This master’s thesis is an introduction to the measurement of changes in tissue impedance of blood flow by impedance plethysmography. Other chapters deal with the kinds of plethysmographs and their principles. The aim is to draft four-electrode impedance plethysmograph to measure changes in tissue impedance depending on blood flow. First, describe the individual blocks of the medical instrument. The practical part of the master’s thesis involves circuit design four-electrode plethysmograph. Given that a very important role in its function plays a constant current source operating at a frequency of 60kHz, this subset was implemented and verified its correct function. To draw component schemes used program EAGLE version 5.10.0.