Blood viscosity corresponds to the viscosity of blood, which depends on parameters such as blood composition and temperature. Blood does not behave like a Newtonian fluid but exhibits nonproportional and erratic viscosity. Pathological changes in viscosity are present, for example, in hyperviscosity syndrome.
What is blood viscosity?
Blood viscosity corresponds to the viscosity of blood, which depends on parameters such as blood composition and temperature. Viscosity is considered a measure of the viscosity of liquids or fluids. The higher the viscosity, the more likely it is to be a viscous fluid. A high viscosity thus characterizes a fluid as having less flowability. The particles within a viscous fluid are bound together to a greater extent and are therefore relatively immobile. The fluids of the human body also have a certain viscosity. Some of them behave as Newtonian fluids and exhibit linear viscous flow behavior. This is not true for human blood. The term blood viscosity is associated with the viscosity of blood, which, unlike other body fluids, does not behave as a Newtonian fluid and is therefore not characterized by linear viscous flow behavior. Rather, the flow behavior of blood is non-proportional and erratic and is sometimes governed by the so-called Fåhraeus-Lindqvist effect. By the term Fåhraeus-Lindqvist effect, medicine refers to the characteristic behavior of blood whose viscosity changes as a function of vessel diameter. Thus, in vessels with a small diameter, the blood is less viscous to prevent capillary stasis (congestion). Thus, blood viscosity is characterized by viscosity differences in different parts of the circulation.
Function and purpose
Because of its characteristic properties, blood is not a Newtonian fluid. Its non-proportional and erratic flow behavior is mainly determined by the Fåhraeus-Lindqvist effect. The Fåhraeus-Lindquist effect is based on the fluidity and thus the deformability of red blood cells. Shear forces are generated near the vessel walls. These shear forces displace the erythrocytes of the blood into the so-called axial flow. This process is also known as axial migration and results in a cell-poor marginal flow, in which the plasma marginal flow around the cell acts as a kind of sliding layer for the blood, making it appear more fluid. This effect reduces the hematocrit influence on peripheral resistance within smaller vessels and reduces frictional resistance. In addition to the Fåhraeus-Lindquist effect, many other parameters determined blood viscosity. For example, the viscosity of human blood depends on hematocrit, erythrocyte deformability, erythrocyte aggregation, plasma viscosity, and temperature. Flow velocity also has an effect on viscosity. Blood viscosity is the subject of viscometry and hemorheology. Viscosimetry determines the viscosity of liquids on the basis of the flow capacity, resistance and internal friction, each of which is dependent on temperature and pressure. The viscosity of plasma can be measured using capillary viscometers. In order to determine blood viscosity, on the other hand, the effect of shear forces must be taken into account. Hemorheology corresponds to the flow properties of blood, which depend on parameters such as blood pressure, blood volume, cardiac output and blood viscosity, as well as on vascular elasticity and lumen geometry. The modification of these individual parameters controls the blood flow to the tissues and organs in such a way that their demand for nutrients and oxygen is ideally optimally met. Control of flow behavior is primarily the responsibility of the autonomic nervous system. Blood viscosity interacts with the flow behavior of the blood and thus also changes in order to ensure an optimal supply of nutrients and oxygen to the tissues. Effects such as erythrocyte aggregation are thus ultimately required for blood supply to the tissues. In medicine, this aggregation is understood to be the agglomeration of red blood cells that occurs as a result of attractive forces between erythrocytes and acts at a slow flow rate of the blood stream. Erythrocyte aggregation essentially determines blood viscosity.
Diseases and ailments
Because there is a close relationship between viscosity, flow dynamics, and the supply of nutrients and oxygen to body tissues, blood viscosity disorders can have serious consequences on the entire organism. A disturbance of blood viscosity underlies, for example, the hyperviscosity syndrome. This clinical complex of symptoms is characterized by an increase in the paraprotein concentration in the blood plasma. As a result, the viscosity of the blood increases and its flowability is reduced. The viscosity of blood depends on the physical and chemical properties within the fluid and accordingly changes with each abnormal concentration of its individual components. For example, the hyperviscosity syndrome characterizes Waldenström’s disease. In this disease, the concentration of IgM in the blood increases. The IgM is a large molecule of Y-shaped units and in a plasma concentration of 40 g/l is sufficient for the development of hyperviscosity syndrome. The hyperviscosity syndrome due to paraproteins also characterizes malignant diseases, such as multiple myeloma. In some benign diseases, the syndrome may also be present, especially in Felty’s syndrome, in the context of lupus erythematosus or in rheumatoid arthritis. Increased viscosity of blood is also associated with phenomena such as thrombosis. In most cases, thrombosis is also related to a change in flow velocity or altered blood composition. Decreased flow velocity may be present, for example, in the context of immobilization, especially in bedridden patients. Abnormal blood viscosity may also be associated with erythrocyte disorders. In the context of spherocytosis, for example, spherical instead of disc-shaped erythrocytes are produced. This change in shape shows effects on blood viscosity because the erythrocytes in this shape no longer have all the necessary properties.
