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I Breathing under water (long snorkel tube and scuba tank)

  1. Aug 2, 2016 #1
    Hello Forum,

    Humans breath by inhaling and exhaling air at 1 atm of pressure (which is ~15lbs/in^2). Air pushes from the outside on every inch of our body with a force of 15lbs but an equal force is pushing from the inside. The net force is zero.

    The water pressure increases linearly with depth (1 extra atm every 10 meters). As far as divers go, why do scuba tanks need to supply air at the same pressure as the external underwater ambient pressure? During the inhaling phase, muscles contract (diaphragm, intercostal muscles) and the lungs expand causing the air pressure inside them to decrease an become less than the external pressure. That automatically forces air inside the lungs.

    What is the a problem if the compressed air supplied by the scuba tank is at a higher pressure than the ambient water pressure at that depth?

    What is the problem breathing with a long snorkel tube which is connected to the outside air where the pressure is 1 atm? Why are the inhaling and exhaling very difficult? I am not sure I understand why the high water pressure at a certain depth pushes "too hard" on the body for the muscles to contract. The ambient and internal body pressure should always equalize otherwise the body would collapse...

    thanks,
    fog37
     
  2. jcsd
  3. Aug 2, 2016 #2

    jbriggs444

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    Have you ever tried to sip from a 30 foot straw? It is hard to inflate your lungs when the air in your lungs is at much less than ambient pressure.

    Have you ever inflated a balloon so much that it popped? That's what happens when the difference between internal pressure and external pressure is more than the balloon can sustain.

    When you suck on a straw or blow up a balloon, the pressure of air in your lungs is different from ambient.
     
  4. Aug 2, 2016 #3
    Thanks jbriggs444.

    I see how sucking from a straw requires the air inside the lungs to be at a lower pressure than atmospheric pressure (opposite for blowing a balloon up).

    "...Have you ever tried to sip from a 30 foot straw? It is hard to inflate your lungs when the air in your lungs is at much less than ambient pressure...."
    To inhale (and inflate the lungs) the internal pressure must be less than the ambient pressure. You mention that the air in the lungs is much less than ambient pressure. That should lake inhaling very easy
     
  5. Aug 2, 2016 #4

    berkeman

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    I tried breathing through a hose and going underwater in our pool when I was about 10 years old. I made it to bout a foot or two underwater before I couldn't breath in any more. Quite an eye opener!

    The water pressure pushing in on your chest and abdoment has to be overcome for you to breathe in.
     
  6. Aug 2, 2016 #5

    cjl

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    When inhaling, the pressure in your lungs must be less than ambient, but it only takes a very small pressure differential to create a fairly strong flow. I would be surprised if your lungs could generate more than a couple psi of pressure above or below ambient, which only corresponds to a few feet of water depth. Consider that trying to breathe through a straw at a depth of 30 feet would be the same pressure differential as trying to create a nearly perfect vacuum by sucking the air out of a container with your lungs.

    As for this statement:

    "The ambient and internal body pressure should always equalize otherwise the body would collapse..."

    Exactly. The problem is, when you connect the lungs to the surface via an air tube, the air in the lungs will equalize with the air on the surface, but the body is trying to equalize to the surrounding water pressure. If the muscles can't support this pressure difference, the lungs will collapse.
     
  7. Aug 2, 2016 #6

    jbriggs444

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    The air in the straw is also much less than ambient pressure. But your muscles are not fighting that. They are fighting to expand against the normal ambient pressure outside the chest walls.

    If you want to make inhalation easy you can use high pressure air delivered via nose and mouth (aka a CPAP machine).
    If you want to make exhalation easy, you can use low pressure air at nose and mouth but not so low that the airways collapse (a BIPAP machine comes close).
     
    Last edited: Aug 2, 2016
  8. Aug 2, 2016 #7

    Dale

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    Can you bench press 1000 kg? Why not?

    Can you lift the same amount with your arms as you can with your legs? Why not?
     
