Seph

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I feel kind of dumb for not understanding this one, but here it goes:


If the total cross sectional area rises from aorta<arterioles< capillaries, why does every book say that pressure drops most in arterioles because they are the highest point of resistance in the cardiovascular system?
If total cross-sectional area is larger in the arterioles than the aorta, and since every resistance vessel is in parallel, shouldn't this be a lower point of resistance comparing to the aorta? Thus with resistance going down, it makes sense that arterial pressure also drops.
 

calvnandhobbs68

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I feel kind of dumb for not understanding this one, but here it goes:


If the total cross sectional area rises from aorta<arterioles< capillaries, why does every book say that pressure drops most in arterioles because they are the highest point of resistance in the cardiovascular system?
If total cross-sectional area is larger in the arterioles than the aorta, and since every resistance vessel is in parallel, shouldn't this be a lower point of resistance comparing to the aorta? Thus with resistance going down, it makes sense that arterial pressure also drops.
The pressure drops most ACROSS aterioles because V=IR. When you increase resistance with the same flow, you have to compensate by increasing the change in potential across the resistor (which is the change in pressure in the circulatory system).

Edit: Said decreasing instead of increasing whoops.
 
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Work

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I feel kind of dumb for not understanding this one, but here it goes:


If the total cross sectional area rises from aorta<arterioles< capillaries, why does every book say that pressure drops most in arterioles because they are the highest point of resistance in the cardiovascular system?
If total cross-sectional area is larger in the arterioles than the aorta, and since every resistance vessel is in parallel, shouldn't this be a lower point of resistance comparing to the aorta? Thus with resistance going down, it makes sense that arterial pressure also drops.
Remember that the flow (Q) = change in pressure (deltaP)/resistance (R), which means R=(deltaP)/Q. The bigger the resistance, the bigger the drop in pressure.

Resistance = 8nl/[(pi)(radius)^4] via Poiseuille's law so that a decrease in radius will increase resistance significantly. Going from the aorta to the arterioles means that the blood vessel radius will decrease by a lot, which means that resistance will be higher and consequently, there will be a huge drop in blood pressure.

Also remember that pressure = force/area, so when you're going to arterioles to capillaries, the radius is decreased in the capillaries but total area is increased due to the extensive capillary network, meaning the pressure is actually decreased (and subsequently resistance).
 
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Seph

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Sorry, I didnt understand that. I dont get why are arterioles resistance vessels in the first place, since they represent a total larger cross-sectional area than the aorta. If I have pipe with a 5 cm2 area, and it than enlarges to a 500 cm2, pressure drops and resistance drops. If I put together all arterioles, I actually get something like that (500).
I can understand that resistance rises if the 5 cm2 pipe goes down to 2.5, by darcy/poiseuille law, but that is what happens when you go from one artery to one arteriole, not when you go from one artery to thousands of arterioles all parallel (total R in a parallel system is lower than any individual R).
 
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Seph

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Also remember that pressure = force/area, so when you're going to arterioles to capillaries, the radius is decreased in the capillaries but total area is increased due to the extensive capillary network, meaning the pressure is actually decreased (and subsequently resistance).
Work, this is exactly what happens when you go from large arteries to a network of arterioles. Following this line of thought, why does the resistance go up from artery-> arterioles, but not from arterioles-> capillaries?
 

Work

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Sorry, I didnt understand that. I dont get why are arterioles resistance vessels in the first place, since they represent a total larger cross-sectional area than the aorta. If I have pipe with a 5 cm2 area, and it than enlarges to a 500 cm2, pressure drops and resistance drops. If I put together all arterioles, I actually get something like that (500).
Work, this is exactly what happens when you go from large arteries to a network of arterioles. Following this line of thought, why does the resistance go up from artery-> arterioles, but not from arterioles-> capillaries?
I think I understand what you're saying. Arterioles do have a larger cross-sectional area than the aorta, but they also have a higher resistance because the diameter of the arterioles (and hence the radius) is way smaller.

Since resistance = 8nl/[(pi)(radius^4), this means that the resistance is inversely proportional to the radius of a vessel to the fourth power, meaning if the radius goes from 4mm to 2mm, the resistance will increase by 16x.

In other words, the cross-sectional area increase in the arterioles is not enough to make up for the increase in resistance going from aorta to arterioles.

This is different in capillaries since their total cross-sectional area is huge, and the change in resistance is smaller since the change in diameter going from arterioles to capillaries is smaller compared to going from aorta to arterioles.
 
