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Rheologic properties of blood

Rheology is the science studying the flows of liquids and gases. The following fluids are distinguished:

— Newtonian fluids — homogeneous the viscosity of which does not alter, when the rate of their movement changes;

— non-newtonian fluids — suspensions for which a dependence of viscosity on the velocity of fluid movement is typical: their viscosity is inversely proportional to the velocity.

Plasma is referred to non-newtonian fluid, since there are formed elements of blood in the form of suspension in it, and, in the first turn, — erythrocytes. Erythrocytes are able to deform while passing through capillaries, the diameter of which is lesser than the diameter of erythrocytes. So, a formation of erythrocytic aggregates in the form of a column of coins is possible in pulmonary capillaries that significantly decreases the blood flow velocty, elevates viscosity of blood and increases a load on the heart because of its efforts for propulsion of such blood along the vessels.

Inside the erythrocyte is positively charged, outside — negatively, and:he additional negative charge is created on its external surface as a result of erythrocyte friction against plasma in the course of its movement. The difference between the charges inside and outside the erythrocyte is normally equal to 35 mV and got the name of Z-potential.

Inside the intima of vessels is charged negatively. While the erythrocytes are moving along the vessels, repulsive forces start to act, therefore, the formed elements of blood move along the vessels in the center, and by the edges — plasma (laminar stream) that serves as if a lubricant for erythrocytes.

In the loss of Z-potential in different pathologic processes, a laminar stream is disturbed that leads to the increase of blood viscosity. The latter, by the law of nonnewtonian fluid, leads to a decrease of blood flow velocity, activation of erythrocyte aggregation processes. Aggregates of erythrocytes close the lumina of capillaries, and areas of tissue that they serve, are left subject to ischemia with all ensuing consequences.

Aggregation of erythrocytes leads to a slowing down of their advance along the vessels. A drop of electric charge contributes to the appearance of protsin molecules with adhesive substances (crystals of bilirubin, uric acid, etc.) on the surface of erythrocytes. All this leads to the deformation of erythrocytes, changes of plasmatic properties of blood and creates some prerequisites for injury of cells with release of degradation products, triggering the mechanisms of blood coagulation. Fibrin precipitates on the aggregations of erythrocytes, i.e. arise conditions for development of DIC and thrombohemorrha.'ic syndrome. On this background ischemia of tissues and their hypoxia are intensified even greater, acidosis grows on. As a result, a disturbance of function of cellular respiratory ferments occurs, metabolism decreases, that, after all,

leads to the destruction of lysosomas with release of lysosomal enzymes that lyse cell membrane leading to its destruction (necrosis).

Disturbances of rheologic properties of blood in all critical conditions pass through four stages:

1. Spasm of vessels of resistance as a result of stress reaction with release of endogenic catecholamines, a decrease of blood flow velocity in the capillaries.

2. Disturbance of microcirculation, decrease and even cessation of blood flow, ischemia of tissues, elevation of content of acidic products of metabolism in the blood, as well as biologically active substances that try without success to restore a blood flow in the capillaries. A stratification of blood flow occurs when plasma still continues to pass through capilaries and carries with itself away metabolytes and BAS, in high concentrations, into a common blood flow. Functional activities of organs sharply decrease, their useful participation in vital activity of the organism ceases.

3. The aggregation, sludge of formed elements and intravascular coagulation in capillaries lead to the sequestration of blood and a decrease of VCB. It is therefore that under any critical states hypovolemia develops, even if the state is not associated with blood or plasma losses.

4. Sequestrated blood has microdots. In case of its return to a common blood flow it passes through a pulmonary-capillary filter causing development of "shock lung" syndrome.

The simplest criteria of microcirculatory disorders are:

— difference of temperature in the rectum and of skin is more than 4°C;

— metabolic acidosis: pH < 7.35, BE is negative;

— decrease of A-V difference by oxygen.

The principles of physiologic therapy of rheologic disorders:

— to increase a blood flow velocity;

— hemodilution down to a decrease of Ht to 0.35;

— anticoagulants;

— mean molecular blood.substitutes (rheopolyglucin or neohemodex) decrease blood viscosity to 40% at the expense of Z-potential increase on the erythrocyte membranes and formation of hypocoagulation complexes with fibrinogen.

The following increase the viscosity of blood:

1. Globulin fractions of protein, fibrinogen, cholesterol. Hence, practical recommendations are the following: in the acute period of critical states do not apply agents increasing immunity before a rheology of blood is not restored.

2. Hypercapnia

3. Acidosis (decrease of pH by 0.5 leads to the triple increase of blood viscosity

4. Nicotine

5. Diabetes mellitus, obesity, hypertensive disease

6. Hypovolemia. When Ht elevates up to 55 a blood viscosity increases 10 times.

Albumin and rheopolyglucin cause a decrease of blood viscosity. There are some data that the electric field around a man exerts its influence upon blood viscosity.

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