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Why does blood pressure drop as you get farther from the left ventricle?

  1. Apr 2, 2012 #1
    Blood is in continuous contact with itself. Thus any pressure on artery blood is pressure on vein blood. So they have the same pressure. What am I missing here?
     
  2. jcsd
  3. Apr 2, 2012 #2

    DaveC426913

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    Which question do you want an answer to? Change in BP at a distance from the ventricle, or change in BP between arterial and veinous blood?

    If the latter, examine the physical proximity between arterial blood and veinous blood. They are certainly not in continuous contact with each other.
     
  4. Apr 2, 2012 #3
    You didn't quite understand the question. Blood IS in contact with itself. There is no gap in blood. I have a straw and I blow water from one end to the other end. Why is the pressure of water in the far end of the tube lower than that in the end closer to my mouth? Pressure is imparted on the entire fluid as a whole.

    Edit: pushing a line of marbles from one end causes a push of the whole line of marbles. To say that pressure decreases is like saying that the marbles closer to the push are faster, which is phsically impossible because they don't go through each other.
     
  5. Apr 2, 2012 #4
    The blood pressure decreases as the blood travels farther away from the heart BECAUSE: the heart is a pump. Imagine with me a water pump. The water pump is pumping water through a smooth straight length of pipe. Each big "pump" makes the pressure go up to 80 to 120mmHg. The inside pressure that is there at all times inside the pipe because it is so full of water is always staying around 45 to 70 mmHg. These numbers are known as systolic and diastolic pressures. Okay? So. If the pipe is really long and not straight but very curvy sometimes then the water pressure at the end of the plumbing will necessarily be lower, will it not? There is less blood pressure in the artery of your big toe, for example than in your left arm.
     
  6. Apr 2, 2012 #5

    DrGreg

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    The answer is friction. Blood is a viscous fluid and rubs against the side of the vessels it passes through. The vessels exert a force on the fluid opposing its motion and reduce the pressure. In the wide, main arteries, the pressure doesn't drop much (but gravity has some effect). In the narrower arteries and arterioles the effect is much greater. The veins (which operate at much reduced pressure) also have valves in them which have an effect, too.

    Wikipedia contributors, "Blood pressure" Wikipedia, The Free Encyclopedia, (accessed 2 April 2012). Text is available under the Creative Commons Attribution/Share-Alike License
     
  7. Apr 2, 2012 #6
    But if pressure is lower in the veins, wouldn't that mean a build up of blood in the veins?
     
  8. Apr 2, 2012 #7
    A fluid in a pipe encounters friction along the pipe walls. If you lengthen the pipe you need more pressure for the fluid to move at the same flow rate.The pressure is greater at the pump than at the farther end, contrary to waht you have stated.

    Your marble analogy is not correct. All marbles have some sort of friction while rolling. The marble you are pushing on has to overcome the rolling friction of all of the marbles down the line. The last marble has to overcome only its rolling resistance, so it has the least amount of force ( or pressure ) on it, while the one you are pushing has the most.
     
  9. Apr 2, 2012 #8

    DaveC426913

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    I understood the question. You mentioned arterial and veinous blood. They are separated by a significant amount of passage through capillaries. Lots of friction.

    In your water and straw analogy, consider what would happen if the middle third of the straw were constricted to 1/10 the normal diameter. You'd have to jam on the water to get it through the constriction at any reasonable flow rate. and even at that it would be at much lower pressure when it came out the other side. Like a trickle.

    In your marble analogy, imagine if the marbles had to all be pushed uphill. And that the tube they were being pushed through was flexible.

    Make sense now?
     
  10. Apr 2, 2012 #9

    boneh3ad

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    No. Blood moves from the arteries to the veins, so the pressure must be lower in the veins for the blood to keep flowing. By the time it reaches the vena cavae, it is at very low pressure, at which point the heart functions as a pump to draw it in and expel it at its higher pressure again.
     
  11. Apr 2, 2012 #10
    Sorry guys. I feel stupid because I simply can't understand this. I have these frequent instances where I hit a wall with certain physics concepts. My success rate in overcoming them is very low (33%). I don't know if I am mentally challenged, because on the surface it doesn't seem that way. And the thing that kills me is that I absolutely love physics.
     
  12. Apr 2, 2012 #11
    Ok, first of all: what is pressure? And then, what is blood pressure?
     
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