An apparatus for measuring the blood parameters in an extracorporeal circuit comprises: a seat (23) suitable for containing a duct (14) for the blood flow of said extracorporeal circuit emitting means (16) and detecting means (18) of electromagnetic radiation that face the seat (23) a control unit (38) to which said emitting means (16) and detecting means (18) are connected. The apparatus according to the invention is characterised in that the emitting means (16) are suitable for producing electromagnetic radiation at different wavelengths, and the detecting means (18) are suitable for detecting the electromagnetic radiation diffused in the blood at said wavelengths. Moreover, the apparatus is characterised in that the control unit (38) is suitable for calculating values of blood parameters through a correlation between reference values and ratios obtained from values of the light intensity of the radiation detected at at least two different wavelengths.
