A measuring cell for receiving a blood sample is designed such that it can be transilluminated with a light source. The light source and a means for determining the luminous intensity corresponding to the light source can be arranged on the measuring cell for determining the luminous intensity of the light of the light source emerging from the measuring cell, wherein the measuring cell has constant, predefined properties with respect to the light of the light source, so that the blood oxygen content of the blood sample can be detected on the means for determining the luminous intensity due to a change of the luminous intensity of the light of the light source.