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fluids/bio question

Discussion in 'MCAT Study Question Q&A' started by holla, Aug 16, 2002.

  1. holla

    holla Junior Member
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    yo gotta question...according to bernoulli's if velocity of the fluid increases the pressure decreases..so now in blood flow such as in the arteries if the arteries constrict, i.e. increasing pressure, does that mean in this case the velocity decreases..which doesnt agree with the continuity equation that if area decreases velocity increases...which would say that pressure decreases in the arteries when it constricts..anyone get any ideas about this
     
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  3. TexasGuy41

    TexasGuy41 Senior Member
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    I think that you can't really relate blood to Bernouli's in the exact sense because with platelets and erythrocytes, blood is not going to behave like a normal fluid (water), so constricting the arteries, though it would make a fluid like water flow faster, since blood has some appreciable solids within it, it won't act the same way. Thats just my two cents though.
     
  4. Sonya

    Sonya Senior Member
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    Hi,

    I remember wondering the exact question so many times during our BME classes. It was for me one of the hardest concepts for me. If you asked me right after one of our exams or if I had my notes, i may be more helpful. I don't exactly remember now what the explanation was. But, i think you have to think along lines of, what are the assumptions of bernouilli's law? I believe it's based on conservation of energy (fluid changing potential to kinetic energy) and conservation of mass.

    I believe the explanation might have something to do also with whether or not blood flow is laminar vs. turbulent....
    or more likely, it was b/c there wasn't exactly conservation of mass , when you're changing the diameter of the vessel. In both situations, are you dealing with the same amount of mass of blood?

    umm, here's something on bernoulli's law:
    http://scienceworld.wolfram.com/physics/BernoullisLaw.html

    But, yes, if you decrease the diameter of a pipe, the fluid will flow across it faster. IF the pressure on both ends remains same. Keep in mind, the continuity equation deals with pressure difference on two ends of a tube, which isn't exactly blood pressure.

    If i cab get it straight, I'll try and write a more helpful explanation.

    Good luck!
    Sonya
     
  5. Neuronix

    Neuronix Total nerd
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    Your confusion stemms from trying to think locally and a systemmically. Let me try to break it down.

    Locally - Yes, if you increase the diameter of a section of pipe, velocity decreases, and pressure increases in that section. If you decrease the diameter, velocity increases, and pressure decreases in that section. Bernoulli's Law.

    Systemically - If one part of the pipe (ie an arteriole) contricts, velocity in that constricted area increases and pressure decreases. However, that increased the RESISTANCE behind the constricted area, which causes an increase in pressure in the system. So, when the arterioles constrict in the body, the heart has to pump harder to overcome the increase in resistance and the blood pressure in the arteries increases.
     
  6. tf2medic

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    haha so this post was 6 years ago, but i'm still somewhat confused by it. i'm having the same difficulty understanding.

    so locally, the pressure does decrease as area decreases and velocity increases, but systemically you're getting a net increase in pressure?

    so basically, Bernoulli's equation or however you spell his name only applies locally and not systemically?? so umm, why does velocity increase when you decrease the area then? I always assumed the velocity increased because reduced area makes the pressure go up and thus speeds up the flow, but what i consider intuitive thinking is wrong according to Bernoulli. the only explanation I can give is then the increased resistance outside the local area causes the velocity to be increased. is that right??

    can someone else explain this?? thanks.
    i never got down fluids well at all. my physics 1 teacher was behind and he left fluids till the very end and barely covered it. i don't think we were even tested on it ehh
     
  7. armybound

    armybound urologist.
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  8. tf2medic

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    okay that link doesn't really help. did you even read it or did you google Bernoulli's equation and paste that link? haha sorry...thanks for trying tho.

    I want to know why does velocity speed up when you decrease the area of the pipe when Bernoulli's equation says the pressure decreases. I would think the pressure would increase causing the velocity to increase.

    ohh wait a second, the whole increase pressure/decrease velocity relationship is only true when the height of the pipe does not change from one end to other end. height is essentially related to area since the height of the flow in the pipe is just twice the radius that you would use to find the area. so if the height changes, then the area does change from one end to the next, thus Bernoulli's equation doesn't apply and the continuity equation or whatever it is called does apply.

    okay, can anyone give me the reason why velocity speeds up then when you decrease the area? is it pressure related? thanks
     
  9. Neuronix

    Neuronix Total nerd
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    I wrote that in 2002? Yeah right like I remember that crap. Good luck :D
     
  10. tf2medic

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    haha, I actually got an answer. these forums are pretty useless asking about physics since everyone is bio i guess haha.

    i asked my BME friend...he said pressure is CONSTANT in the continuity equation: v1A1=v2A2. the reason velocity increases as area decreases is because of increased force. Pressure = Force/Area. since pressure is constant, as you decrease area, you must increase force to balance it out and keep it at equilibrium. thus, when you like squeeze a hose and the water moves through it faster, it's not more pressure that you are applying that makes it flow faster but rather more FORCE you are applying. it's really simple. this explanation doesn't contradict Bernoulli's **** whatsoever either. makes perfect sense.

    i think engineers are best prepared for this test.

    btw, admins...you pretty much destroy the thread when you move from mcat discussion to this more specific forum. no one comes here and thus you won't get replies. just my 2 cents. lata
     
  11. BerkReviewTeach

    BerkReviewTeach Company Rep & Bad Singer
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    from another thread:
    Blood pressure versus blood speed and flow rate questions and misconceptions are extremely common. It comes down to the perspective and definitions. There is a simple explanation for both speed/area relationship in a narrowing pipe and the blood flow versus pressure questions. But, I can't help but be a little leery to post after the last two people who tried to help got replies like the ones above. Maybe a bit later.
     
