## 2 Topics and Results
### Blood Flow
#### 2.3.2 Blood Viscosity as a Regulator of Vasodilation
It should also be emphasized that the physiological consequences for the body, and especially for the circulatory system, during a single intense physical exercise are poorly studied and are most likely influenced and depend on the individual characteristics and fitness characteristics of the exerciser. In his studies, [PERSON] (2010) [71] investigated the relationships between changes in blood viscosity, vascular resistance, NO synthesis and oxygen consumption induced by a single (submaximal) physical exercise in a healthy human population. From his findings, it appears that the established increase in blood viscosity during physical exercise may be a necessary condition for NO production and induction of vasodilation to achieve thehighest aerobic efficiency of working muscles [71]. However, any inappropriate increase in hematocrit can also increase blood viscosity--even to very high levels, thereby affecting cardiac and aerobic performance and activity, independent of increased arterial oxygen (O\({}_{2}\)) content. In another study, [PERSON] et al. (2010) [70], demonstrated that each individual can reach an optimal level of hematocrit and/or increased blood viscosity--in order to achieve the highest aerobic efficiency when exposed to the corresponding physical load. In an interesting analysis [PERSON] et al. (2011) [72], reported that whole blood viscosity (WBV) in volleyball players increased about 15% after 15 min of exercise. At the same time, the blood viscosity of athletes is lower compared to the WBV of untrained individuals. The short-term effect of physical (training) efforts on viscosity (hemoconcentration, hyperviscosity) is transient and does not affect the general condition of athletes whose blood and blood cells are characterized by normal rheological (hemorheological) parameters, including a lower viscosity of the overall blood (WBV), compared to untrained individuals (healthy subjects, controls). The rheological behavior of whole blood (reported by measuring WBV) has been comparatively studied in different categories of elite athletes (nationally and internationally famous athletes in football, volleyball and karate, including in the \"overtraining\" syndrome) with a view to the thesis that the positive influence of fitness is associated with decreased blood viscosity. In parallel, basic biological parameters such as zinc in serum, fibrinogen in blood plasma, plasma viscosity (PV), hematocrit and other biomarkers were also investigated [73]. Positive fitness effects on hemodynamics are reported: increased blood volume and decreased whole blood viscosity. The low level of serum zinc and plasma ferritin associated with the term \"zinc-deficient athletes\" correlates with erythrocyte deformability and aggregability, but also requires the planning of additional future studies [73; 74; 75]. Hemorheological changes in blood and blood cells, such as increased blood and plasma viscosity, decreased erythrocyte and platelet deformability and increased cell (intercellular) aggregation, high levels of hematocrit and fibrinogen in the blood, are accurate and up-to-date hemorheological biomarkers. They are successfully used in modern biomedicine for the purposes of prevention, diagnosis, monitoring and prognosis of the outcome of socially significant diseases [5]. Increased blood and plasma viscosity (WBV, PV) correlate with the development of cardiovascular and neurovascular diseases, including hypertension, atherosclerosis, peripheral vascular diseases, metabolic diseases such as diabetes (type 1 and 2), various pathological metabolic processes as a result of old age (often associated with problems in cholesterol and lipoprotein levels, with smoking, alcoholism, drug addictions and other addictions, with autoimmune reactions, psychiatric disorders, malignant (neoplastic) diseases (cancer and leukemia), metabolic syndrome and others).