Physical laws are reliable means of data processing techniques and particularly in medical engineering. Specifically, biological objects demand a deep analysis of some regularly used formulas and defined conditions, the lack of which will nurse some doubt as for the effectiveness of the formulae particularly used in rheography. The basic formulas used in bioimpedance measurements are considered in this work. The insufficiency in the expression of blood volume change as a function of total resistance (or biological impedance) relative to variation is being considered, while new expressions are being proposed. For instance, it has been demonstrated that the relative blood volume variation DV/V is directly proportional to the relative impedance variation (DZ/Z) within the body segment under investigation and inversely proportional to the natural logarithm of the total impedance of the segment (lnZ).
