An apparatus for extracorporeal treatment of blood including a filtration unit, a blood withdrawal line, a blood return line, an effluent fluid line, a dilution fluid line connected to the blood withdrawal line, and a dialysis fluid line. Pumps act on the fluid lines for regulating the flow of fluid. A control unit sets initial values for a fluid flow rate(s) through the lines and periodically executes a flow rate update procedure to adjust the fluid flow rate(s) to deliver a set dose (Dset) in a reference time interval.