An arrangement quantitatively determines the blood flow within blood vessels through which blood flows in a volume of a tissue defining a surface. Three-dimensional first image data of a first volume portion of the volume is detected and optical second image data is detected continuously in time of a first surface portion of the surface. A calibrating unit of the arrangement calibrates the relative value of the flow speed and/or the volumetric flow of the blood flowing through the blood vessels disposed directly below the first surface portion based on the absolute values of the flow speed and/or of the volumetric flow of the blood. An output unit outputs the absolute values of the flow speed and/or the volumetric flow of the blood flowing through the blood vessels arranged directly below the first surface portion.