Distensibility is a metric of the stiffness of blood vessels.
It is defined as
{\displaystyle D={\frac {d_{sys}-d_{dias}}{(p_{sys}-p_{dias})d_{dias}}}}
{\displaystyle d_{sys}}
are the diameter of the vessel in systole and diastole, and
{\displaystyle p_{sys}}
are the systolic and diastolic blood pressure.