The field of mechanics that studies the features of the deformation and flow of real continuous media, some of which are non-Newtonian fluids with structural viscosity, is rheology. In this article, we consider the rheological properties of blood. What is it, it will become clear.
Definition
A typical non-Newtonian fluid is blood. Plasma is called if it is devoid of uniform elements. Blood serum is a plasma in which fibrinogen is absent.
Hemorheology, or rheology, studies the mechanical laws, especially how the physical properties of blood change during circulation at different speeds and in different parts of the bloodstream. Its properties, the functional state of the bloodstream, and the contractility of the heart determine the movement of blood in the body. When the linear flow velocity is small, blood particles are displaced parallel to the axis of the vessel and to each other. In this case, the flow is layered, and the flow is called laminar. So what are rheological properties? About it - further.
What is the Reynolds number?
In the case of an increase in the linear velocity and the excess of a certain value, different for all vessels, the laminar flow will turn into a vortex, disordered, called turbulent. The rate of transition of laminar to turbulent motion determines the Reynolds number, which is approximately 1160 for blood vessels. According to the Reynolds numbers, turbulence can only occur in places where large vessels branch, as well as in the aorta. In many vessels, the fluid moves laminarly.
Shear rate and stress
Not only the volumetric and linear blood flow velocity are important, two other important parameters characterize the movement to the vessel: speed and shear stress. Shear stress is characterized by the force acting on a unit of the vascular surface in the tangential direction to the surface, measured in pascals or dyne / cm 2 . Shear rate is measured in seconds reverse (s-1), and it means the magnitude of the gradient of the speed of movement between parallel moving fluid layers per unit distance between them.
What indicators depend on the rheological properties?
The ratio of stress to shear rate determines blood viscosity, measured in mPas. For a whole fluid, viscosity depends on a shear rate range of 0.1-120 s-1 . If the shear rate is> 100 s-1 , the viscosity does not change so markedly, and when shear rate of 200 s-1 is reached, it does not change much. The value measured at high shear rate is called asymptotic. The principal factors that influence viscosity are the deformability of cell elements, hematocrit and aggregation. And taking into account the fact that there are much more red blood cells compared with platelets and white blood cells, they are mainly determined by red cells. This is reflected in the rheological properties of the blood.
Viscosity factors
The most important determinant of viscosity is the volume concentration of red blood cells, 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. Viscosity is most affected by globulins and fibrinogen. Some researchers believe that the more important factor that leads to a change in plasma viscosity is the ratio of proteins: albumin / fibrinogen, albumin / globulins. The increase occurs during aggregation, determined by the non-Newtonian behavior of whole blood, which determines the aggregation ability of red blood cells. Physiological red blood cell aggregation is a reversible process. This is what the rheological properties of blood are.
The formation of aggregates by red blood cells depends on factors of mechanical, hemodynamic, electrostatic, plasma and others. Nowadays, there are several theories that explain the mechanism of red blood cell aggregation. The most famous theory today is the bridge mechanism by which bridges of large molecular proteins, fibrinogen, Y-globulins are adsorbed on the surface of red blood cells. The aggregation force is pure - this is the difference between the shear force (causes disaggregation), the layer of electrostatic repulsion of red blood cells, which are negatively charged, by the force in the bridges. The mechanism responsible for the fixation of negatively charged macromolecules on red blood cells, that is, Y-globulin, fibrinogen, is not yet fully understood. There is an opinion that molecules are linked due to the dispersed van der Waals forces and weak hydrogen bonds.
What helps to evaluate the rheological properties of blood?
What is the reason for red blood cell aggregation?
The explanation of erythrocyte aggregation is also explained by depletion, the absence of high molecular weight proteins close to red blood cells, in connection with which there is a pressure interaction similar in nature to the pressure of a macromolecular solution osmotic, leading to the approach of particles suspended. In addition, there is a theory linking the aggregation of red blood cells with red blood cells, leading to a decrease in the zeta potential and a change in the metabolism and form of red blood cells.
Due to the relationship between the viscosity and aggregation ability of red blood cells, in order to evaluate the rheological properties of blood and the peculiarities of its movement through the vessels, it is necessary to conduct a comprehensive analysis of these indicators. One of the most common and widely available methods for measuring aggregation is to evaluate the rate of red blood cell sedimentation. However, the traditional version of this test is uninformative, since it does not take into account the rheological characteristics.
Measurement methods
According to studies of rheological blood characteristics and factors that influence them, we can conclude that the aggregation state affects the assessment of rheological properties of blood. Nowadays, researchers pay more attention to the study of the micro-rheological properties of this fluid, however, viscometry also has not lost relevance. The main methods for measuring the properties of blood can be divided into two groups: with a uniform stress and strain field - con-plane, disk, 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 vibrations, devices that work according to the Stokes method, capillary viscometers. This measures the rheological properties of blood, plasma and serum.
Two types of viscometers
Two types of viscometers are most widespread now : rotational and capillary. Viscometers are also used, the inner cylinder of which floats in the fluid that is being tested. Now they are actively engaged in various modifications of rotational rheometers.
Conclusion
It is also worth noting that the noticeable progress in the development of rheological technology just allows us to study the biochemical and biophysical properties of blood in order to control microregulation in metabolic and hemodynamic disorders. Nevertheless, the development of methods for the analysis of hemorheology that would objectively reflect the aggregation and rheological properties of Newtonian fluid is relevant at the moment.