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# Compliance and elastance

Learn about compliance (and elastance) of arteries, veins, and lead pipes! Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

## Want to join the conversation?

• Does this mean that the lead pipe (discussed at the end of the video), which has low compliance, therefore has high elasticity? A bit confused by this.
• Yes, that is right. The term elastance is very confusing. In daily life we often think that sometihing that stretches easily is very elastic, but something being very elastic actually means it recoils easily. The stretching part is compliance. If you are having a hard time with these concepts, just try to remember that elastance is a stiffness-index.
• Is "elastance" from the protein "elastin?"
• Etymonline.com says the word elastic is from the Greek elastos "ductile, flexible," I would venture to guess that elastin was named for its elastic properties rather than the other way around.
• How do we reconcile this linear relationship between volume and pressure with Boyle's law, stating that pressure and volume are inversely proportional?
• Boyle's law refers to the behavior of gasses in a closed system, if I remember correctly. Here we have an open system. If volume goes up, that means molecules are added to the system and not that the space between molecules is expanded. Also, since this topic concerns the circulatory system, the behavior of blood and blood vessels is studied. Blood is considered incompressable, so Boyle's law is not that usefull here.
• What units are compliance and elastance measured in?
• Compliance is measured in terms of volume/Pressure so micrometers^3 divided by mmHg might be one way. Another way would be cm^3 divided by Pascals. Basically, any length cubed divided by any unit of pressure (Pascal, mmHg, cmH20, atmosphere, bar, etc.). Elastance is just the Pressure/volume and is the reciprocal of compliance such as Pascals/meters^3
• when will we use elasticity and compliance in real life? Why is it so important?
• An great 'real life' example that I like to think of is with aortic dissections. Aortic dissections are due to a tear in the inner layer (tunica intima) of the aorta, so blood gets between that layer and the aorta's middle layer (tunica media). Since the aorta is not elastic enough, or compliant, to compensate for this extra stress on the aortic wall, it tares. Thus termed an aortic dissection, which is often fatal very quickly.