Blood viscosity

The Blood viscosity corresponds to the viscosity of the blood, which depends on parameters such as blood composition and temperature. The blood does not behave like a Newtonian fluid, but shows a non-proportional and erratic viscosity. Pathological changes in viscosity are present, for example, in hyperviscosity syndrome.

What is the blood viscosity?

Viscosity is a measure of the viscosity of liquids or fluids. The higher the viscosity, the more likely it is to speak of a thick liquid. A high viscosity thus characterizes a fluid as less flowable. The particles within a viscous fluid are to a greater extent bound to one another and, as a result, are relatively immobile.

The fluids in the human body also have a certain viscosity. Some of them behave as Newtonian liquids and show linear viscous flow behavior. This does not apply to human blood. The term blood viscosity is associated with the viscosity of the blood, which, unlike other body fluids, does not behave as a Newtonian fluid and is therefore not characterized by linearly viscous flow behavior.

The flow behavior of blood is rather non-proportional and erratic and is sometimes determined by the so-called Fåhraeus-Lindqvist effect. With the expression of the Fåhraeus-Lindqvist effect, medicine refers to the characteristic behavior of the blood, the viscosity of which changes depending on the vessel diameter. In vessels with a small diameter, the blood is therefore less viscous in order to prevent capillary stasis (congestion). Thus, the blood viscosity in different areas of the blood circulation is characterized by viscosity differences.

Function & task

Due to 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 arise near the vessel walls. These shear forces displace the erythrocytes of the blood in the so-called axial flow. This process is also known as axial migration and results in a low-cell edge flow, in which the plasma edge 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 the peripheral resistance within smaller vessels and the frictional resistance decreases.

In addition to the Fåhraeus-Lindquist effect, many other parameters determine blood viscosity. The viscosity of human blood depends, for example, on hematocrit, erythrocyte deformability, erythrocyte aggregation, plasma viscosity and temperature. The flow rate also has an influence on the viscosity.

Viscometry and hemorheology deal with blood viscosity. Viscometry determines the viscosity of liquids based on the temperature and pressure-dependent fluidity, resistance and internal friction. The viscosity of the plasma can be measured using a capillary viscometer. To determine the blood viscosity, however, the effects 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 the vascular elasticity and the lumen geometry. Changing these individual parameters controls the blood flow in the tissues and organs in such a way that their need for nutrients and oxygen is ideally met.

The control of the flow behavior is primarily the responsibility of the vegetative nervous system. The 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 therefore ultimately necessary for the blood supply to the tissue. Medicine understands this aggregation to be the agglomeration of red blood cells, which is created due to the forces of attraction between erythrocytes and which work at a slow flow rate of the blood stream. The erythrocyte aggregation essentially determines the blood viscosity.

Illnesses & ailments

Since there is a close connection between the viscosity, the flow dynamics and the supply of body tissues with nutrients and oxygen, disturbances in the blood viscosity can have serious consequences for the entire organism. A disorder of the blood viscosity, for example, is the basis of the hyperviscosity syndrome. This clinical complex of symptoms is characterized by an increase in the concentration of paraprotein in the blood plasma. This increases the viscosity of the blood and reduces its ability to flow.

The viscosity of the blood depends on the physical and chemical properties within the fluid and changes accordingly with any abnormal concentration of its individual components. The hyperviscosity syndrome, for example, characterizes Waldenström's disease. With this disease, the IgM concentration in the blood increases. The IgM is a large molecule made up of Y-shaped units and, in a plasma concentration of 40 g / l, is sufficient for the development of a hyperviscosity syndrome.

The hyperviscosity syndrome due to paraproteins also characterizes malignant diseases such as multiple myeloma. The syndrome can also be present in some benign diseases, especially in Felty's syndrome, lupus erythematosus or rheumatoid arthritis.

An increased viscosity of the blood is also associated with symptoms such as thrombosis. In most cases, thrombosis is also related to a change in the flow rate or a change in the composition of the blood. A reduced flow rate can occur, for example, in the context of immobilization, especially in bedridden patients.

An abnormal blood viscosity can also be associated with diseases of the erythrocytes. As part of spherocytosis, for example, spherical instead of disk-shaped erythrocytes are produced. This change in shape has an effect on the blood viscosity, since the erythrocytes no longer have all the necessary properties in this shape.