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How can hydrostatic pressure drop to zero?

  1. May 6, 2010 #1
    Hello everyone,


    If this is a vessel the hydrostatic pressure would drop along the vessel due to resistance. My question is why does hydrostatic pressure drop if hydrostatic pressure is due to gravity. Gravity is always there, even if the molecules lose hydrostatic energy, they would gain it back due to gravity?

    Also why is this called hydrostatic pressure, the gravity is acting down, and the pressure is acting other way. I don't get it.

    Thanks :smile:
  2. jcsd
  3. May 6, 2010 #2


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    Staff: Mentor

    Hydrostatic pressure is due to gravity only, not due to motion, so resistance is irrelevant and it doesn't drop to zero.

    If you prop a dead person up against a wall, the only pressure (gauge pressure) left in their blood vessels is hydrostatic.
    Last edited: May 6, 2010
  4. May 6, 2010 #3
    Hey thanks Russ I appreciate the help :smile: but in my text book it say if constrict the area where blood enters the vessel, the hydrostatic pressure drops. Ok I don't want to know why it is just that hydrostatic pressure keeps changing in certain conditions. If it is due to gravity why is it changing, I'm starting to doubt that hydrostatic pressure in this diagram is not due to gravity?

    Here the diagram shows that if hydrostatic pressure if greater molecules inside the vessel would go out, but I'm getting confused how a moving fluid can have hydrostatic pressure. If water sits on a cup at rest, I can understand hydrostatic pressure? Why are they getting hydrostatic pressure involved with vessels, when static pressure is more appropriate?
  5. May 6, 2010 #4


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    Staff: Mentor

    [edit] Er, wait - if blood is flowing, then this is not a static situation and you can indeed exchange [hydro]static and dynamic pressure. You do still have the wiki for Bernoulli's principle handy, right...?
  6. May 6, 2010 #5
    Thanks again. This is a quote

    Capillary Hydrostatic Pressure (PC )

    This pressure drives fluid out of the capillary (i.e., filtration), and is highest at the arteriolar end of the capillary and lowest at the venular end.

    Edit: Didn't see your edit, I understand bernoulli, static and dynamic pressure but your sentence is bit unclear for me. Are you saying hydrostatic is actually static pressure when blood is flowing?
  7. May 8, 2010 #6


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    Staff: Mentor

    Googling, it looks like they are using the term differently than is my understanding for fluid dynamics. Here's the whole link: http://www.cvphysiology.com/Microcirculation/M012.htm

    The rest of the quote related to the issue:
    It looks to me like they are describing static pressure, not hydrostatic pressure. The way I understand the definitions for fluid dynamics, hydrostatic pressure is a subset of static pressure. "Static pressure" is the pressure of a static fluid - meaning without direct influence from motion. But that pressure can be created in several ways:
    1. By gravity.
    2. By pressurization of a closed system (ie, a squeezed balloon).
    3. By a pump.
    4. By a device that converts velocity pressure to static pressure.

    Here's a dictionary reference for "hydrostatic pressure" that seems to imply the engineering field and medical field use the same term to mean two different things: http://www.answers.com/topic/hydrostatic-pressure

    But anyway, since you asked about the gravity part of the definition...

    Imagine a pair of reservoirs, one above the other. A pipe extends from the bottom of the upper one down to just below the surface of the bottom one. A valve is in the middle of the pipe. The pipe is full of water (and is less than 35 feet long).

    Case 1: The valve is closed. when the valve is closed, you have two pressures at work. Hydrostatic pressure at the valve would be from the weight of the water sitting on the valve. You also have the static pressure due to the atmosphere pushing down on the reservoirs. This is often treated as a regular static pressure (ie, a pressurized vessel), but if the altitude changes enough, it is more like a hydrostatic pressure. This pressure increases the pressure at the valve - from both above and below. From above, it is obvious. From below, it is the air pressure pushing down on the lower reservoir and pushing water up the pipe to the valve.

    Case 2: Open the valve completely. The hydrostatic pressure still exists, but now it acts all the way down the pipe. The static pressure at the bottom of the pipe won't be due to the weight of the water though: since the valve is open, much of that pressure is converted to velocity pressure in making the water move from the top reservoir to the bottom one. And since the valve is open and the atmospheric pressure pushes against both reservoirs equally, it cancels out and you can ignore it.

    Case 3: Close the valve partway. Now the valve creates a restriction that again causes it to support the weight of the water above it, similar to case 1....except this time it only supports some of the weight. Some of that hydrostaticpressure also goes to making the water move. How does that relate to the question in the OP? Because of the restriction, there is a static pressure loss (drop) across the valve. The hydrostatic pressure below the valve is lower than it is above the valve because the valve is supporting some of the water column above it.
    Last edited: May 8, 2010
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