P = QR driving me nuts (Cardiovascular Phys)

[AnonymousD.O.]

I have decided to post this question to the masses, I have been struggling to find an answer to this for a while now.

The equation of flow in the vessels analogous to Ohms law is of course

Pressure = Flow X Resistance.

Its pretty obvious that If resistance increases, than Flow decreases. What is driving me insane is what about certain situations like the capillaries?

Capillaries have low Pressure, that is pretty well defined. Also then, they have low flow compared to say an artery. BUT, there resistance is high.

Resistance however is proportional according to P=QR, so how can P and Q decrease, while R increases?

The concepts make sense to me but this simple equation pisses me off. I fee like there is some fundamental assumption or principle that I am missing.

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[Dharma]

Just throwing this out there, but I'm thinking it has to due with the capillary bed being a group of circuits in parallel (relative to one another), resulting in a total lower resistance for the capillary bed.

 

[redblue]

Yea, capillaries have the lowest resistance because they're in parallel. remember arterioles have the highest resistance

 

[CherryRedDracul]

Just throwing this out there, but I'm thinking it has to due with the capillary bed being a group of circuits in parallel (relative to one another), resulting in a total lower resistance for the capillary bed.

This is correct. Also, I want to throw out there that the P in the equation is the pressure gradient, i.e, the difference in pressure between two points. The greater the gradient, the faster the flow.

Also, this site is a gift from God. I used it extensively when I took cardiovascular physiology back in grad school: http://www.cvphysiology.com/Hemodynamics/H004.htm

 

[Dharma]

@CherryRedDracul sweet link… and scary new avatar! Nice!

 

[AnonymousD.O.]

This is correct. Also, I want to throw out there that the P in the equation is the pressure gradient, i.e, the difference in pressure between two points. The greater the gradient, the faster the flow.

Also, this site is a gift from God. I used it extensively when I took cardiovascular physiology back in grad school: http://www.cvphysiology.com/Hemodynamics/H004.htm

Ok say let me pose this question then. What about a stenotic vessel, say the Aorta.

With the decreased radius of the vessel, Pressure would go up. But because the vessels has a smaller lumen now, flow would decrease right? Now this gets confusing because obviously with stenosis, physiologic changes occur to increase flow against this increased pressure (say LV hypertrophy). But if a question asked what happened to pressure or flow, what would you put? (((my physio teacher has confused our whole class on this)

 

[CherryRedDracul]

Ok say let me pose this question then. What about a stenotic vessel, say the Aorta.

With the decreased radius of the vessel, Pressure would go up. But because the vessels has a smaller lumen now, flow would decrease right? Now this gets confusing because obviously with stenosis, physiologic changes occur to increase flow against this increased pressure (say LV hypertrophy). But if a question asked what happened to pressure or flow, what would you put? (((my physio teacher has confused our whole class on this)

I'll talk about it in terms of aortic valve stenosis.

I can understand that CV physiology gets confusing because so many things are happening which can affect variables in multiple ways. When looking at the deltaP (pressure gradient) = QR equation, it's best to hold one variable constant while examining the relationship between the other two variables. So what's happening with aortic valve stenosis?

The valve orifice area is decreased causing an obstruction. Obstruction is synonymous with resistance. If we hold resistance constant, what's the relationship between deltaP and Q? If there's an obstruction, you can expect flow across the valve to decrease for the moment. Meanwhile, the obstruction will cause pressure to build up proximal to it as the heart puts the Frank-Starling mechanism to work (against the afterload) while the lack of flow distal to the obstruction means pressure will be lower. The difference in the increase pressure before the obstruction and the decreased pressure after the obstruction means that the pressure gradient, deltaP, has increased. Eventually, enough pressure builds up that flow will be restored albeit at a higher deltaP.

In mild aortic stenosis, flow may remain the same but may be at a higher deltaP. In severe aortic stenosis, however, pathological changes in LV (i.e., hypertrophy) may cause cardiac output AKA flow to decrease because at that point, the heart simply cannot keep up.

I hope I have this all correct, btw. You might want to check this paper out though because I think it can clear up a lot of confusion that many students have about CV physiology. Actually, I highly recommend it: http://advan.physiology.org/content/25/2/8

 

[AnonymousD.O.]

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[AnonymousD.O.]

I'll talk about it in terms of aortic valve stenosis.

I can understand that CV physiology gets confusing because so many things are happening which can affect variables in multiple ways. When looking at the deltaP (pressure gradient) = QR equation, it's best to hold one variable constant while examining the relationship between the other two variables. So what's happening with aortic valve stenosis?

The valve orifice area is decreased causing an obstruction. Obstruction is synonymous with resistance. If we hold resistance constant, what's the relationship between deltaP and Q? If there's an obstruction, you can expect flow across the valve to decrease for the moment. Meanwhile, the obstruction will cause pressure to build up proximal to it as the heart puts the Frank-Starling mechanism to work (against the afterload) while the lack of flow distal to the obstruction means pressure will be lower. The difference in the increase pressure before the obstruction and the decreased pressure after the obstruction means that the pressure gradient, deltaP, has increased. Eventually, enough pressure builds up that flow will be restored albeit at a higher deltaP.

In mild aortic stenosis, flow may remain the same but may be at a higher deltaP. In severe aortic stenosis, however, pathological changes in LV (i.e., hypertrophy) may cause cardiac output AKA flow to decrease because at that point, the heart simply cannot keep up.

I hope I have this all correct, btw. You might want to check this paper out though because I think it can clear up a lot of confusion that many students have about CV physiology. Actually, I highly recommend it: http://advan.physiology.org/content/25/2/8

Thanks CherryRedDracul haha......answers like that are why I like studentdoctor. You helped out a lot, I hope other people see your answers.