Endothelium dependent control of arterial diameter by blood viscosity.

The local control of arterial diameter by blood flow rate has been attributed to the ability of the endothelial cells to sense the shear stress exerted on them by flowing blood. Since shear stress is proportional to the product of flow rate and fluid viscosity, an increase in the latter will augment shear stress and lead to arterial dilatation as well. We therefore designed experiments to reveal the control of the arterial lumen by blood viscosity and to compare it with the control by blood flow rate. Changes in external diameter of a segment of feline femoral artery caused by variations in blood viscosity and/or flow rate were recorded during the perfusion in situ under stabilised transmural pressure. Decrease in blood viscosity evoked by haemodilution at a constant flow rate led to arterial constriction whereas an increase in viscosity due to haemoconcentration caused arterial dilatation. These viscosity induced changes in arterial diameter depended on intact endothelium. Responses to the changes in blood viscosity and flow rate of the same magnitude were practically identical. These results show that blood viscosity is a factor affecting vascular smooth muscle tone and confirm the suggestion that shear stress is the key signal in the endothelium dependent control of the arterial lumen by blood flow rate.