Summary of Compliance of blood vessels
Transcript for Compliance of blood vessels
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.
Both of these concepts are super relevant for large elastic arteries like the aorta, which literally comes right off the heart. Now, first off, heart cells take ATP and use that chemical energy to contract during systole, turning it into mechanical energy. This propels blood out into the aorta. Since it’s moving blood, it has kinetic energy, but also since it’s exerting pressure on the walls of the vessel, it also has pressure energy, a form of potential energy, and the sum of these two is equal to the total energy E, which we’ll represent by this little health bar thing, which is filled with kinetic energy in blue and pressure energy in purple. So, let’s just say that during this time that pressure energy corresponds to a pressure of 140 mmHg. After the heart contracts, it relaxes and is filling with blood, called diastole. Now, during this time the blood’s still moving and there’s still pressure, but it’s just much less energy. So the pressure energy drops significantly, let’s say to something that corresponds to 50 mmHg. The difference between these, in this case 90 mmHg, is called the pulse pressure.