Blood viscosity and hemodynamics during exercise.

We tested the effects of submaximal exercise on blood viscosity (η(b)), nitric oxide production (NO) and hemodynamics. Relationships between the exercise-induced changes that occurred in these parameters were investigated. Nine subjects performed exercise for 15 min at 105% of the first ventilatory threshold. Mean arterial pressure (MAP) and cardiac output (Qc) were measured, allowing the determination of systemic vascular resistance (SVR). Blood was sampled at rest and at the end of exercise. The η(b) was determined at high shear rate and was used to calculate systemic vascular hindrance (VH). NO production was estimated by measuring plasma concentrations of NO stable end products (NOx). Qc, MAP, η(b) and NOx, increased with exercise, whereas SVR and VH decreased. The changes between rest and exercise were calculated and tested for correlations. We observed: 1) a positive correlation between the increase in η(b) and the increase in NOx; 2) a negative correlation between the increase in NOx and the decrease in VH; 3) a negative correlation between the increase in η(b) and the decrease in SVR. Although the increase in Qc and blood flow during exercise probably promoted NO production due to shear dependent stimulation of the endothelium, the present results also support that the rise in η(b) during exercise may be necessary for NO production and adequate vasodilation.