of the dimension of the coupling and detection section. - a duct penetrating through the measuring cuvette along a duct path, where the duct path is conceived as penetrating through the duct centrally, - a first connector formation for connecting a first fluid-conveying line, - a second connector formation for connecting a second fluid-conveying line, - a coupling and detection section between the first and the second connector formation (12, 16), which • Extends along a straight longitudinal axis between its first longitudinal end which is nearer to the first connector formation and its second longitudinal end which is nearer to the second connector formation, • Is configured at least for irradiation of electromagnetic radiation into a duct region of the measuring cuvette surrounded by the coupling and detection section, and • Is configured for detachable mechanical coupling with a radiation component, comprising a radiation source and/or a radiation sensor of the electromagnetic radiation, • Extends along a straight longitudinal axis between its first longitudinal end which is nearer to the first connector formation and its second longitudinal end which is nearer to the second connector formation, • Is configured at least for irradiation of electromagnetic radiation into a duct region of the measuring cuvette surrounded by the coupling and detection section, and • Is configured for detachable mechanical coupling with a radiation component, comprising a radiation source and/or a radiation sensor of the electromagnetic radiation, where the measuring cuvette exhibits a physical orientation structure which allows detachable mechanical coupling of the measuring cuvette with the radiation component in a predetermined desirable relative orientation and prevents it in an undesirable relative orientation, wherein the physical orientation structure is arranged and configured between the first and the second longitudinal end (22a, 22b) of the coupling and detection section at a distance (b) to the longitudinal axis which varies along the longitudinal axis and/or with a dimension (a) orthogonally to the longitudinal axis which varies along the longitudinal axis, characterized in that the orientation structure extends along the longitudinal axis over more than one third of the dimension of the coupling and detection section.