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#1
02-10-2016
 USMLE Forums Scout Steps History: Not yet Posts: 21 Threads: 6 Thanked 3 Times in 3 Posts Reputation: 13
fall of blood pressure

What is the real explanation why the pressure dicreases when you go through the central circulation?

For example: the pressure in proximal part of aorta is higher than distal part becouse if you use the formula:R=(:eeta:x L)/(r^4) the L increases so R also increases(resistance). And if you then use the formula of Q=deltaP/R, becouse the Q is constant, if the R increases delta P must also increaes.
So this means you get a bigger pressure drop that's why the pressure at the beginning is higher than at the end.

But if I try to use this logic to explain why the pressure dicreases if you go from the arteries to the arterioles then it doesn't work out.
Becouse the total radius increases if you go from the arteries to the arterioles.
So if you take that segment and use the formula of resistance, becouse the radius increases the resistance should decrease then you should get an decrease in delta P. So the pressure drop should be lower and not higher.

I know this can't be true but what goes wrong and how should it be done is what I try to understand.

#2
02-10-2016
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 86 Threads: 1 Thanked 55 Times in 37 Posts Reputation: 65

Quote:
 Originally Posted by Ewaa What is the real explanation why the pressure dicreases when you go through the central circulation? For example: the pressure in proximal part of aorta is higher than distal part becouse if you use the formula:R=(:eeta:x L)/(r^4) the L increases so R also increases(resistance). And if you then use the formula of Q=deltaP/R, becouse the Q is constant, if the R increases delta P must also increaes. So this means you get a bigger pressure drop that's why the pressure at the beginning is higher than at the end. But if I try to use this logic to explain why the pressure dicreases if you go from the arteries to the arterioles then it doesn't work out. Becouse the total radius increases if you go from the arteries to the arterioles. So if you take that segment and use the formula of resistance, becouse the radius increases the resistance should decrease then you should get an decrease in delta P. So the pressure drop should be lower and not higher. I know this can't be true but what goes wrong and how should it be done is what I try to understand.
First of all, the pressure in the proximal part of the aorta and in the distal part of the aorta is virtually same. In fact, blood pressure in the legs is higher than in the aorta when you are standing up. The only significant pressure drop occurs in the arterioles. There is minimal resistance in the aorta and the big arteries, the pressure remains high throughout.

In the first formula L is constant. It is the length of the blood vessel which only changes when you are growing. What it says is that the longer the vessel, the higher the resistance to flow. You can toss that out of the window because the resistance is never going to change due to changes in length.

The major determinant of resistance is radius. I have never heard of a concept of total radius. Total surface area, yes. Anyways, arterioles can alter their radius and therefore alter resistance. So, by using your formula Q=deltaP/R pressure difference has to be high if the resistance is high. It makes sense too, the pressure will of course be higher behind a resistor as compared to pressure past the resistor.
#3
02-10-2016
 USMLE Forums Scout Steps History: Not yet Posts: 21 Threads: 6 Thanked 3 Times in 3 Posts Reputation: 13

Quote:
 Originally Posted by Nodo First of all, the pressure in the proximal part of the aorta and in the distal part of the aorta is virtually same. In fact, blood pressure in the legs is higher than in the aorta when you are standing up. The only significant pressure drop occurs in the arterioles. There is minimal resistance in the aorta and the big arteries, the pressure remains high throughout. In the first formula L is constant. It is the length of the blood vessel which only changes when you are growing. What it says is that the longer the vessel, the higher the resistance to flow. You can toss that out of the window because the resistance is never going to change due to changes in length. The major determinant of resistance is radius. I have never heard of a concept of total radius. Total surface area, yes. Anyways, arterioles can alter their radius and therefore alter resistance. So, by using your formula Q=deltaP/R pressure difference has to be high if the resistance is high. It makes sense too, the pressure will of course be higher behind a resistor as compared to pressure past the resistor.
With the total radius I mean that although the individual arterioles have a smaller radius than the aorta/arteries if you take the sum of them all it will be bigger than that of the aorta/arteries.

But what couses that if you go from the arteries to arterioles you get that big pressure drop? I don't mean in the case that you alter the radii of the arterioles, but just in a normal resting state.

#4
02-10-2016
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 86 Threads: 1 Thanked 55 Times in 37 Posts Reputation: 65

Quote:
 Originally Posted by Ewaa With the total radius I mean that although the individual arterioles have a smaller radius than the aorta/arteries if you take the sum of them all it will be bigger than that of the aorta/arteries. But what couses that if you go from the arteries to arterioles you get that big pressure drop? I don't mean in the case that you alter the radii of the arterioles, but just in a normal resting state.
I understand what you mean by total radius, I just do not think that the concept is of physiologic importance.

In a normal state arterioles are the portions of the circulation with the highest resistance. Therefore, arterioles will provide the largest pressure drop.
#5
02-10-2016
 USMLE Forums Scout Steps History: Not yet Posts: 21 Threads: 6 Thanked 3 Times in 3 Posts Reputation: 13

Quote:
 Originally Posted by Nodo I understand what you mean by total radius, I just do not think that the concept is of physiologic importance. In a normal state arterioles are the portions of the circulation with the highest resistance. Therefore, arterioles will provide the largest pressure drop.
But is there not an explanation why the biggest resistance is in the arterioles?

If you compare what is changing if you go from arteries to arterioles is the many many branches. So that leads to a much bigger total radius and thereby also of the area.

If you consider that now you have suddenly a much bigger area then it is logical why the pressure drops, becouse P=F/A.

But if you try to use the concept of the resistance than the increase of the radius should lead to decrease of resistance and then it doesn't make sense
#6
02-10-2016
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 86 Threads: 1 Thanked 55 Times in 37 Posts Reputation: 65

Quote:
 Originally Posted by Ewaa But is there not an explanation why the biggest resistance is in the arterioles? If you compare what is changing if you go from arteries to arterioles is the many many branches. So that leads to a much bigger total radius and thereby also of the area. If you consider that now you have suddenly a much bigger area then it is logical why the pressure drops, becouse P=F/A. But if you try to use the concept of the resistance than the increase of the radius should lead to decrease of resistance and then it doesn't make sense
A transition from the arteries to the arterioles is the site of the biggest decrease in the vessel diameter. That is how I have always thought but now you made me think about it.

The pressure does not drop because of the increased surface area. The pressure drops because the resistor causes the blood to back up and there is a small amount of blood going through. A smaller amount of blood produces less pressure.

I have no idea how to explain this. None of the physiology books that I have used went into any more detail. Try not to overwhelm yourself with this. This stuff can seriously make your head explode.

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