# Compliance of blood vessels

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### USMLE® Step 1 style questions USMLE

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A graduate student is conducting an experiment to determine the effects of arterial compliance and stroke volume on pulse pressure. Which of the following best describes the effects that changes in the independent variables will have on the dependent variable?

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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.

### Summary

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.

### Sources

1. "Medical Physiology" Elsevier (2016)
2. "Physiology" Elsevier (2017)
3. "Principles of Anatomy and Physiology" Wiley (2014)
4. "Venous Function and Central Venous Pressure" Anesthesiology (2008)
5. "Definition of arterial compliance" American Journal of Physiology-Heart and Circulatory Physiology (2000)
6. "Definition of arterial compliance. Aortic pressure-diameter relationship assessed by intravascular ultrasound: experimental validation in dogs" Am J Physiol Heart Circ Physiol (2000)
7. "Human Anatomy & Physiology" Pearson (2018)

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