The area of mechanics that studies the features of deformation and flow of real continuous media, one of the representatives of which are non-Newtonian fluids with structural viscosity, is rheology. In this article, we consider the rheological properties of blood. What it is will become clear.
Definition
A typical non-Newtonian fluid is blood. It is called plasma if it is devoid of formed elements. Serum is plasma that lacks fibrinogen.
Hemorheology, or rheology, studies mechanical patterns, especially how the physical and colloidal properties of blood change during circulation at different speeds and in different parts of 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 flow velocity is low, the blood particles move parallel to the axis of the vessel and towards each other. In this case, the flow has a layered character, and the flow is called laminar. So what arerheological properties? More on that later.
What is the Reynolds number?
In the event of an increase in the linear velocity and exceeding a certain value, which is different for all vessels, the laminar flow will turn into a vortex, chaotic, called turbulent. The rate of transition from laminar to turbulent motion determines the Reynolds number, which is approximately 1160 for blood vessels. According to Reynolds numbers, turbulence can only occur in those places where large vessels branch, as well as in the aorta. Fluid moves laminar through many vessels.
Speed and shear stress
Not only the volumetric and linear velocity of blood flow is important, two more important parameters characterize the movement towards the vessel: velocity and shear stress. Shear stress characterizes the force acting on a unit of the vascular surface in the tangential direction to the surface, measured in pascals or dynes/cm2. The shear rate is measured in reciprocal seconds (s-1), which means it is the magnitude of the gradient of the velocity of movement between layers of fluid moving in parallel per unit distance between them.
On what indicators do rheological properties depend?
The ratio of stress to shear rate determines blood viscosity, measured in mPas. For a solid fluid, the viscosity depends on the shear rate range of 0.1-120s-1. If the shear rate is >100s-1, the viscosity changes not so pronounced, and after reaching the shear rate of 200s-1 almost nois changing. The value measured at high shear rate is called asymptotic. The principal factors that affect viscosity are the deformability of cell elements, hematocrit and aggregation. And given the fact that there are much more red blood cells compared to 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 factor determining 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, the viscosity of which determines the composition of proteins, and it depends on temperature. Viscosity is most affected by globulins and fibrinogen. Some researchers believe that a 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 aggregation of erythrocytes 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 well-known today is the bridging theory.the mechanism by which bridges from large molecular proteins, fibrinogen, Y-globulins are adsorbed on the surface of erythrocytes. The net aggregation force is the difference between the shear force (causes disaggregation), the electrostatic repulsion layer of erythrocytes, which are negatively charged, and the force in the bridges. The mechanism responsible for the fixation of negatively charged macromolecules on erythrocytes, that is, Y-globulin, fibrinogen, is not yet fully 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 assess the rheological properties of blood?
Why does erythrocyte aggregation occur?
Explanation of erythrocyte aggregation is also explained by depletion, the absence of high-molecular proteins close to erythrocytes, in connection with which a pressure interaction appears, similar in nature to the osmotic pressure of a macromolecular solution, leading to the convergence of suspended particles. In addition, there is a theory linking erythrocyte aggregation with erythrocyte factors, leading to a decrease in the zeta potential and a change in the metabolism and shape of erythrocytes.
Due to the relationship between the viscosity and aggregation ability of erythrocytes, in order to assess the rheological properties of blood and the features of its movement through the vessels, it is necessary to conduct a comprehensive analysis of these indicators. One of the most common and quite accessible methods for measuring aggregation is the assessment of the rate of erythrocytesedimentation. However, the traditional version of this test is not very informative, since it does not take into account rheological characteristics.
Measurement methods
According to studies of blood rheological characteristics and factors that affect them, it can be concluded that the assessment of the rheological properties of blood is affected by the aggregation state. Nowadays, researchers pay more attention to the study of the microrheological properties of this liquid, however, viscometry has also not lost its relevance. The main methods for measuring the properties of blood can be divided into two groups: with a homogeneous stress and strain field - cone-plane, disk, cylindrical and other rheometers with 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 is how the rheological properties of blood, plasma and serum are measured.
Two types of viscometers
Two types of viscometers are currently the most widespread: rotational and capillary. Viscometers are also used, the inner cylinder of which floats in the liquid 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 makes it possible to study biochemical and biophysicalblood properties to control microregulation 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 currently relevant.
