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 larger change in pressure for a given change in volume. 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, when a heart structurally remodels in response to chronic volume or pressure overload 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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