Compliance of blood vessels

Compliance, which is sometimes called capacitance or distensibility, refers to the ability of a vessel to respond to an increase in pressure by to distending or swell and increase the volume of blood it can hold, or with decreased pressure, a decrease in volume. The way that this applies to blood vessels is to remember that they are stretchable tubes like rubber hoses rather than lead pipes. So if the pressure increases, the walls of the tube can actually stretch out a bit to accommodate a larger volume, and exactly how much they stretch out depends on their compliance.

We can calculate a given blood vessel’s compliance, C, by dividing the volume of blood, V, in mL by the amount of pressure (P) in mmHg, that the blood is experiencing. And so we measure compliance in mL / mm Hg.

So we can plot out volume as a function of pressure, where the slope, volume over pressure, is the compliance. The veins have high compliance, meaning they’re high-volume, low pressure vessels, and even a small increase in pressure expands the volume a loti. The arteries, on the other hand have low compliance, and are low-volume, high pressure vessels, meaning with same amount of pressure, their volume doesn’t expand as much. Furthermore, a hardened artery would be even less compliant, and is like a lead pipe, in other words it takes an incredible amount of pressure to change the volume even a tiny bit.

With that in mind, since veins are more compliant, the majority of the blood in the body at any given time is in the veins, whereas less blood is in the thicker, less compliant arteries at any given time. Now, when the arteries harden due to arteriosclerosis, they become even less compliant over time, which means they can’t hold as much blood volume at the same pressure. That volume of blood is going to wind up in the veins. In this situation, blood simply moves away from the even higher pressure arteries to the area of lowest pressure, typically where the compliance is highest, like the veins.

Now, if compliance, or volume over pressure, is it’s tendency to stretch out with pressure, than its inverse would be it’s tendency to not stretch, or another way to think about it is its tendency to recoil back to its original shape, which is a concept known as its elastance E.