Blood vessel compliance can be determined in the following way. The two ends of a section of aorta can be tied off and fluid injected into it through a hypodermic needle attached to a syringe and a pressure transducer. The volume of fluid forced into this little balloon can be controlled and the subsequent increase in pressure observed.
For example, 80 ml might be injected, resulting in an increase in pressure of 40 mmHg. The compliance over the range of volume explored is 2 ml/mmHg. If another 80 ml of fluid were injected, the pressure would probably increase more than 40 mmHg, since the wall is already stretched and is in the steep portion of the length-tension curve. Consequently the compliance would be less, i.e. the wall is stiffer because the collagen fibers are being stretched.
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