Originally Posted by samusmle33
Thanks. I have few more questions. How would a decrease in vessel compliance affect pulse pressure? I understand that this would lead to a high SBP..but I'm not clear about its impact on DBP. Acc to kaplan LNs, the measured DBP would be falsely low. Hence the pulse pressure will be wide/high.
And as v move more distally from heart, the resistance to blood flow increases and DBP would increase or decrease? Why does the pulse pressure increase in distal vessels?
For the first part of the question the answer is this. At first, I thought you were talking about venous compliance because that is the one that can actually be modulated. However, as I looked into Kaplan LN's it seems like they are talking about decreased compliance of the arteries (mainly aorta) due to mostly aging. First of all, you need to understand what compliance means. Simply put, it is how "stretchable" something is. If something is very "stretchable" (not elastic, stretchable) then you can put a high amount of fluid in there without affecting the pressure. Compare to something that is very stiff, the same amount of volume would create a larger pressure. A simple analogy would be trying to fill up a balloon. If the balloon wall is thin it is easier to fill it with air because it is more compliant and accommodates more volume at less pressure if you compare it to a thicker balloon which is way harder to fill up. Now, in addition to that, the thicker balloon would want to collapse more than the thinner one, right? It pushes the air out with more "force". That is whats called elastence (or elasticity?), wanting to recoil. There is an inverse relationship between elastence and compliance, the more compliant something is, the less elastic it is. Back to arteries. Same principles apply, the same volume of blood would produce less pressure in a compliant artery compared to a less compliant one. So, as people age, their arteries become stiffer (less compliant, more elastic), meaning that the same stroke volume, produces a higher systolic BP. Since the arteries are also more elastic and generate more pressure, that increases the pressure gradient and drives more blood out of the aorta towards the peripheral arteries (flow is directly proportional to the pressure difference). Decreased vessel compliance will produce an increased PP. By increasing the SBP (more pressure at the same stroke volume) and decreasing DBP (more blood leaving the aorta).
For the second question, I honestly have never heard of that. As far as I know, the farther away you get from the left heart, the less pressure difference there is and past the arterioles there is no such thing as SBP or DBP. Even down in the arterioles pulse pressure is very small.
The biggest site of resistance are the arterioles. They are the ones that control how much blood will go over to the venous side. Think of them as faucets. There are no major faucets until you hit the arterioles, so the blood flows freely until it hits a faucet. If you open the faucet (dilate the arterioles) more blood crosses to the venous side, if you close the faucet (constrict the arterioles) less blood crosses to the venous side. Remember, that small diameter=high resistance and high resistance=less flow.
DBP that we measure with a BP cuff is almost
the same as the aortic DBP because there is not a big pressure difference between the aorta and the major arteries (because there is no big resistance and the blood is allowed to flow freely, almost
equilibrating the pressure everywhere). However, blood pressure does decrease as we move away from the heart, pressure difference is
the driving force for blood flow.
Just remember that DBP is directly proportional to the amount of blood remaining in the aorta right before the aortic valve opens. It is easy to work through scenarios if you get the basic principles down.
I am not sure if I explained it well or not, I might have created more questions than I have answered. Ask me if anything needs clarification.