Arterial Stiffening

Increased arterial stiffness accelerates atherosclerosis by increasing peak arterial blood velocity, increasing Reynolds number, which indicates the propensity for a flowing fluid to develop pools and eddy currents in association with changing arterial geometry. In the normally compliant aorta, a portion of each stroke volume is stored in systole and propelled with lower velocity in diastole creating blood flow throughout most of the cardiac cycle. An area of pooling created by high velocity would disappear during slow diastolic flow, and any accumulated microthrombus would be dispersed. In a perfectly stiff aorta, the entire stroke volume would be expelled in systole. Given constant stroke volume, conservation of mass requires increasing peak arterial blood velocity with increasing arterial stiffness.8 Additionally, increased peak arterial velocity will augment shear-mediated platelet activation.  No low velocity diastolic flow will occur, so that a pool formed during high velocity systolic flow will persist throughout the cardiac cycle.

Unfortunately, the elastin molecules in the aorta, the large vessel which distributes blood to the rest of the body, fatigue and fracture over time, like any other material exposed to repetitive stress.  Thus, the aorta becomes stiffer with aging.  With progressive loss of aortic compliance, diastolic flow decreases.  Diastolic flow is important because it minimizes the “residence time” or amount of time a thrombus has undisturbed to undergo organization.  This increases the likelihood of the thrombus becoming an atherosclerotic plaque.  In other words, blood flow during diastole minimizes stasis, which Virchow recognized as a risk factor for thrombosis.

In summary, arterial, particularly aortic, compliance serves to slow peak arterial velocity, dampening the effect of changing arterial geometry and minimizing residence time.  Loss of compliance, as with aging, hypertension, cigarette smoking, etc., creates the possibility of areas of stasis and residence time throughout the cardiac cycle.