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Physics behind blood vessel constriction and blood pressure?

by sameeralord
Tags: blood, constriction, physics, pressure, vessel
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sameeralord
#1
Jan4-10, 07:02 AM
P: 640
First of all I have to say I have really confused myself on this one by overthinking and I'll ask my questions step by step. Any help would be greatly appreciated.

General rule is when blood vessels constrict blood pressure increases.

1. Ok there is a container and gas molecules in it. If I decrease the volume of the container the pressure increases. This analogy can not be used for blood vessels right? I mean it is a liquid and has a blood flow.
2. It is said that when blood vessels constrict blood pressure increases. Blood pressure means the pressure at which the heart pumps the blood right? So when they say an increase do they mean an increase in blood pressure overall(increase in heartbeat) or just at the place where blood vessel was constricted? I mean does a contraction of one vessel increase the overall heart beat or just the pressure of that vessel?
3.Why does blood pressure increase when blood vessels constrict? Is it because that when blood vessels constrict there is an increase in resistance and sensory neurones tell heart to increase blood pressure? If heart did not have sensory neurones and was unable to change its pumping rate willl the constriction of blood vessel decrease pressure?
4. Now how does this all apply when you consider heart as a network of vessels. Does it work like a circuit? If you have two branches and if one branch was dilated, more blood would follow the dilated branch right (path of least resistance), in this case how is there a more blood flow to that branch if heart decreases pressure. I mean isn't more blood countered by low pressure so no change.

Thank you so much for anyone who is going to attempt to answer these for me.
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Andy Resnick
#2
Jan4-10, 07:53 AM
Sci Advisor
P: 5,510
If vasoconstriction occurs, increased pressure is not the only possible outcome- decreased flow can also occur.

You are right to think in terms of 'lumped elements' for the circulatory system- vasoconstriction is equivalent to increasing the resistance.
sameeralord
#3
Jan4-10, 08:03 AM
P: 640
Quote Quote by Andy Resnick View Post
If vasoconstriction occurs, increased pressure is not the only possible outcome- decreased flow can also occur.

You are right to think in terms of 'lumped elements' for the circulatory system- vasoconstriction is equivalent to increasing the resistance.
Thank you Andy however could you please elaborate on your answer. I know you are like to give short and precise answers but I'm bit confused. I have a misunderstanding I think. What I'm thinking is that

When a blood vessel constrict, obviously more blood hits the walls and there is higher pressure, but I'm thinking this higher pressure is countered by the increased resistance, meaning at the end increased resistance decreases pressure. Then why does constricting the blood vessel increase pressure? Thanks

sameeralord
#4
Jan4-10, 04:39 PM
P: 640
Physics behind blood vessel constriction and blood pressure?

If a blood vessel consticts, the resistance increases and kinetic energy decreases, if kinetic energy decreases hyrdrostatic pressure must go up but why does other occur?

"The average capillary hydrostatic pressure is determined by arterial and venous pressures (PA and PV), and by the ratio of post-to-precapillary resistances (RV/RA). An increase in either arterial or venous pressure will increase capillary pressure; however, a given change in PA is only about one-fifth as effective in changing PC as the same absolute change in PV. Because venous resistance is relatively low, changes in PV are readily transmitted back to the capillary, and conversely, because arterial resistance is relatively high, changes in PA are poorly transmitted downstream to the capillary. Therefore, PC is much more influenced by changes in PV than by changes in PA. Furthermore, PC is increased by precapillary vasodilation (particularly by arteriolar dilation); precapillary vasoconstriction decreases PC. Venous constriction increases PC, whereas venous dilation decreases PC."

If anyone can explain this I'll be very greatful.
Moonbear
#5
Jan4-10, 07:14 PM
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Quote Quote by sameeralord View Post
Blood pressure means the pressure at which the heart pumps the blood right? So when they say an increase do they mean an increase in blood pressure overall(increase in heartbeat) or just at the place where blood vessel was constricted? I mean does a contraction of one vessel increase the overall heart beat or just the pressure of that vessel?
They're talking about a local effect, of the blood passing through that section of a blood vessel. It has nothing to do with what the heart is doing, and in fact, you have to assume nothing is changing about the heart for the whole rule to apply.

When liquid goes from a larger tube into a small tube, and the flow going in is the same, then the pressure inside or flow leaving (or both) has to increase to accommodate that large volume in a smaller space.

In reality, if you got vasoconstriction everywhere, rather than just locally, and your nervous system is properly functioning, blood pressure is maintained relatively constant by adjusting either the heart rate or stroke volume, or both.
sameeralord
#6
Jan4-10, 07:27 PM
P: 640
Quote Quote by Moonbear View Post
They're talking about a local effect, of the blood passing through that section of a blood vessel. It has nothing to do with what the heart is doing, and in fact, you have to assume nothing is changing about the heart for the whole rule to apply.

When liquid goes from a larger tube into a small tube, and the flow going in is the same, then the pressure inside or flow leaving (or both) has to increase to accommodate that large volume in a smaller space.

In reality, if you got vasoconstriction everywhere, rather than just locally, and your nervous system is properly functioning, blood pressure is maintained relatively constant by adjusting either the heart rate or stroke volume, or both.
Thanks a lot Moonbear I just overthought again . Thanks Andy as well.
sameeralord
#7
Jan5-10, 05:45 AM
P: 640
Hey moderator has edited my topic. I still have one question remaining

If a blood vessel consticts, the resistance increases and kinetic energy decreases, if kinetic energy decreases hyrdrostatic pressure must go up but why does other occur?

"The average capillary hydrostatic pressure is determined by arterial and venous pressures (PA and PV), and by the ratio of post-to-precapillary resistances (RV/RA). An increase in either arterial or venous pressure will increase capillary pressure; however, a given change in PA is only about one-fifth as effective in changing PC as the same absolute change in PV. Because venous resistance is relatively low, changes in PV are readily transmitted back to the capillary, and conversely, because arterial resistance is relatively high, changes in PA are poorly transmitted downstream to the capillary. Therefore, PC is much more influenced by changes in PV than by changes in PA. Furthermore, PC is increased by precapillary vasodilation (particularly by arteriolar dilation); precapillary vasoconstriction decreases PC. Venous constriction increases PC, whereas venous dilation decreases PC."

If anyone can explain this I'll be very greatful.


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