  12. tf2medic

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    i already got my answer on the fluid mechanics of the pipe from my BME friend, and i know how blood flow and blood pressure work so i don't need your help. i'm a bio major so i get all the bio stuff no prob, but my physics is limited since i only took physics 1 and 2 and they didn't cover fluids in either. i'm good now. thanks though!
     
  13. tncekm

    tncekm MS-1
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    Well, your friend is wrong. Given your tone, my better judgement tells me not to help you, but I think I'd like to rise above that to show you how to analyze pressure, volume, flow, and continuity appropriately. :D

    Pressure is NOT constant in the continuity equation. FLOW rate is constant in the continuity equation. That is, the amount (volume) of liquid that passes a given point will be constant.

    E.x. If you've got a 5cm area pipe that is seeing the transfer of 3L/s of fluid and you reduce the area to 2cm, the flow rate is still 3L/s. However, given:

    A1V1=A2V2 then
    V2 = (A1/A2)V2 = (5/2)V2. So, the velocity will increase by a factor of 2.5!

    Now, lets assume this pipe is horizontal and that there is no pump, so there is no change in potential energy associated with it. Bernouli's equation is now:

    ΔP + 1/2ρΔV^2 = 0

    Rearrange and

    ΔP = -1/2ρΔV^2

    ΔP = -1/2*ρ*(6.25-1)v^2
    ΔP = -2.625*ρ*v^2
    Pf = Pi - 2.625*ρ*v^2 (note: ρ & v^2 are positive by definition)

    In other words, Pressure Initial (Pi) is greater than Pressure Final (Pf). So, pressure surely isn't constant, but flow is.

    Glad I could help correct your misunderstandings, again :D
     
  14. RSAgator

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    Just to further clarify, while pressure is not constant, change in pressure IS constant (in this scenario). That is an assumption that is made when considering the continuity equation. There must always be some sort of driving force in order for fluid flow to occur. That force is generally from the pressure difference, or from potential energy (vertical column of water). This is why fluid in a horizontal pipe with both ends open to the atmosphere (aka both ends have the same pressure) wouldn't have water flow (aside from the water collapsing out due to gravity) whereas that same pipe, if vertical, would.
     
  15. tncekm

    tncekm MS-1
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    Yeah, so in this case just assume that the driving force is something like opening a faucet, or having a small tube exit the bottom of an extremely large water bin.
     
  16. tf2medic

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    yeah, okay that's what my friend was implying. he just didn't say it in as simple terms. he was just like "how can the pressure change of one small area effect and change the pressure of the entire system??" so basically, he was saying one area's change in pressure is too minimal to change the overall change in pressure of the entire system. i mean, yeah his explanation was a lil flawed (he actually said he HOPED he was right cuz he thought he forgot everything from fluids class) but he was close enough. using his logic wouldn't have made me miss it on the mcat haha.

    thanks for the clarification
     
  17. tncekm

    tncekm MS-1
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    All the poster above me said was that the fluid didn't move by itself--there had to be a driving force (gravity, pump, pressure difference, etc) and that when applying the continuity equation you're assuming the changes in pressure at the two points you're examining are constant.

    I think you misinterpreted what he said. He was just building on how to understand continuity.

    Although he did use "pressure" and "constant" in the same sentence, I don't think what he said is in agreement with your BME friend.

    Whew, explaining these concepts to you could be a full time job, but, I'm happy to do it since it helps me with my MCAT prep :luck:
     
  18. tf2medic

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  19. tncekm

    tncekm MS-1
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    Sorry, I'm not following...
     
  20. tf2medic

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    this concept makes perfect sense now AFTER reading EK physics!

    **** kaplan's explanation. (edit: okay actually i just went back and looked at kaplan and it says volume flow rate and doesn't equate area and velocity to pressure using cont. equation. it just doesn't explain it very well compared to EK. i LOVE EK)
    (edit again: to be even more fair, my stupid physics 1 barely covered fluids, so most of my first exposure to fluids was thru kaplan. EK was technically 2nd exposure to the topic so you could argue that's why i understood much better since it was my 2nd time reading over the material. but nonetheless, i still LOVE EK)

    it comes down to the fact that the cont. equation has nothing to do with pressure. the cont equation and its relationships deals with volume/mass flow rates being affected by area and velocity, NOT pressure. the pressure relationships are found using bernoulli's equation. also, the whole P=F/A is for fluids at rest and does not really apply here. the cont. and bernoulli's equations are for flowing ideal fluits, not ones at rest. w00t makes sense
     
  21. RHC1987

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    oops sorry didnt mean to post here.
     
  22. DrMattOglesby

    DrMattOglesby Grand Master
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    did anybody mention UPSTREAM vs DOWNSTREAM???
    because once you vasoconstrict, the pressure DOWNSTREAM will decrease whereas the pressure UPSTREAM will increase. and when it passes through the constricted portion of the vessel the velocity is higher.
    i dont know why everyone is tossing around all the equations for something so intuitive. I think someone started to mention a garden hose example though...muchos props to them
     

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