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Blood flow depends on driving pressure and a resistance factor, the latter being related to geometrical hindrance and to the intrinsic viscosity of the blood. Since whole blood is non-Newtonian in nature, blood viscosity is strongly dependent on shear conditions. Low-shear areas occur in cardiovascular disease, and therefore the interaction between blood viscosity and flow conditions may affect vascular disorders. Increased shear stress secondary to increased viscosity may produce endothelial activation and release of endothelium-derived relaxing factors, leading to flow-dependent vasodilation. All the determinants of blood rheology, including plasma protein and erythrocyte factors may be altered in patients with arterial hypertension.A hyperviscosity state is created which is associated with an unfavourable prognosis, since it is correlated with blood pressure levels and the severity and complications of the disease including left ventricular hypertrophy. The mechanisms of haemorheological abnormalities in hypertension are still unclear. It is not known whether blood rheology is an independent variable in patients with hypertension or whether it is a covariable with other established indices of heterogeneity. However, many aetiopathological changes identified in hypertensive disease may contribute to the observed changes in blood rheology. Haemorheological changes in hypertension, through complex interactions with platelet activation and endothelial function, may contribute to the development of thrombosis and atherosclerosis. Moreover, in acute and chronic ischaemia and other conditions where compensatory mechanisms such as collateral formation and vasodilation are limited, rheological factors may become important determinants of blood flow and tissue oxygenation.Many antihypertensive agents have direct or indirect potential effects on haemorheological variables. However, to date, most studies that have investigated the effects of therapy on rheological variables have not been performed in clinically relevant situations. Controlled studies that monitor both the acute and longterm effects of antihypertensive drugs on relevant haemorheological variables are required to show whether specific therapeutic approaches can correct abnormalities in blood rheology.