Apparatus for measuring the blood parameters in an extracorporeal circuit, comprising: a seat suitable for containing a duct for the blood flow of said extracorporeal circuit; electromagnetic radiation emitting means; electromagnetic radiation detecting means, said means (16, 18) facing the seat; a first temperature sensor for measuring the operating temperature of the detecting means; a second temperature sensor suitable for measuring the blood temperature, a central body suitable for containing inside it the means and connected to a base and a control unit to which said emitting means and said detecting means are connected; wherein the electromagnetic radiation emitting means and electromagnetic radiation detecting means are contained inside the space between the body and the base, holes (24, 26) are formed on the seat for optical connection between the duct and the detecting means and emitting means, said electromagnetic radiation emitting means are suitable for producing electromagnetic radiation at different wavelengths, said electromagnetic radiation detecting means are suitable for detecting the electromagnetic radiation diffused in the blood at said wavelengths, and in that the control unit 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, wherein the duct is any section of a disposable extracorporeal circuit wherein the values of the blood parameters calculated are corrected based on the blood temperature and/or are corrected based on the value of the operating temperature of the detecting means and wherein the apparatus comprises a cover, said cover able to rotate around an axis, being parallel to a side of the body, said cover having a projection, which is adapted to squash the duct, inserted into the seat in the body, when the cover is closed, wherein two parallel flat surfaces are created in the duct in a way, that one surface is in contact with the cover and the other surface is in contact with the body at the holes (24, 26) for the radiation emitting means and radiation detecting means. a seat suitable for containing a duct for the blood flow of said extracorporeal circuit; electromagnetic radiation emitting means; electromagnetic radiation detecting means, said means (16, 18) facing the seat; a first temperature sensor for measuring the operating temperature of the detecting means; a second temperature sensor suitable for measuring the blood temperature, a central body suitable for containing inside it the means and connected to a base and a control unit to which said emitting means and said detecting means are connected; wherein the electromagnetic radiation emitting means and electromagnetic radiation detecting means are contained inside the space between the body and the base, holes (24, 26) are formed on the seat for optical connection between the duct and the detecting means and emitting means, said electromagnetic radiation emitting means are suitable for producing electromagnetic radiation at different wavelengths, said electromagnetic radiation detecting means are suitable for detecting the electromagnetic radiation diffused in the blood at said wavelengths, and in that the control unit 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, wherein the duct is any section of a disposable extracorporeal circuit wherein the values of the blood parameters calculated are corrected based on the blood temperature and/or are corrected based on the value of the operating temperature of the detecting means and wherein the apparatus comprises a cover, said cover able to rotate around an axis, being parallel to a side of the body, said cover having a projection, which is adapted to squash the duct, inserted into the seat in the body, when the cover is closed, wherein two parallel flat surfaces are created in the duct in a way, that one surface is in contact with the cover and the other surface is in contact with the body at the holes (24, 26) for the radiation emitting means and radiation detecting means.