The invention relates to a device (1) for determining the blood volume and/or blood volumetric flow of a circulation section, comprising a fluid storage unit (2) for storing a measuring fluid (3), an injection device for the injection of the measuring fluid into the circulatory system, at least one pump device (5) disposed between the fluid storage unit and the injection device, means for determining at least one property of the measuring fluid before injection into the circulatory system, further means for determining the respective properties in the circulatory system downstream of the injection site, and an analysis device for calculating the blood volume or volumetric flow as a function of the properties determined. The invention further relates to such a device having means for conditioning the measuring fluid. The invention provides that the measuring fluid can be supplied from the fluid storage unit to a second pump device (9) by means of a first pump device (5), and from the second pump device to the catheter. The invention further provides that the measuring fluid can be supplied to the catheter (11) at a first volumetric flow provided by the first pump device, said first volumetric flow able to be temporarily, in particular regularly, superimposed by a second volumetric flow provided by the second pump device, and/or that the means for conditioning the measuring ring fluid act upon at least one pump device. The invention also relates to a method for operating such devices.
