Compliance of blood vessels

Definitions & Key takeaways

Compliance of blood vessels also referred to as distensibility or capacitance, is defined as the ability of a blood vessel to adjust the blood pressure and increase the volume of blood that it can hold. When the pressure increases, vessels distend or swell, and when the pressure decrease, their volume decreases. Compliance is low in blood vessels that are less elastic, such as large arteries, and high in more elastic vessels, such as small veins. In general, compliance decreases with age because the walls of larger arteries tend to become more rigid.

The decrease in compliance with age can cause several problems. For example, increased stiffness of the large arteries can lead to hypertension because the heart has to work harder to push blood through these stiffened vessels. The reduced flexibility of arterial walls may also impede blood flow to different body parts, which can result in tissue damage.

Chapters:

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Introduction0:00–0:40

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.

Calculating Compliance0:40–0:52

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.

Arteries and Veins0:52–2:20

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.

Elastance E2:20–2:39

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.

Pulse Pressure2:39–4:03

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. Okay, but in reality even though the aorta isn’t very compliant, during systole it still stretches out with these high-pressure contractions, some of that volume is held in this distended space, since it’s very elastic, the walls store some of that energy of contraction as elastic energy in the walls, another form of potential energy.

Windkessel Effect4:03–6:02

Since the total energy has to stay the same, there’s less pressure and kinetic energy to make way for that elastic energy.
so less pressure energy means the blood pressure isn’t as high, let’s say 120 mmHg. Now, during diastole, the elastic walls recoil or snap back, moving that blood that was held during systole out into the circulatory system and that stored elastic energy gets converted back to pressure energy and kinetic energy, so now pressure energy gets added, yielding a pressure of say 60 mmHg, which means the new pulse pressure is lower—60 mmHg.
So, with elastic arteries, the blood pressures are 120 over 60, with a difference or pulse pressure of 60 mmHg, and remember without the elastic properties, it was 140 / 50 with a pulse pressure of 90.
So here we see these large elastic arteries buffer or dampen the pulse pressure, and this whole phenomenon is known as the Windkessel effect!
As a person ages, healthy compliant arteries become stiff and non-compliant, resulting in blood vessels that behave more like the non-elastic case, which means pulse pressure ultimately goes up.
All right, as a quick recap: compliance refers to the volume of blood a vessel can hold at a certain pressure, and this is related to distensibility.

Review6:02–6:30

Arteries, are low-volume vessels high-pressure,, and therefore have low compliance, and veins are high-volume vessels, low-pressure, meaning they have high compliance.
Aged arteries become much less compliant, leading to higher blood pressure in the elderly.