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Old Style Nanny

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If the total cross sectional area rises from aorta<arterioles< capillaries, why does every book say that pressure drops most in arterioles because they are the highest point of resistance in the cardiovascular system?
If total cross-sectional area is larger in the arterioles than the aorta, and since every resistance vessel is in parallel, shouldn't this be a lower point of resistance comparing to the aorta? Thus with resistance going down, it makes sense that arterial pressure also drops.
Sorry, I didnt understand that. I dont get why are arterioles resistance vessels in the first place, since they represent a total larger cross-sectional area than the aorta. If I have pipe with a 5 cm2 area, and it than enlarges to a 500 cm2, pressure drops and resistance drops. If I put together all arterioles, I actually get something like that (500).
I can understand that resistance rises if the 5 cm2 pipe goes down to 2.5, by darcy/poiseuille law, but that is what happens when you go from one artery to one arteriole, not when you go from one artery to thousands of arterioles all parallel (total R in a parallel system is lower than any individual R).
Work, this is exactly what happens when you go from large arteries to a network of arterioles. Following this line of thought, why does the resistance go up from artery-> arterioles, but not from arterioles-> capillaries?
I can understand your confusion. You are wondering why resistance goes down in the arterioles even though their total cross-sectional area is lower than that of the arteries.

It is simple really, the resistance is not dependent on the cross-sectional area; rather it is dependent on the inherent tone of the vessels (which in turn is partly dependent on the diameter). Arterioles have the highest tone.

Also remember, capillaries do have lower pressure than arterioles. BUT, the pressure drop is greatest when going from arteries to arterioles than when going from arterioles to capillaries because the reduction in diameter in the first transition is far greater (~5:1) than the reduction in diameter in the second transition (~1:1).
 

Phloston

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I've seen a practice question on this.

They want you to know that arterioles have the greatest wall thickness to total vessel diameter ratio, where vessel diameter = luminal diameter + (2x wall thickness).

This does not mean they have the greatest absolute wall thickness. The elastic arteries, for instance, have greater absolute wall thickness. But the arterioles have the greatest relative wall thickness.

On the other hand, capillaries have the greatest cross-sectional area overall. But their wall thickness is minimal.

The fact that the arterioles have the greatest wall thickness to vessel diameter ratio means they have the greatest capacity to alter their luminal size. Since change in resistance is a function of luminal diameter, the arterioles' ability to change their luminal size more greatly than other vessels makes them the location of the greatest drop in resistance.
 
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Seph

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Thanks work, nany and pholston. Work, I completely got what you said, thanks a bunch.

So now I understand why arterioles are the highest point of resistance in the CV system, but now the mathematics screw it up: if resistance is larger in the arterioles, why does the pressure DROP instead of going up? P=COxR, if resistance rises, pressure rises.

I understand why pressure drops across the resistance, as in comparing the point before the resistance (the aorta) and the point after. But the point were resistance actually takes place should have a spike in pressure, no?



So why this drop in the arterioles?
 

Old Style Nanny

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So now I understand why arterioles are the highest point of resistance in the CV system, but now the mathematics screw it up: if resistance is larger in the arterioles, why does the pressure DROP instead of going up? P=COxR, if resistance rises, pressure rises.

I understand why pressure drops across the resistance, as in comparing the point before the resistance (the aorta) and the point after. But the point were resistance actually takes place should have a spike in pressure, no?

So why this drop in the arterioles?
If you study this image below, you can reason it out yourself.

 
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Work

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Thanks work, nany and pholston. Work, I completely got what you said, thanks a bunch.

So now I understand why arterioles are the highest point of resistance in the CV system, but now the mathematics screw it up: if resistance is larger in the arterioles, why does the pressure DROP instead of going up? P=COxR, if resistance rises, pressure rises.

I understand why pressure drops across the resistance, as in comparing the point before the resistance (the aorta) and the point after. But the point were resistance actually takes place should have a spike in pressure, no?

So why this drop in the arterioles?
If you study this image below, you can reason it out yourself.
You're right in that if resistance goes up for the same amount of flow, then pressure would have to increase, but what Mrs. Doubtfire was getting at was this: that would be true if you're going from the aorta to one arteriole, but remember that the arterioles are arranged in parallel, so the pressure actually decreases; in simpler terms, the same pressure from the aorta gets distributed to all of the arterioles in parallel, so you actually get a huge drop in pressure. I hope that makes sense. This is why I am not an educator.