Afterload and Wall Stress

[virajpatel]

How does afterload work into wall stress? Increase in arterial BP puts more load on the heart so there will be more ventricular dilation, hence greater wall stress?

Compliance is the ability to resist the returning to original dimensions after being stretched. So how does compliance of the aorta play a role in maintaining diastolic blood pressure so that pulse pressure from the ventricle is reduced?

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[Instatewaiter]

How does afterload work into wall stress? Increase in arterial BP puts more load on the heart so there will be more ventricular dilation, hence greater wall stress?

Compliance is the ability to resist the returning to original dimensions after being stretched. So how does compliance of the aorta play a role in maintaining diastolic blood pressure so that pulse pressure from the ventricle is reduced?

Wall tension is the (ventricular pressure x radius/wall thickness. So as your aortic pressure rises, your ventricular pressure will have to rise to maintain your cardiac output. Therefore your wall tension will rise.

Some use wall tension as an esimate for afterload.

Compliance and its role on diastolic blood pressure is a more complex and much longer answer that you can google.

 

[DrPicard]

T = Pr/w, where T = wall tension, P = pressure, w = wall width.

Aortic pressure increases ==> ventricular pressure must increase to maintain the pressure gradient for flow ==> if P increases in the equation above, you can see wall tension also increases. The heart compensates by undergoing hypertrophy which increases w and reduces wall tension while working at a higher pressure.

Compliance of the aorta allows its wall to stretch under pressure from the stroke volume, thus buffering the increases in systolic pressure. During diastole, the wall compresses back thus generating diastolic pressure. In other words, compliance functions to decrease systolic and increase diastolic pressure, hence decreasing pulse pressure. This is a simplified explanation obviously, but should be good enough for the USMLE.

 

[virajpatel]

Thanks for the explanation guys.
One more thing, as you age complaince decreases so from your explanations the 'buffering' capacity for systolic pressure would decrease so it would be higher everytime the ventricles pumps blood into the aorta while the diastolic would be lower, therefore increasing pulse pressure. But since elastance is the inverse of complaince it would increase, elastance is the ability to recoil so how would an increase in its value lead to higher blood pressures in people of older age.

 

[wjs010]

Thanks for the explanation guys.
One more thing, as you age complaince decreases so from your explanations the 'buffering' capacity for systolic pressure would decrease so it would be higher everytime the ventricles pumps blood into the aorta while the diastolic would be lower, therefore increasing pulse pressure. But since elastance is the inverse of complaince it would increase, elastance is the ability to recoil so how would an increase in its value lead to higher blood pressures in people of older age.

Higher compliance = higher unstressed (diastolic ) BP . Stressed pressure would be systolic pressure. Basically, when you have high compliance, you are able to "hold" more and more (increase DBP) but as you age and compliance decreases, that volume shifts from more DBP to more SBP because less compliance means you can hold less blood in vessels. So pulse pressure would increase

 

[DrPicard]

Thanks for the explanation guys.
One more thing, as you age complaince decreases so from your explanations the 'buffering' capacity for systolic pressure would decrease so it would be higher everytime the ventricles pumps blood into the aorta while the diastolic would be lower, therefore increasing pulse pressure. But since elastance is the inverse of complaince it would increase, elastance is the ability to recoil so how would an increase in its value lead to higher blood pressures in people of older age.

Depends on how you choose to define compliance and elastance. They're opposites. Something that stretches easily is compliant; or, to put it in other words, something that recoils weakly is compliant. Elastance is how strongly a structure recoils; or, in other words, how difficult it is to stretch.

A compliant rubber band would be easy to stretch, but once you let go, it would also snap back slower. Another rubber band with higher elastance (or lower compliance) would be harder to stretch (so you wouldn't stretch it as much), but would also snap back faster.

If you want to figure this out from an elastance point of view, as we increase this value, it becomes difficult for blood during systole to expand vessels outwards. This means less buffering, and greater SBP. Less expansion also means less blood pushed by the vessel during diastole, hence lower DBP.

 

[DrPicard]

Its just that we choose to talk about compliance from the perspective of stretch-ability and elastance from the perspective of recoil, when in fact both these terms qualify both these characteristics.

 

[virajpatel]

Okay I understand this concept. But I am also having confusion with wall tension in blood vessels. You have passive components and active components. AND I know that according to the laplace relationship if you increase the radius you have more wall tension but I don't get how the elastin, collagen and smooth muscle play a role and the idea that vessel walls actually have the ability to withstand wall tension not transmural pressure.