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Pressure in a glass bottle

  1. Aug 7, 2005 #1


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    I have been looking at some discussion questions in a textbook and there is a question about blood transfusion from inverted glass bottle rather than a flexible bag. The question" What would happen if blood were transfused from an inverted glass bottle rather than a flexible bag?

    I was thinking along the lines in the volume difference but then I think I am missing a link to or an idea to connect it to pressure and how the velocity of the blood coming out will change. Could it also be due to the air bubble it will have changing the pressure of the bottle of the inside compared to the outside thus stopping it from letting any more blood out? I'm not to sure what principles I need to look at to try and explain this to myself as I am a little bit confused.
    Could someone help me straighten out what ideas i need to look at in particular?

    Thanks in advance :smile:
  2. jcsd
  3. Aug 7, 2005 #2


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    The difference between a "bag" and a "bottle" is that the bag will collapse. As blood comes out of the bag, air pressure inside the bag is reduced so air pressure outside "collapses" the bag- result, air pressure inside becomes the same as air pressure outside (you are right- PV= nRT. n is decreasing but volume is also decreasing so P stays constant. Of course, it's the same as if there were NO bag there.)
    As blood came out of a bottle, the air pressure inside the bottle would be reduced because "n" is decreasing. The bottle is rigid so V cannot change. Another way of saying that is that the air pressure outside is NOT "transmitted" to the blood. Pretty soon the pressure on the blood would drop below what is necessary for blood to come out.

    When you are pouring water from a bottle, you hear the "glug, glug" because when the pressure inside the bottle becomes too low the water has to stop pouring long enough to allow air in the bottle to restablish the air pressure inside the bottle. Since a blood transfusion hose goes directly into a blood vessel, that can't happen.
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