Physical properties of flowing blood

The changes of viscosity, optical reflection and electrical resistivity of blood due to flow are dependent on the orientation and deformation of red cells. From electrical point of view, it can be assumed that blood is suspension of small insulating particles (red cells) in conductive fluid (plasma) when the frequency of supplied voltage is lower than several hundreds KHz. When blood flows, red cells deform and orient in flow direction. Therefore, flowing blood shows anisotropie electrical and optical properties. In steady flow, blood resistivity longitudinal to flow decrease with flow rate, and transverse one increases. Blood flow in living body is not steady but pulsatile. We measured both longitudinal and transversal resistivity changes, optical reflection change and viscosity change of sinusoidally flowing blood in a rectangular conduit. The results are 1) the longitudinal resistivity change is opposite to that of transverse one, 2) the waveform of reflected light change is similar to that of resistance change, and 3) minimum points of both longitudinal resistivity and viscosity changes do not appear at the moment when flow is zero but are delayed. When the amplitude of sinusoidal flow is small and oscillation frequency is high, the phase difference between the zero crossing period of flow and the period of minimum change in resistivity, increases up to 90 degrees. Viscosity of blood decreases with increase of amplitude and frequency of sinusoidal flow. From these results, it is concluded that blood behave like elastic material when the shear of blood is less than certain value (about 5 for hematocrit of 40%), and above this value blood behave as viscous material. We think that blood make structure at low shear, and this structure suddenly break at the certain shear.