The visco-elastic properties of the arterial wall.
The visco-elastic properties of the arterial wail are of importance in determining the behaviour of the circulatory system for the pulse wave is a relatively rapid event. It has been reported that arteries show very little change in length with each heart beat. There are no reports of studies on arterial behaviour carried out under conditions that take account of these two factors. Accordingly an investigation has been made of the pressure-diameter relations of various types of dogs' arteries under static and dynamic conditions. All experiments were performed with the vessel held at the length which it occupied in vivo. Measurements were also made of the relative wall thickness and of the amount of shortening which occurred when an artery was excised. Under static conditions all arteries show an elastic modulus which increases with distension; this is less marked, in the low pressure range, for the elastic thoracic aorta. This increasing modulus results both from the presence of various wall constituents arranged in parallel and from the non-linear elastic behaviour of these elements themselves. Under dynamic conditions the artery shows increased stiffness, the magnitude of this increase is proportional to the muscularity of its wall. This increase occurs at low frequencies and is compatible with what is known of the time-dependent properties of smooth muscle. Little further increase occurs at frequencies between 2 and 20 c/a. The dilatation also lags behind the pressure changes by a small amount. These factors will influence both the velocity and the attenuation of the pulse wave. The variation with frequency of these two factors has been calculated using the data obtained in these experiments. The results of this calculation stand comparison with published values for the pulse wave velocity
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