Do you know this physio..?

Discussion in 'Step I' started by sparkplug, May 6, 2008.

  1. sparkplug

    Jan 23, 2008
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    Medical Student
    Do some one explain this please?

    By the effect of Autonomic nerv. system or other will increase blood pressure and flow like by vasoconstriction, however in bernoulli's principle, it says that pressure and flow volume will decrease during constriction and increase in dilation...this is what we seen during stenosis or clogging of blood vessels. Simmilarly, in autoregulation--like by vasomotion, blood flow and pressure will decreased by constriction and vice versa..this will agree with bernoulli's but the vasoconstriction/vasodilation effect that the body uses to increase or decrease the blood pressure + flow will not agree with this rule...would you please explain these

    One other thing...assume a small capillaries with same crossectional area will branch out from a single arteriole(with higher crossection). I know that the speed at each branches are equal and less than that in the artery/arteriole, but do you think the flow or volume will be same both in the arteriole and at each capillary branches regardless of the area difference? I.e Q1 vs Q2

    Artery, Q1.........Arterioles, with Q2.......................branches of capillaries, with Q3

    Thank you very much
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  3. MattD

    MattD Curmudgeon

    Oct 9, 2003
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    Resident [Any Field]
    to answer question #1, you get a decrease in pressure AFTER a local constriction, ie. when flow goes from the constricted area to the higher diameter area, pressure drops. There is an energy loss associated with passing through the constriction, so the pressure after the constriction will be lower than the pressure before the constriction. Think about the constricting valve in a refrigeration system (if you took thermo), or, consider a constriction of the afferent arteriole of the nephron. If you constrict the afferent arteriole, the capillary pressure drops. However, this is a local constriction. If you constrict ALL the vessels in the body systemically, then you have a lower total volume for the fluid to exist in, thus increasing the pressure.

    So you're right, in Bernoulli's equation, when you go from a large diameter pipe to a small diameter pipe, the speed goes up and the pressure goes down. Locally this is what happens. But, this is assuming a static system. You're not taking a pipe network at steady state and then CLAMPING DOWN on the pipe, and seeing a decrease in pressure. If you did this, you would still see the pressure drop locally as compared to the pre-constricted area, but GLOBALLY the average pressure throughout the system would go up.


    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 psi
    50 psi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 psi



    ---------------------------------\ . . . . . . . . . . . . . . ./-------------
    . . . . . . . . . .. . . . . . . . . .. . . . .\_________________/ . . . .49 psi
    55 psi . . . . . . . . . . . . . . . . . . . . . . . . . 33 psi


    make sense? (please forgive the periods, i don't know how to insert whitespace.....)

    I didn't understand the 2nd question.
  4. docmd2010

    docmd2010 Senior Member

    Apr 17, 2005
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    great answer.
    another way i look at it, is if arterioles are being constricted, blood is being backed up into the arterial system because the constriction doesnt permit it go any further. So now there's more blood in the artery that supplied that arteriole, and thus more blood in aorta. Now, the heart must pump against a significantly higher resistance (increased afterload) because, in effect, it must pump blood into the more-than-normal-filled aorta. Imagine going to someone's apartment and trying to open their main door to get in, if the people on the inside are all pushing against the door while you're trying to push in, u'll have to push a lot harder now right? So if you're a heart in this situation, what do you do? you undergo hypertrophy, and get stronger, and push harder and harder, and pull through, and push the door open against all that blood...You can see now, how HTN leads to heart failiure-eventually the heart can't push anymore, and essentially gives up. A similar effect is noted when you have, say atherosclerosis, (or anything that decreases vessel compliance), now the vessels can't distend as much, and the heart as to work even harder (blood pressure goes up).

    Hope this helps.

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