The following conditions that affect the flow properties of blood are discussed in brief from the rheological point of view.
(1) Volume fraction of erythrocytes-hematocrit value: -
The simple Einstein relation between relative viscosity and volume concentration of suspended particles is not valid for blood. The observed relative viscosity is greater than that calculated, owing to the interaction between the erythrocytes and a increase in the effective hematocrit value produced by enclosing and immobilizing of a certain amount of plasma within the collided erythrocytes groups. The asymptotic minimum viscosity of blood is given by a modified Hatschek's equation, over a wide range of hematocrit values.
(2) Temperature: -
The relative viscosity of a disperse system should not be affected by temperature unless the volume fraction and the shape of the dissolved or suspended particle changes. The relative viscosity of blood is, however, affected by temperature. It rises by about 10per cent by decrease in temperature from 37°C to 17°C. This rise is considered to be due to a small increase in the volume of individual erythrocytes, together with a change of shape towards a more spherical and less disc-like form.
(3) Perfusion pressure: -
If the perfusion pressure head is sufficiently large, the rate of flow of blood through a rigid vessel increases in proportion to the increase in the applied pressure head. As the pressure head is lowered, the plotted points indicating the relation between the rate of flow and the applied pressure lie on a smooth curve, which is convex to the pressure axis. The convexily becomes more and more prominent with decrease in the pressure head. Blood, therefore, is not a Newtonian fluid. It will appear to behave as a Newtonian fluid only in the limiting conditions in which the mean shearing stress is quite large and the flow still remains laminar. In order to explain the non-Newtonian flow of blood under low pressure head-the reduction in its apparent viscosity with increase in the shearing stress and rate of shear-the following factors should be taken into account.
(a) Orientation of the erythrocytes. Increase in the rate of shear leads to an increase in the fraction of erythrocytes which are orientated parallel to the flow axis and, as a result, it brings about a decrease in apparent viscosity.
(b) Coherence resistance and marginal slippage layer.“Coherence resistance”between the erythrocytes in addition to the viscous resistance and plug flow of unsheared blood moving down the vessel within a thin, peripheral plasmatic zone under the low pressure head might be responsible for the observed non-Newtonian property of blood. The plasmatic zone is considered to be produced by the wall effect and the axial accumulation of flowing erythrocytes.
(4) The radius of the vessel: -
In a narrow vessel of which radius is less than 300μ, the apparent viscosity of the blood is found to be less than the value observed in larger vessels. The smaller the radius is, the lower becomes the apparent viscosity. This effect (sigma effect) is observed not only in the blood but also in various suspensions in which the suspended particles are large enough to be comparable in size with the radius of the vessel.
