# Why do you get pressure drop in central circulation?

## Main Question or Discussion Point

What is the reason that the pressure drops in the central circulation?especially if you go from arteries to arterioles you get a huge pressure drop.

If you look what is changing if you go from arteries to arterioles then it is that you get many many branches this leads to an increase of total radius.(the individual radii of the arterioles are smaller but the sum of them all is larger than aorta/arteries). So because of this increase in total radius you could say that the resistance should drop. So if the resistance is dropping(in stead of increasing) how is it that you get this huge pressure drop?

## Answers and Replies

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BvU
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Hello Joojo,

Has to do with viscosity and the fact that 1000 small ducts have a lot more nearby surface to stick to than one big one with the same area. In the big duct there is e.g. a 0.1 mm 'boundary layer' between velocity zero at the wall and the plug flowing bulk. In a 0.2 mm arteriole it is all boundary layer.

cf resistance to flow in Poiseuille flow
Here they even have blood flow examples

The radius dependence is very strong for laminar flow; with bigger ducts there will be plug flow, perhaps even some turbulence and then the dependence is less drastic.

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Thank you. Never thought about it from that perspective.
So what you are saying is that the increase in radius does decrease the resistance(even thoug it is not as big a factor as in the arterial system, becouse the flow in the arterioles is not laminar). But at the same time you have a much bigger area([PLAIN]http://www.mathgoodies.com/lessons/vol2/images/area_formula.gif) [Broken] and this area is cousing for a much bigger friction which increases the resistance.
And this increase in resistant(from the increased area) is much bigger than the decrease of resistance(from the bigger radius) so overal the resistance increases and this leads to that big pressure drop?

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BvU
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You have lost me somewhat. Perhaps because I am unfamiliar with the medical terms. Increase in radius decreases resistance, yes. Bigger area (cross section area) means less pressure drop.

The part 'at the same time' eludes me completely. It seems you now expect a pressure drop when the radius increases ?

In post #1: "from arteries to arterioles you get a huge pressure drop". I interpreted that as: the pressure in the bigger ducts is higher than in the narrower ones. Is that what you meant ?

In post #1: "from arteries to arterioles you get a huge pressure drop". I interpreted that as: the pressure in the bigger ducts is higher than in the narrower ones. Is that what you meant ?
Yes that is what i meant.
You have lost me somewhat. Perhaps because I am unfamiliar with the medical terms. Increase in radius decreases resistance, yes. Bigger area (cross section area) means less pressure drop.
With the area i mean because al those vessels now have much more surface area for friction so this will lead to increase resistance and this increase resistance should lead to a big pressure drop.

And even though the increased radius should have caused and decrease in resistance and thereby pressure difference should decrease, this is not going to happen because the increased resistance by way of the increased friction(due to increased area for friction to happen)is playing a much bigger role. So overall the resistance in increasing in the arterioles and this leads to that big pressure drop.

Can anybody confirm this please?