THE TRIPHASIC EFFECTS OF EXERCISE ON BLOOD RHEOLOGY - WHICH RELEVANCETO PHYSIOLOGY AND PATHOPHYSIOLOGY

The life-extending effects of regular exercise are related to a decrea se in both coronary and peripheral vascular morbidity, associated with some improvements in cardiovascular risk factors. A possible link bet ween the beneficial metabolic and hemodynamic effects of exercise coul d be blood rheology, which is markedly affected by exercise. We propos e here a description of the hemorheological effects of exercise as a t riphasic phenomenon. Short-term effects of exercise are an increase in blood viscosity resulting from both fluid shifts and alterations of e rythrocyte theologic properties (rigidity and aggregability). Increase d blood lactate, stress, and acute phase play a role in this process. Middle-term effects of regular exercise are a reversal of these acute effects with an increase in blood fluidity, explained by plasma volume expansion (autohemodilution) that lowers both plasma viscosity and he matocrit. Long-term effects further improve bleed fluidity, parallel w ith the classical training-induced hormonal and metabolic alterations. While body composition, blood lipid pattern, and fibrinogen improve ( thus decreasing plasma viscosity), erythrocyte metabolic and theologic properties are modified, with a reduction in aggregability and rigidi ty. On the whole, these improvements reflect a reversal oi the so-call ed ''insulin-resistance syndrome'' induced by a sedentary lifestyle. S ince impaired blood rheology has been demonstrated to be at risk for v ascular diseases, the hemorheologic effects of exercise can be hypothe sized to be a mechanism (or at least a marker) of risk reversal. This latter point requires further investigation. The physiological meaning of the triphasic pattern of exercise-induced alterations of blood rhe ology is uncompletely understood, but increased blood fluidity may imp rove several steps of oxygen transfer to muscle, as clearly demonstrat ed in hypoxic conditions. Increasing evidence emerges from the literat ure, that blood fluidity is a physiological determinant of fitness.