An extracorporeal blood system (10) is described. The system comprises: an extracorporeal blood machine (20, 30) including an extracorporeal blood circuit (16, 35)； an access device (36) enabling the extracorporeal blood circuit (16, 35) to be connected to a patient (18)； electrical contacts (22, 24) configured to contact blood as the blood flows through the extracorporeal blood circuit (16, 35)； and a controller (52) configured to model a total impedance Z according to at least two lumped impedances including (i) a lumped impedance ZD due to components (40, 42, 44, 46) of the extracorporeal blood machine (20, 30) and (ii) a lumped impedance ZP due to access of the extracorporeal blood circuit (16, 35) to the patient (18).
