Compliance of Blood Vessels

In short, Compliance refers to the “ease” of the blood vessel, either artery or vein, to DISTEND on the change of volume contained or pressure applied. Formulated as Δ Volume/Δ Pressure, it’s directly related to amount of change in the volume of blood within the vessel, and inversely proportional to the amount of change in the Pressure in the contained blood.

In other words, If you compare the Aorta, with a thick wall containing massive amounts of collagen and extremely compliant, with the radial artery, a thin-walled artery with excessive smooth muscles making it more elastic or less compliant, you can deduce that, for a certain amount of blood;

• Fixate The Pressure Applied On Both Vessel Walls To A Certain Value, them to have equal value of pressure, the more DISTEND-able compliant aorta must have MORE volume to overcome its likelihood to grow bigger, while the radial artery will do with small volume on which it’ll constrict, due to absent compliance.

• Fixate The Volume Contained Within The Two Vessels To A Certain Value, then to obtain the same volume in both arteries, Aorta will distend more (compliant), thus decreasing pressure, while Radial artery will distend less (elastic), thus increasing pressure.

Now, apart from the different patterns of compliance, an important concept is the Elasticity, which is the inverted property of Compliance. The former is caused by collagen fibers within the vessel wall that allow it to properly distend upon increase in vascular volume or pressure, similar to the wall of a balloon. However, the Elasticity depends on presence of Smooth muscles within the vascular wall, thus increased volume or pressure will partially increase the diameter and thus elevates the pressure within the vessel.

The term Elastic Recoil means the ability of the vessels to rapidly return to its baseline caliber once the offending pressure subsides, with the cardiac cycle as a proper example. With each stroke, the systolic B.P. distends the Aorta more than the radial artery due to different compliance. However, with the end of Systole and the beginning of Diastole, the diastolic B.P., being lower than the systolic, causes the vessel caliber to return gradually to its previous value, with the more compliant artery returning from a higher ground than the stiff one, thus increasing the diastolic B.P. more than the latter.

And, as a side note, the different factors controlling the four different patterns of Blood pressure are as follows, where S.V. is Stroke Volume, H.R. is Heart Rate, and T.P.R. is Total Peripheral Resistance;

• Systolic BP increases with +S.V. and +H.R. as they both add up to Cardiac Output directly; And again, -Compliance causes less distention of vessel wall with each stroke, thus elevating the Systolic BP. And this latter is the pathophysiology of Elderly ATH-Induced Isolated Systolic Hypertension.
  

• Diastolic BP increases with +S.V. and +H.R. due to same previous argument. Also, +P.V.R. brought upon by peripheral vasoconstriction increases collision probability of kinetic cells with the vascular wall, hence the pressure exerted on them. Finally, +Compliance will lead to marked Elastic Recoil, thus elevating the otherwise low DBP.

• Pulse Pressure increases with +S.V., as it leads to marked rise in SBP, and less marked rise in DBP, widening the gap between them. -Compliance directly causes Higher PB, as the SBP becomes higher, while the DBP becomes lower.

• Mean B.P., or mean A.B.P. taken over time interval, increases with +S.V., +H.R. and +T.P.R. These are the factors that actually formulate the Poiseuille Equation Form of A.B.P.
  

Thus, in summary, a high-compliance artery would have a LESS SYSTOLIC pressure, HIGH DIASTOLIC pressure, and therefore LOW PULSE pressure, with the opposite argument in stiff arteries.