The ability of a blood vessel wall to expand and contract passively with changes in pressure is an important function of large arteries and veins. This ability of a vessel to distend and increase volume with increasing transmural pressure (inside minus outside pressure) is quantified as vessel compliance (C), which is the change in volume (ΔV) divided by the change in pressure (ΔP).
The volume-pressure relationship (i.e., compliance) for an artery and vein are depicted in Figure 1. Two important characteristics stand out. First, the slope is not linear because the blood vessel wall is a heterogeneous tissue. Therefore, compliance decreases at higher pressures and volumes (i.e., vessels become "stiffer" at higher pressures and volumes). Second, at lower pressures, the compliance of a vein is about 10 to 20-times greater than an artery. Therefore, veins can accommodate a large changes in blood volume with only a small change in pressure. However, at higher pressures and volumes, venous compliance (slope of compliance curve) becomes similar to arterial compliance. This makes veins suitable for use as arterial by-pass grafts.
There is no single compliance curve for a blood vessel. For example, vascular smooth muscle contraction, which increases vascular tone, reduces vascular compliance (Figure 2); conversely, smooth muscle relaxation increases compliance. This is particularly important in the venous vasculature for the regulation of venous pressure and cardiac preload. Contraction of smooth muscle in arteries reduces their compliance, thereby decreasing arterial blood volume and increasing arterial blood pressure. Another example of changing compliance is reduced aortic compliance with age or disease (e.g., arteriosclerosis). When this occurs, there is a qualitatively similar downward shift in the compliance curves as shown in Figure 2 for veins. Such compliance changes in the aorta are responsible in large part for the increase in aortic pulse pressure with advanced age or arterial disease.
Compliance, as depicted in Figure 1, represents static compliance that is generated by expanding a vessel by a known volume and measuring the change in pressure at steady-state. However, prior to achieving a steady-state pressure, the pressure will actually be initially higher than the steady-state pressure when the volume of fluid is first added. The transient fall in pressure at a constant volume is called stress relaxation and is related to the viscous properties of biological tissues. If the initial pressure increase is used instead of the steady-state pressure when the vessel volume is suddenly increased, the compliance will be lower (i.e., the vessel will appear more stiff). Therefore, the compliance of the vessel is also dependent upon the rate by which the change in volume occurs i.e., there is a dynamic component to compliance.