# PHYSIO Cardio Help!! Velocity, Resistance, Radius.....

Discussion in 'Step I' started by Proverbs31, May 25, 2008.

1. ### Proverbs31 5+ Year Member

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I cant figure out where my brain is not making the connection. Can anyone help. This is loaded detail with Physio so be forewarned.

So I'm trying to "Understand" all this, so it makes sense and its not memorization. Im trying to relate all the equations in Cardio Phys and I seem to keep tripping up on one main thing. Here goes:

If ...and it is because Im copying from BRS Physio Cardio Chp 3 equations....

If Velocity of Blood Flow can be expressed by V= Q/A, so Velocity is directly proportional to Blood Flow (Q) and is inversely proportional to the cross sectional area (A) at any level of the cardiovasc system,
And
Turbulance (non straight blood flow) is INCREASED by 1. DEC blood viscosity, and 2. INC blood Velocity (with the example of narrowed vessels beside it in the book),
AND
Resistance is Directly proportional to viscosity, and Inversely proportional to the radius (by a power of 4)......by Poiseuilles eq. R= 8nl/ (pie symbol)r^4

Then you get the relationship that Blood Flow (Q) is inversely proportional to Resistance...so it cant make sense for Velocity to be increased if vessel radius is smaller.

Now look...I get it in my head ..I can almost hear my old Kaplan MCAT instructor talking about the example of putting your finger on a hose and how much faster the water comes out but the equations dont add up?

See below:
If Velocity (V) increases w/ small or dec area, and Resistance is proportional to Viscosity, and Inversely prop. to radius, then as radius decreases, resistance increases and velocity increases (bc its inversely prop to surf area), so you think Velocity is proportional to Q (Blood flow) and Q is INVERSELY proportional to resistance, then WAIT!! if you Inc resistance, you decrease Q, and thus Decrease Velocity.

I probably just lost all of you in that translation but its not making the connection all the way across so Im getting confused. It makes sens that dec a diameter will inc the velocity of the blood thru it, but that doesnt hold true if you look at the example in the book, where its saying velocity is prop to flow, well flow is inversely prop to resistance which is inversely prop to radius...so DEC radius, Inc Resistance and thus Dec Flow and Thus dec velocity....
So a smaller or narrowing of the vessel CANT inc Velocity.

Can anyone help or did I just confuse the masses?

2. ### Proverbs31 5+ Year Member

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Sorry let me try to clarify this.

If you look at Resistance equation (Poiseuilles equation), and all the things that go with that ...i.e. what is inversely vs directly proportional to resistance then it doesnt make sense or follow true for the example they give in the book for Turbulance being Increased by Increasing Velocity and they put in parenthesis next to Inc velocity: (Narrowing of a vessel) for the reason youd have inc velocity. Bc Velocity is proportional to Blood Flow and Blood Flow clearly decreases with resistance. So how is true

Can anyone explain that?

3. ### Monica Lewinsky 2+ Year Member

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Poiseuille's Law assumes laminar flow, so you really can't daisy chain those equations together as it makes a paradoxical situation as you pointed out.

The velocity will increase if the vessel is smaller and the flow rate is the same. The reason why BRS goes into viscosity is that it INCREASES resistance (e.g. higher hematocrit = higher BP) and that it DECREASES turbulence (you are more likely to hear a murmur in someone with anemia).

4. ### Proverbs31 5+ Year Member

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Thanks...I ended up calling my best friend at Harvard whos in anesthesiology bc it bugged me so much...and as you said...He said I wasnt taking into account Pressure either and he said I couldnt relate the two equations...
Basically if you have the same amt of volume and you constrict the hole it needs to get out...1. its going to increase velocity to try and get it out and thus youll have higher pressure...and if it cant...you get backup...

I appreciate you helping answer it for me.
Thanks!

5. ### LUBDUBB Freakaholic 10+ Year Member

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Well it's a good thing you didn't call me, because I'm only at GWU Just messin', glad you figured it out.

6. ### osli Senior Member 5+ Year Member

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To put your mind at ease, you didn't do anything terribly wrong in your examination of the equations. But you glanced over a few details that would have helped clear up your confusion.

1. Yes, resistance is proportional to velocity since higher velocity means higher turbulence which means higher resistance. However, you have to keep in mind that there is a proportionality constant in there, so it isn't anything like a 1:1 increase (i.e., doubling velocity does not double resistance, it only increases it slightly).

2. Yes, if you string the equations together it is TRUE that when you increase velocity (whether by pressure increase or diameter decrease) there is a corresponding increase in resistance. But that is an effect, not a cause. You can't ignore all of the other contributing variables... there are many in the big picture, and changing any one has effects on most all of the others. If you increase resistance WITHOUT changing any other variable (e.g., increased viscosity), then you get a decreased velocity. If resistance increases BECAUSE you changed another variable (e.g., decreased radius) then you can't assume that velocity will similarly decrease, because now you have to consider that the change in radius has a direct effect on the velocity. As pointed out above, we use equations that are based on laminar flow, and then just remember that in the real world increased turbulence increases resistance in most cases. If you were looking at the turbulent flow equations, it wouldn't have looked like a paradox.

End result: the increase in resistance as velocity increases has the effect of reducing the amount of increase in velocity as a result of vessel narrowing. In other words, V=Q/A would predict that as area halves velocity doubles, but in the real world the effect of increased turbulence limits that increase such that it is somewhat less than double. That is what you discovered in your exploration of the details.

7. ### Proverbs31 5+ Year Member

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Not sure how you did that, but you made me feel slightly *smart*.... dare I say....while I was feeling like a ****** earlier.

Thanks...I appreciate all the help. Def makes sense now.

So for the Cardio section and all the wonderful charts and graphs with it in FA that make me want to puke...how much detail do I need to understand in them? For instance, the beautifully simple (insert sarcasm) Cardiac Cycle on bottom of pg 234 FA 2007-with everything from the ECG, to Heart sounds, to Phases.....Do I just need to understand where each one occurs and what is going on in that pathway..i.e. Isovolumetric relaxation: period bw Aortic Valve closure and Mitral Valve opening....Or do I need more details.

Why I ask is because Im "that" DETAIL person....I will kill something and look up so much stuff about it youd think I was a cardiologist. I will continually look things up until I have an answer to every question I had. I know I cant afford to do that for the USMLE on every subject and with ensuing time constraints. SO I guess Im trying to fig out how much I need to elaborate on with BRS Phys or other sources because the cardio section is really alot of graphs and short liners. Starling Curve? Many things affect Preload/afterload/SV etc and only a few things are listed. CO variables, EF....etc Thoughts?

8. ### Proverbs31 5+ Year Member

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hahah! Sorry... extra piece of info that wasnt necessary...
I should be thankful hes even my friend...
Just so happens hes the only one of my "already doctor" friends that I got a hold of.

9. ### Monica Lewinsky 2+ Year Member

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Understanding details were really for your time during coursework, you gotta triage information if you are getting close to taking your boards.

10. ### LUBDUBB Freakaholic 10+ Year Member

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If you understand mechanisms, you can apply your knowledge to any problem. If you don't understand mechanisms, you better hope the patient comes to you with a b c d taped to his/her forehead
With that kind of respect for learning, proverbs can be my dr. any day.

11. ### Monica Lewinsky 2+ Year Member

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Thats why we take coursework before we study for step 1.

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