In the quiescent state, normal red blood cells will aggregate in a space efficient manner. In the low shear rate region, the cells are in large aggregates and as the shear rate increases, the size of the aggregates diminish. In this range of shear rates, the viscoelasticity is dominated by the aggregation properties of the red blood cells. Deformability plays a lesser role.

In this region the internal stress due to pressure is sufficient to separate aggregated cells causing breakage of aggregates. Then the cells are progressively disaggregated with increasing shear rate. Increasing shear rate causes the cells to orient in the direction of flow. Above a unit strain, a cell is forced to move past its adjacent neighbor. This is accomplished by orientation and deformation. In this region, the influence of aggregation properties on the viscoelasticity diminish and the influence of red cell deformability begin to increase.

In this region the increasing shear rate causes normal red blood cells to stretch or deform and align with the flow. The blood will begin to form layers of stretched and packed red blood cells sliding on layers of plasma. In this region the viscoelasticity of the blood is dominated by the deformability of the red blood cells.

6.  How does red blood cell aggregation affect blood viscoelasticity?

As a result of changes in plasma protein concentrations and the influence of certain diseases, the tendency of red blood cells to aggregated can be enhanced or diminished.

The figure at right shows an idealized example of how altered aggregation tendencies can affect viscoelasticity. The viscosity and elasticity of blood with low aggregation tendencies (RED) are below the values for normal blood (BLUE) at low shear rates. The viscosity and elasticity of blood with elevated aggregation tendencies (GREEN) are above those for normal blood at low shear rates. But, in the region of high shear rates, where aggregation effects no longer dominate, the viscosity and elasticity approach the same values in each case.

Normal red blood cells are deformable but many conditions of disease and trauma can reduce their deformability. Red blood cells with reduced deformability will have difficulty forming layers at high shear rates. In the quiescent state cells with an extreme diminishment of deformability will also have difficulty aggregating.

The idealized example at right shows how the viscoelasticity blood containing cells with low deformability (RED) can differ from the viscoelasticity of blood with normal cells (BLUE).

The red blood cell is an elastic element that dominates the way blood flows in the microcirculation as well as in larger vessels. The red blood cell is also the primary structural element responsible for blood viscoelasticity. Consequently, the viscoelastic properties of blood also govern the flow through the microcirculation.

Variations in blood viscoelasticity among normal individuals is very small. Thus, changes due to disease or surgical intervention can be readily identified, making blood viscoelasticity a useful clinical parameter. Variations in blood viscoelasticity are seen in such conditions as cardiovascular disease, peripheral vascular disease, sickle cell anemia, diabetes, stroke and other conditions.

As an example, the viscoelasticity of an individual's blood with sickle cell disease is markedly different from the viscoelasticity of normal blood. This is clearly seen at high shear rates where the Patient's Elasticity (Red) is significantly higher than the Normal Elasticity (BLACK).

The measurement of blood viscoelasticity requires oscillatory measurements with high precision and sensitivity. Conventional rotational rheometers cannot meet these requirements. A rheometer must be able to operate at frequencies near the pulse rate. A well designed rheometer should also require small sample volumes, minimally expose the operator to the blood sample and be simple to operate. A rheometer designed for the measurement of blood must meet the unique challenges posed by blood.

Yes, the BioProfiler and Vilastic-3 provide the required sensitivity and precision that is unavailable with conventional rotational rheometers. Small sample volumes of 0.5 to 1 ml, simple sample handling procedure and simple system operation have made these instrument   an important part of research at such institutions as:

We at Vilastic Scientific can. With over 30 years of experience studying the viscoelastic properties of many biological fluids such as blood, plasma, synovial fluid and as well as in kinetics of coagulation, we can provide expert advice on measurement protocols and consultation on data interpretation.

by phone: +1 512-327-4134

by fax: +1 512-327-0655

by email: rheology@vilastic.com

Please visit our Technical Note: "Plasma Viscosity and Blood Viscoelasticity".

A list of publications on the rheology of blood and other biofluids can be found at,  Blood and Biofluid Bibliography.

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