## 2 Topics and Results
### Blood Flow
#### 2.3.2 Blood Viscosity as a Regulator of Vasodilation  
The shape of normal red blood cells is well known: at rest they resemble a double concave disc. Erythrocytes can undergo deformation and acquire the shape of echinocytes and stomatocytes under the influence of endogenous and exogenous factors, as well as return to their original resting shape when these factors are removed [62; 63; 64]. Echinocytes and stomatocytes are the most common morphological forms of red blood cells (in vitro and in vivo under the influence of various agents, as well as in various physiological and pathophysiological conditions) [65]. [PERSON]'s (1990) data show that when red blood cells are suspended in buffered saline, they can be transformed into echinocytes [66]. On the other hand, the shape and arrangement of erythrocytes transversely in the vascular lumen depend on the kinematics and dynamics of blood flow, strain rate and \"shear\" stress [67]. Any transformation in the shape of red blood cells, including those associated with changes in the biochemistry and morphology of their cell membrane, can cause specific hemorheological changes that are of great importance in clinical practice [65]. There are data in the literature on the influence of drug-induced echinocytosis and stomatocytosis on the viscosity of blood cell suspensions. Blood viscosity has been found to be increased in echinocytosis. [PERSON] et al. (2008) [65] found that under experimental conditions with erythrocyte aggregation (4 g/dL Dx 70, at a low shear rate of 0.1 s\({}^{-1}\)) a low degree of echinocytosis resulted in a greatly increased suspension viscosity. It is interesting that in all cases the viscosity of the suspension can be normalized by repeated \"transformation\" of echinocytes into normal red blood cells-discocytes. A lower suspension viscosity is then achieved and thus the best oxygen transport efficiency is obtained. According to Poiseuille's Law (1), vascular geometry is an extremely important factor determining blood flow, and blood viscosity is often ignored (except in borderline cases, such as polycythemia). With polycythemia, the hyperviscosity of the blood can contribute to the development of thrombotic complications and increase the pulse rate (tachycardia, tachyarrhythmia). The general \"rheological behavior\" of blood (including its rheological properties and those of its individual components-cells and blood plasma) is a key regulator of vascular functions and vascular resistance to flow. While increased blood viscosity is generally accepted as a risk for increasing blood flow resistance, several studies have shown that moderate increases in blood viscosity can positively influence endothelial vascular function and vasomotor vascular tone [68; 69]. Furthermore, on average, increased blood viscosity positively affects the physical endurance of healthy individuals during exercise [70]. An increase in blood viscosity leads to an increase in the \"shear\" stress (tangential stress) applied to the endothelial cells of the vessel wall. This leads to increased production of endothelial-derived nitric oxide (NO), which facilitates vasodilation, tissue perfusion, and increases the amount of O\({}_{2}\) delivered to cells and tissues [60; 68]. It should be noted here that patients with chronic vascular disease and endothelial dysfunction may not experience the beneficial effects of a modest increase in blood viscosity as healthy individuals. Intense exercise typically increases blood viscosity by 10-12%. This increase is mainly due to the reduced plasma volume, which leads to a higher hematocrit and an increase in viscosity. It is known that during prolonged training the hematocrit decreases (phenomenon of auto-hemodilution), and hence the viscosity of the blood also decreases [71].