# TBR: Blood Flow and Pressure

Discussion in 'MCAT Study Question Q&A' started by justadream, Sep 5, 2014.

1. ### justadream 7+ Year Member

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TBR Physics II page 96 #16

"Compared to a healthy neighboring section of artery, a narrowed section (as a result of arteriosclerosis) yields"

Answer: decreased height for the manometer reading and a larger blood flow velocity through the constriction.

Explanation: "Using Bernoulli's equation, an increased flow velocity leads to a decreased local pressure against the walls in the narrowed region"

Translation: Less pressure, increased speed

My understanding was that in the unideal fluid dynamics in the body:

Dilating blood vessels:more blood flow, decreased pressure, decreased speed

Constricting blood vessels:less blood flow, increased pressure, increased speed

@Cawolf

2. ### Cawolf 5+ Year Member

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This again!

Yes, narrowing of a localized section of artery results in changes that produce less laminar flow and increase the chance of ischemia beyond the point (increase speed and decreased pressure).

You can't compare this problem to the highly generalized systemic changes you listed. You need to examine the situation more closely. A small narrowed section of artery does not equate to constricting blood vessels throughout the body.

3. OP

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@Cawolf

lol it's that time (TBR Fluids last 3/3 is today)

1) So do the generalized systemic changes only apply to larger systems (aka the entire body?)

2)
In localized system, are these true:

LOCALIZED:
Dilating blood vessels:more blood flow, increased pressure, decreased speed

Constricting blood vessels:less blood flow, decreased pressure, increased speed

4. ### Cawolf 5+ Year Member

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1) Yes, such as systemic vasoconstriction with sympathetic stimulation.

2) If by local, we are looking at a small section with normal flow before and after. . . then yes, except for the "more blood flow part". The reason the velocity changes is to keep the blood flow constant.

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@Cawolf

Okay so blood flow is the same in localized systems.

In whole-body systems, why does constriction/dilation change the volume of blood flow? Like where does the blood go? To the tissues/lymph?

New Summary:

LOCALIZED:
Dilating blood vessels: same blood flow, increased pressure, decreased speed

Constricting blood vessels: same blood flow, decreased pressure, increased speed

SYSTEMATIC:
Dilating blood vessels:more blood flow, decreased pressure, decreased speed

Constricting blood vessels:less blood flow, increased pressure, increased speed

6. ### Cawolf 5+ Year Member

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I am not sure that your generalizations for systemic blood flow are accurate.

I don't believe that systemic vasoconstriction alters the flow rate - it generally would supply more blood to the tissues in demand - such as skeletal muscle.

7. OP

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@Cawolf

So when people say that sympathetic innervation causes dilation of blood vessels to the skeletal muscles (providing them with more blood) but also causes constriction of blood vessels to digestive system (providing them with less blood), is that systematic or local?

Is that question above what you're trying to say (that whether something is systematic or local is like subjective)?

8. ### Cawolf 5+ Year Member

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Those would be systemic changes, that effect the whole body.

You can also look at them at a local level and be asking what happens right before and right after a certain spot that is changed.

It's not cut and dry - so yes, it is subjective and there isn't a general rule you can write because everything is different.

Just take what you know and reason it out based on what they are asking.

9. ### mehc012 Big Damn Hero 5+ Year Member

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Well, yes, but the venous system is a large reservoir of blood volume while at rest. When you exercise, the increased muscle activity can increase venous return. So you could see an increase in actual flow in times of need.

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10. OP

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@mehc012

Can you summarize when constriction/dilation of blood vessels leads to more or less blood (by that I mean volume) in the body?

11. ### mehc012 Big Damn Hero 5+ Year Member

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You're not gaining blood volume...that would require actually physically generating more fluid and blood cells.
However, if you have a large amount of blood in the venous system, and then you move it out of there more quickly than it is being put back in, even just for a short bit until things balance out, the amount of blood flowing/second increases and the amount sitting in 'reserve' decreases. If, subsequently, you do the opposite, the amount of blood flowing/second decreases and your reserve increases.

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12. OP

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@mehc012

So locally, if you constrict blood vessels, you would agree that (at least temporarily) you have less blood?

Also, how do you determine the effect on dilation/constriction on blood pressure/speed?

13. ### mehc012 Big Damn Hero 5+ Year Member

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You should never phrase it as 'having less blood' because you simply cannot alter the amount of blood in your body by changing the diameters of the blood vessels. So, no, I would not agree.

You only need to understand very simple cases of this such as the one described in the OP. For ideal fluids, you must assume constant volume flow rate, because that is all that the MCAT physics questions expect you to know. This will be Bernoulli, etc. For real fluids (e.g. Poiseuille's Law) the most important thing to understand is that length is inversely proportional to flow rate while the radius of the vessel to the fourth power is proportional, meaning that the radius is the biggest effector.

They are not going to ask you about changes in overall blood pressure, because that starts going into a whole host of non-MCAT topics, such as the Frank-Starling principle, and other various homeostatic mechanisms by which your body responds to and regulates changes in blood pressure. Still, no matter what, you need to know that the volume of blood in your body only changes rapidly in the case of acute hemorrhage.

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14. OP

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@mehc012

So for blood vessel problems on the MCAT, since you are saying to assume

1) Constant total volume
2) Ideal conditions

Then constriction should always lead to decreased pressure (since constriction leads to smaller cross-sectional area ==> faster velocity ==> less pressure because of Bernoulli effect)?

15. ### mehc012 Big Damn Hero 5+ Year Member

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I did NOT say to always assume ideal conditions. You will generally be told whether you are doing an ideal fluid problem (Bernoulli's) or a real fluid problem (Poisseuile's).

Constant total volume is not really an assumption - it is just what is true in the human body in any short-term timeframe excluding a case of severe hemorrhage.

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16. OP

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@mehc012

Gotcha, but for the question in the OP, for example, how do you know it is an ideal fluid problem? I mean like I doubt a question is gonna be like "this is an ideal fluid problem"

Also, if in ideal situations: constriction leads to smaller cross-sectional area ==> faster velocity ==> less pressure

What about in non-ideal situations (e.g., have viscosity)?

Constriction leads to smaller cross-sectional area ==> [Faster or smaller?] velocity ==> [Greater or less?] pressure

17. ### DrDreams Captain of the U.S.S. Enterprise Gold DonorVerified Account 7+ Year Member

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When you constrict a blood vessel, prior to the arteriolar point of restriction, the pressure is higher, and velocity is lower, post arteriolar point of constriction, the velocity is higher and pressure is lower. I think you have to ASSUME it is an ideal fluid (I've been burnt in genetics by not assuming true breed) so that is why I say assume it is.

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18. ### mehc012 Big Damn Hero 5+ Year Member

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First of all, yeah, almost all of the MCAT problems do specify 'assume an ideal fluid' or something equivalent (assume zero viscosity, etc). Second, if they do not give you a viscosity, you cannot be expected to calculate anything which requires a viscosity, no?

And finally, yes, it's reasonable to assume an ideal fluid unless specifically told otherwise, just as it is reasonable to ignore air resistance unless asked about it or to presume that a spring follows Hooke's Law.

Honestly, for most questions on the MCAT the real trick is to figure out what it is that they are trying to test your knowledge on. That's the real point. Once you do that, it just comes down to whether you know the equation.

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