Aortic compliance is calculated over the pressure range from 0 - 300 cm of water (Figure 08). Volume increased from 1 to 4, or 3 units. Thus, the calculated compliance is 3 divided by 300, or 0.01. Elastance, the reciprocal, is 1 over 0.01, or 100.
The vena cavae, being thin-walled vessels, behave differently. They can collapse to a nearly flattened state at low transmural pressure, while arteries have thicker walls which prevents them from collapsing, even though transmural pressure may be near zero. Consequently, the compliance of the vena cavae is calculated over two ranges, one in which the vessel is filled to a circular cross-section from the collapsed state, and another where the wall is actually stretched.
In the first range for the vena cavae the volume is increased by 2 units resulting in an increase in pressure of 8 cm of water. This yields a compliance of 0.25, and an elastance of 4. Thus the compliance for the vena cavae over this pressure range is 25 times greater than that of the aorta.
When the vena cavae are actually stretched by a further increase in pressure, the compliance is lower, i.e. 0.047. This is only 4 or 5 times greater than the aorta over this pressure range.
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