Rheological properties of blood - what is it?

A region of mechanics that studies the features of deformation and flow of real continuous media, one of whose representatives is non-Newtonian fluids having structural viscosity, is rheology. In this article, we consider the rheological properties of blood. What it is, it becomes clear.

Definition

A typical non-Newtonian fluid is blood. Plasma it is called, if it is devoid of uniform elements. Blood serum is a plasma in which there is no fibrinogen.

Hemorheology, or rheology, studies mechanical patterns, especially how the physicochemical properties of the blood change with circulation at different rates and at different sites in the vascular bed. Its properties, the functional state of the bloodstream, the contractility of the heart determine the movement of blood in the body. When the linear velocity of the flow is small, the blood particles move parallel to the axis of the vessel and to each other. In this case, the stream has a layered character, and the flow is called laminar. So what are the rheological properties? About this - further.

What is the Reynolds number?

In the case of an increase in the linear velocity and exceeding a certain value, which is different for all the vessels, the laminar flow will turn into a vortex, disorderly, called turbulent flow. The rate of transition of laminar motion to turbulent determines the Reynolds number, which is about 1160 for blood vessels. According to the Reynolds numbers, turbulence can only occur in places where large vessels branch, and also in the aorta. In many vessels, the liquid moves laminarly.

Shear rate and shear stress

Not only the volumetric and linear velocity of blood flow are important, two more important parameters characterize the movement toward the vessel: speed and shear stress. The shear stress is the force acting per unit of the vascular surface in the tangential direction to the surface, measured in pascals or dynes / cm ² . The shear rate is measured in seconds of the inverse (s-1), and it means the magnitude of the velocity gradient between the liquid layers moving in parallel per unit distance between them.

On which indicators do rheological properties depend?

The ratio of stress to shear rate determines the viscosity of the blood, measured in mPas. In a whole liquid, the viscosity depends on the shear rate range of 0.1-120 ^s-1 . If the shear rate is> 100 ^s-1 , the viscosity varies not so much, and after reaching a shear rate of 200 ^s-1 it remains almost unchanged. The value measured at a high shear rate is called asymptotic. The principal factors that affect viscosity are the deformability of cell elements, hematocrit and aggregation. And taking into account the fact that there are much more red blood cells in comparison with platelets and leukocytes, they are mainly determined by red cells. This is reflected in the rheological properties of the blood.

Viscosity factors

The most important factor determining viscosity is the volume concentration of erythrocytes, their average volume and content, this is called hematocrit. It is approximately 0.4-0.5 l / l and is determined by centrifugation from a blood sample. Plasma is a Newtonian fluid whose viscosity determines the composition of proteins, and it depends on temperature. The viscosity is most affected by globulins and fibrinogen. Some researchers believe that the most important factor that leads to a change in the viscosity of the plasma is the protein: albumin / fibrinogen, albumin / globulin ratio. The increase occurs with aggregation, determined by the non-Newtonian behavior of whole blood, which determines the aggregative capacity of red blood cells. Aggregation of erythrocytes physiological is a reversible process. That's what it is - the rheological properties of blood.

The formation of aggregates by erythrocytes depends on mechanical, hemodynamic, electrostatic, plasma and other factors. Nowadays there are several theories that explain the mechanism of erythrocyte aggregation. The most known today is the theory of the bridge mechanism, according to which bridges from large-molecule proteins, fibrinogen, and Y-globulins are adsorbed on the surface of erythrocytes. The aggregation strength is pure - this is the difference between the shear force (causes disaggregation), the layer of electrostatic repulsion of erythrocytes, which are charged negatively, by the force in the bridges. The mechanism responsible for fixing negatively charged macromolecules on erythrocytes, that is, Y-globulin, fibrinogen, is not yet completely understood. There is an opinion that the molecules are linked due to the dispersed van der Waals forces and weak hydrogen bonds.

What helps to evaluate the rheological properties of the blood?

What is the reason for the aggregation of red blood cells?

The explanation of erythrocyte aggregation is also explained by the depletion, the absence of high-molecular proteins close to erythrocytes, in connection with which there appears a pressure interaction, similar in nature to the pressure of the macromolecular solution by osmotic, resulting in the approach of the suspended particles. In addition, there is a theory linking the aggregation of erythrocytes with erythrocyte factors, leading to a decrease in the zeta potential and a change in the metabolism and form of erythrocytes.

Because of the relationship between the viscosity and the aggregation capacity of erythrocytes, in order to evaluate the rheological properties of the blood and the features of its movement through the vessels, it is necessary to conduct a complex analysis of these indicators. One of the most common and quite accessible methods for measuring aggregation is the evaluation of the rate of erythrocyte sedimentation. However, the traditional version of this test is not very informative, since it does not take into account the rheological characteristics.

Methods of measurement

According to studies of rheological blood characteristics and factors that affect them, it can be concluded that the evaluation of the rheological properties of blood is affected by the aggregation state. In our time, researchers are paying more attention to the study of microrheological properties of this liquid, however, and viscometry has also not lost relevance. The main methods for measuring blood properties can be conditionally divided into two groups: with a field of stresses and deformations, a homogeneous cone, disc, cylindrical and other rheometers having different geometry of the working parts; With a field of deformations and stresses relatively inhomogeneous - according to the registration principle of acoustic, electrical, mechanical oscillations, devices that work by the Stokes method, capillary viscometers. This is how the rheological properties of blood, plasma and serum are measured.

Two types of viscometers

Two types of viscosimeters are now the most widespread : rotational and capillary. Also viscometers are used, the inner cylinder of which floats in the liquid that is being tested. Now actively engaged in various modifications of rotational rheometers.

Conclusion

It is also worth noting that the noticeable progress in the development of rheological techniques allows one to study the biochemical and biophysical properties of blood in order to control micro-regulation in metabolic and hemodynamic disorders. Nevertheless, the development of methods for the analysis of hemorheology, which would objectively reflect the aggregation and rheological properties of the Newtonian fluid, is relevant at the moment.