C = ΔV / ΔP
For example, if an isolated segment of an artery is cannulated and the volume of fluid within the segment is slowly increased while measuring pressure, a plot of volume versus pressure can be generated (see figure). The slope of the curve at any given pressure and volume (dV/dP) represents the compliance of the vessel at that pressure and volume. This figure shows that the compliance of the artery is not constant and decreases as the vessel expands – at higher volumes and pressures there is a disproportionate decrease in the volume change for a given change in pressure. Another way to think of this is that the "stiffness" of the vessel wall (reciprocal of compliance) increases at higher volumes and pressures. This non-linearity in the relationship between volume and pressure is common in biological tissues.
Compliance is a fundamental property of a tissue; however, the compliance can be modified by histological changes in the tissue. This occurs, for example, in the heart when it remodels in response to chronic volume or pressure overloaded conditions. In a blood vessel, compliance can also be altered by contraction of smooth muscle in the vessel wall, which decreases the compliance of the vessel..
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