The present review focuses on the past and recent knowledge in the field of exercise hemorheology and presents some unresolved issues for opening discussion. Acute exercise is associated with a rise in hematocrit which results in an increase in blood viscosity. Whereas increased blood viscosity was previously viewed as having negative consequences for cardiovascular function and aerobic performance, recent findings suggest dynamic changes in blood viscosity might be useful for vascular function during exercise by increasing nitric oxide production. Other determinants of blood viscosity are altered by exercise (e.g., decreased red blood cell deformability, increased red blood cell aggregation and plasma viscosity) and may, independent of the associated effect on blood viscosity, directly modulate aerobic capacity. However, the data published on the effects of exercise on the hemorheology are not consistent, with some studies showing decreased, unchanged, or increased red blood cell deformability/aggregation when compared with rest. These discrepancies seem to be related to the exercise protocol investigated, the population tested or the methodogy utilized for hemorheological measurements. Finally, this review focuses on the effects of training exercise training (i.e. chronic physical activity) on the hemorheological profile of healthy individuals and patients with cardiovascular and metabolic disorders.

Since the identification of the elusive endothelium-derived relaxing factor as nitric oxide (NO), much attention has been devoted to understanding its physiological effects. NO is a free radical with many roles, and owing to its neutral charge and hi

Previous studies have demonstrated that red blood cells (RBC) either lyse or at least experience mechanical damage following prolonged exposure to high shear stress (≥100 Pa). Conversely, prolonged shear stress exposure within the physiological range (5-20 Pa, 300 s) was recently reported to improve RBC deformability. This study investigated the relationships between shear stress and RBC deformability to determine the breakpoint between beneficial vs. detrimental exposure to shear stress (i.e., "subhemolytic threshold"). A second aim of the study was to determine whether the frequency of intermittent application of shear stress influenced the subhemolytic threshold. RBC were exposed to various levels of shear stress (0-100 Pa) in a Couette type shearing system for 300 s. RBC deformability was then immediately measured via ektacytometry. Parallel experiments were conducted at the same shear stresses, except the application time differed while keeping constant the total exposure time: shear stress was applied either for 30 s and repeated 10 times (10×30 s) or applied for 15 s and repeated 20 times (20×15 s). For a range of donors, the subhemolytic threshold with constant shear stress application was between 30-40 Pa. When physiological shear stress was applied in an intermittent manner, more frequent applications tended to improve (i.e., increase) RBC deformability. However, when supra-physiological shear stress was applied, both continuous and intermittent protocols damaged RBC. Changes of RBC mechanical behavior occurred without increases of hemoglobin in the suspending media, thus attesting to the absence of hemolysis. Shear stress has a biphasic effect on the mechanical properties of RBC, with the duration and rate of exposure appearing to have minimal impact on the subhemolytic threshold when compared with the magnitude of applied shear stress.