# blood flow,pressure,resistance

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#### SamarEsawy

##### Full Member
10+ Year Member
"very strong sympathetic stimulation can constrict the vessels so much that bloodflow occasionally decreases to as low as zero for a few seconds DESPITE HIGH ARTERIAL PRESSURE" I found this phrase in Guyton's textbook
I don't understand how ..flow is directly proportional to pressure gradient pressure increses and resistance also increases here so why not flow is maintained constant?
thanks

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i don't understand the question

the diameter of the lumen will eventually be so little that blood won't get through regardless of pressure.

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Arterial pressure is the pressure in your arteries in general. If sympathetic stimulation is constricting vessels...it can be working on specific localized areas. I don't know what the entire section says but I would assume it's talking about localized tissue. It doesn't just constrict vessels everywhere in the body all at the same time. So the flow would be zero at that constriction point - likely making the arterial pressure even higher.

It could be that blood flow is so low that the vessel collapses. This happens in the IVC at low pressures.

Flow = Pressure / Resistance

Increase in resistance decreases flow. Not rocket science here.

Flow = Pressure / Resistance

Increase in resistance decreases flow. Not rocket science here.

The equation you posted shows that flow will never drop to zero as long as you have a non-zero pressure difference.

The equation you posted shows that flow will never drop to zero as long as you have a non-zero pressure difference.

It will if the vessel is totally occluded, i.e. resistance = infinity.

It will if the vessel is totally occluded, i.e. resistance = infinity.

I think Rothbard's point is that the OP has already made it clear that the book is stating something contrary to this equation and yet Visionary mindlessly posted that equation. Worries me when I see such oversights by people in medicine.

I think Rothbard's point is that the OP has already made it clear that the book is stating something contrary to this equation and yet Visionary mindlessly posted that equation. Worries me when I see such oversights by people in medicine.

.... Not at all. VT's equation explains OP's question perfectly.

If you increase pressure by a factor of x and resistance by a factor of x, then flow will maintain.

If you increase pressure by a factor of x and resistance by a factor of 1,000x, then flow will decrease extremely sharply.

The book states a high (meaning increased) arterial pressure. It doesn't state that the increase in arterial pressure = the increase in resistance. It seems to suggest that the increase in resistance > increase in arterial pressure.

I think Rothbard's point is that the OP has already made it clear that the book is stating something contrary to this equation and yet Visionary mindlessly posted that equation. Worries me when I see such oversights by people in medicine.

-edit- Also, OP didn't ask "why isn't flow a low but non-zero number?" He wondered why flow didn't stay constant with high arterial pressure and vascoconstriction.

It worries me when I see such oversights by people in medicine.

-edit- Also, OP didn't ask "why isn't flow a low but non-zero number?" He wondered why flow didn't stay constant with high arterial pressure and vascoconstriction.

It worries me when I see such oversights by people in medicine.
I don't understand the controversy. All those equations are assuming ideal flow, but RBCs aren't point particles and blood vessels aren't perfectly round tubes that can get to any arbitrary diameter. Eventually the vessel constricts to a point where it either collapses or a RBC can't fit through it anymore. Or one of the many other gazillions of explanations of how our flow is not "ideal".

I don't understand the controversy. All those equations are assuming ideal flow, but RBCs aren't point particles and blood vessels aren't perfectly round tubes that can get to any arbitrary diameter. Eventually the vessel constricts to a point where it either collapses or a RBC can't fit through it anymore. Or one of the many other gazillions of explanations of how our flow is not "ideal".

Exactly. I've come up with a model, but it only works for a spherical chicken in a vacuum.

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Flow = Pressure / Resistance

Increase in resistance decreases flow. Not rocket science here.
I know. but also increased resistance increases pressure as well which in turn increases flow

I know. but also increased resistance increases pressure as well which in turn increases flow

It says "despite high pressure", not "despite increasing pressure". The blood pressure is not increasing.

Pressure stay same. Resistance go up. Flow go down.

mate, when they refer to pressure they refer arterial pressure or aortic (typically). this does not mean pressure in the capillaries. if you can imagine the capillaries constricting, blood flow into them from the arteries will go down even though their pressure is high. increased blood pressure does not CAUSE increased blood flow. increased blood flow CAUSES increased blood pressure. this means in the capillaries, increased resistance increases pressure if flow remains the same, however since they are constricting, blood flow decreases, this decreases its blood pressure.

It says "despite high pressure", not "despite increasing pressure". The blood pressure is not increasing.

Pressure stay same. Resistance go up. Flow go down.

why pressure stays same?I don't think so...increasing resistance increases pressure

increased blood pressure does not CAUSE increased blood flow
whynot?F=P/R

let's generalize my question guys because I'm getting confused
In general,what is the effect of vasoconstriction or vasodilatation on blood flow?here I'm not talking about this great constriction which decreases the diameter to an extent that blood can't pass

increasing resistance increases pressure

No. Increasing resistance increases pressure drop over the arteriole.

In general,what is the effect of vasoconstriction or vasodilatation on blood flow?

When the arteriole constricts:
• Pressure at the aorta side of the arteriole does not change
• Pressure at the capillary side of the arteriole decreases
• Flow decreases.

This is really basic stuff. You might want to seek out a tutor if this is giving you that much difficulty.

why pressure stays same?I don't think so...increasing resistance increases pressure

whynot?F=P/R

let's generalize my question guys because I'm getting confused
In general,what is the effect of vasoconstriction or vasodilatation on blood flow?here I'm not talking about this great constriction which decreases the diameter to an extent that blood can't pass

Christ do you even read your questions before you post them? The least you could do is make them legible...

When the arteriole constricts:

Pressure at the aorta side of the arteriole does not change
that's not true.when an arteriole constricts, TPR increases which increases pressure in big vessels this is according to Guyton and it seems logical to me
actually I wish I can understand this well do you have any article which can help me?
thank you guys

sometimes you tell me pressure stays the same other times you tell me pressure decreases
that's confusing

guys I've come up with this conclusion :
VC in arterioles will increase resistance to flow , so more blood will flow to large arteries ,this increases pressure in large arteries while pressure in arterioles remains the same (since increased resistance makes blood flow to large arteries and decreases the amount of blood in arterioles)..accordingly blood flow to the organ decreases..
Is this true?

Sure, whatever, man.

are you sure that I get it right?does pressure in arterioles decreases or remains the same?

I don't even understand what you're asking anymore.

.