The impedance is the measure of the opposition that a circuit presents to a current when a voltage is applied. Resistivity is the real part of Impedance (the other part is Reactance) but when Direct Current (DC) is applied there is no difference between Impedance and Resistivity.
In hematology the measurement of impedance variation is applied to blood cells through a calibrated micro aperture. Two electrodes are placed on either side of the aperture. Electric current
passes through the electrodes continuously. When a cell passes through the aperture, the electric resistance between the two electrodes increases proportionately to the cell volume.
The impedance principle is the reference method for detecting and enumerating the total amount of red blood cells, white blood cells and platelets.