Compliance is the ratio of change in volume of a structure when fluid is added, over the change in pressure. A rubber hose, for example, has greater compliance than an iron pipe.
The compliance of arteries is decreased with increasing age (i.e. greater stiffness), even in the absence of atherosclerosis (Figure 3.13). This is due to an increasing concentration of collagen in the wall, as well as a decreasing degree of slackness in the arrangement of collagen fibers in the wall. This can be seen in a length tension diagram for the arterial wall, where resisting tension rises earlier and to a greater extent in older vessels as compared to young vessels.
This results in a decreased ability of the wall to store pressure energy as stretch, and to give it back when pressure declines (Windkessel effect). This results in an increased pulse pressure and an increased pulse wave velocity.
The pulse wave is a wave of distension that passes down the vessel following each ventricular ejection. Pulse wave velocity is approximately ten times that of flow velocity, several meters per sec rather than cm/ sec.
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