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The effects of manipulations of blood pressure and flow on the diameter of the femoral artery in dogs were studied. Diameter measurements were made utilizing real time ultrasound scanning. Although diameter of a flexible tube in a mechanical system varies with internal pressure, in the intact femoral artery the diameter varied with changes in blood flow rate rather than pressure. Increase in blood flow rate resulted in increase in vessel diameter, whether the pressure changed or remained the same. Decrease in blood flow rate resulted in decrease in diameter, irrespective of the pressure. However, when a segment of the femoral artery was isolated from the femoral artery by interposition between plastic tubes, the diameter of the isolated segment no longer increased when flow increased. These findings suggest that arterial diameter is regulated by a conducting mechanism that depends on vascular continuity.