Electrical Characteristics of Flowing Blood

Abstract

The electrical properties of blood are of practical interest in medical and biological engineering, such as in impedance plethysmography and ECG, because the impedance of blood is less than that of all other living tissues.
It is well-known that when blood flows, its impedance changes from its resting, value. The change in the resistivity of flowing blood is principally due to three factors-the axial accumulation, the orientation of red cells and the deformation of red cells. These phenomena occur simultaneously.
In this paper, the effects of these three phenomena upon the blood resistivity are theoretically and experimentally discussed. The resistivity of blood and its changes have been measured. The reflected light from blood was simultaneously detected in order to discuss the phenomenon because the light is mainly reflected from the red cells and is affected by the orientation of the red cells.
Not all of the red cells are oriented along the flow line of the blood. They always rotate. However, the resistivity of blood changes with the change of flow because the rotating velocity depends on the angle between the red cell and the flow line. Therefore, it seems that a part of red cells is equivalently oriented along the flow line of the blood.
Only about 50% of the red cells are equivalently oriented in high shear flow (1 000 s^-1). In the living body, the maximum shear rate is considered to be about 100 s^-1. In this case, about 20 %-30% of the red cells are oriented. And relative change in resistance due to the orientation is about 10%.
The thickness of the plasma layer in actual blood is considered to be about 5 μm. Therefore, only when the physical properties of the blood in capillaries is discussed, this phenomenon becomes highly important.
From the experimental and theoretical results, it is assumed that the change of resistivity in the flowing blood depends mainly upon the orientation of the red cells.