This is about compliance in blood vessels and the heart specifically.
I have looked this up multiple times, and whenever I think I have it, I get another question wrong because I apparently don’t have it down pat yet. I have been told that ‘compliance is the inverse of elasticity’ but that just makes me go through more mental hoops to get to the same answer.
Can someone explain this concept to me? I’ve tried everything short of eating my textbook.
In: 2
you can think of a blood vessel as if it were a rubber tube. now, rubber can be harder or softer, and more or less bouncy.
“compliance” refers to how much that rubber tube allows itself to expand when the liquid inside it is a LOT or under high pressure.
“elasticity” is the ability of that tube to bounce back to its original shape.
so for example if you have a tube that is low compliance, high elasticity, it will resist expanding, and even when it does expand a little, it will tend to revert back to its original shape. this means that the pressure inside it will get higher.
By its definition, compliance is a property of a vessel to increase the volume of blood inside of it without the increase of the pressure inside.
Take two diffrently compliant vessels. If vessel A is more compliant that vessel B, that means that vessel A can take more blood for the same increase of pressure than vessel B.
In numbers, let’s say the compliance of the vessel A is 200mL/mmHg, and vessel B is 50mL/mmHg. That means that if we want to increase pressure in both vessels by 1mmHg, we would need to give vessel A 200mL, while vessle B only 50mL. The opposite is olso true, if both vessels get 400mL of blood, the pressure in vessel A will increase by 2mmHg, while the pressure in the vessel B will i crease by 8mmHg.
Because of histological reasons, veins are much more compliant than arteries meaning that they can fill up with blood while keeping the pressure inside them relatively stable. That’s why we say that the veinous circulation is the blood reservoir; excess blood is kept there when it’s not needed. We can’t keep all that in the arterial circulation because our blood pressure would dangerously rise. Why? *Because arteries are less compliant than veins*
Histology also makes aorta the most compliant artery because its elastic fibers helps aorta to deform and expand during systole. The blood from the LV goes into the aorta, stretching the aortal wall. However, the elastic fibers of the aortal wall help it deform which “prevents” the blood from pushing on the aortal wall and creating pressure i.e. the pressure falls. That’s why people with arteriosclerosis have high fluctuations of blood pressure (pulse pressure is increased). Their arteries are like lead pipes, which are so not compliant meaning that pressure rises with small increases of blood volume.
So to summarize. High compliance means that we can shove blood into a vessel without it increasing pressure inside. One highly compliant vessel is VCI. It “stores” a certain volume of blood for when it gets needed (blood loss or high physical activity).
If you have further questions, please ask.
Compliance = Will it stretch?
Elasticity = Will it snap back?
High compliance and high elasticity mean it will stretch easily to accommodate, and then go back to its shape.
High compliance and low elasticity mean it will stretch easily but after stretching it won’t be as easy to go back to how it was before stretching.
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