Elastic porous tube model of reactive hyperaemia.

Reactive hyperaemia is the term given to the temporary increase in blood flow that follows release of an occlusion of the arterial supply. Measurement of reactive hyperaemia in the leg below the knee is useful in assessment of the vascular system, as resting flows remain unaffected even in the presence of quite severe occlusive arterial disease. An elastic porous tube representation of the vascular system is used to develop equations for the variation of the mean pressure, flow and vessel calibre in the vascular system. The tube represents the arteries and large arterioles, which respond passively to changes in pressure. Leakage through the tube walls represents flow into the small arterioles, which respond actively to the rise in pressure following release of the occlusion by constricting (the myogenic response). The capillaries are represented by rigid tubes, and the venous system is represented by a single compliant vessel. The model predicts variations in the flow, pressure and vessel calibre that are in agreement with experimental observations, and identifies that the pressure gradient is important in determining the initial transient increase in the flow following release of the occlusion. The subsequent development in the flow is governed by the small arteriolar flow, which is determined by the magnitude and duration of the myogenic response.