  9. Aug 2, 2016 #8
    ok, thanks everyone. I am almost there (but not yet).

    As cjl mentions, when inhaling, the air pressure in your lungs must be less than the pressure of the ambient air (very small pressure differential).
    When considering a diver under water, we need to consider two pressures: the water pressure applied on the body and its muscles and the air pressure of the air we breath. To inhale or exhale we need to be able to create air pressure differentials. The muscles must be able to contract and relax to produce those air pressure differentials.

    Using a snorkel tube at a depth under water: the water pressure on the external surface of body is much larger than 1 atm.

    "...The lungs are connected to the surface via an air tube, the air in the lungs will equalize with the air on the surface..." OK. Now, to inhale, we lungs must have a lower pressure. The large water pressure should not be a problem to do that....what about exhaling?
    "...but the body is trying to equalize to the surrounding water pressure. If the muscles can't support this pressure difference, the lungs will collapse..." how does the body equalize the surrounding water pressure?
     
  10. Aug 2, 2016 #9

    berkeman

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    Of course it is a problem. Do you know how you inhale? The muscles attached to your diaphragm muscles pull the diaphragm down, creating a lower pressure in your lungs than the 1 atm ambient air pressure. But with > 1 atm of pressure pushing in on your abdomen, your diaphragm muscles are not strong enough to pull down for you to inhale air from the surface.
     
    Last edited: Aug 2, 2016
  11. Aug 2, 2016 #10

    berkeman

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  12. Aug 2, 2016 #11

    Filip Larsen

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    When the pressure of air in the lungs are sufficient higher than the ambient (water) pressure at the chest, it is very easy to sustain potential fatal injury to the lungs from over expansion (search for pulmonary barotrauma). Not surprising then, one of the most important rules when diving with supplied air is not to hold your breath, especially during any ascents. If you were to supply a diver with unregulated high pressure air (a free-flow valve) then it would be necessary with an open system such that a free water surface was near head or chest to allow the supplied air to drop to ambient water pressure. This means that most of the supplied high pressure air will just escape and be wasted.

    That said, a slight over pressure in air supply can be useful without being harmful when done right. The usual modern diving regulator has an demand valve in the mouth piece that typically is set to deliver air slightly above ambient pressure when you create a small under pressure in the piece. This ensure you can breath in very easily and since it is a valve that closes when you stop maintaining a demand pressure difference in the mouth piece you do not have to fight this small over pressure when exhaling.
     
  13. Aug 2, 2016 #12

    Dale

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    Why not? Have you tried to estimate the forces involved?
     
  14. Aug 2, 2016 #13

    OCR

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    Lol, I tried that also, only it was in a stock pond, but a nice stock pond... looked almost exactly like this one .[COLOR=#black]..[/COLOR] :oldsmile:
    Lol... seems I made it about a foot or two underwater before I couldn't breath anymore either.[COLOR=#black]..[/COLOR]:oldwink:

    What You and I should've done, was to take some 55 gallon barrels, and made a couple of super nifty... diving bells, man, that would have worked great![COLOR=#black]..:oldcool:[/COLOR]


    I couldn't swim back then, and I still can't swim now; in fact, swimming to me means... "staying alive in the water." [COLOR=#black]..[/COLOR]:approve:

    However, I can usually keep from breathing water by using the ... "floundering technique" .[COLOR=#black]..:oldlaugh:[/COLOR]
     
  15. Aug 3, 2016 #14
    ok, thanks everyone.

    Berkeman and Filip Larsen, let me see if I have the right idea:

    There are two fluids (air and water) and their pressures: the air pressure in the air and the water pressure inside the water.
    At the surface, the human body experiences an external ambient pressure of 15 lbs/in^2. That same pressure is exerted also from the inside of the body so we don’t get crashed. The act of breathing involves inhaling and exhaling air. To inhale, we contract certain muscles (intercostal, diaphragm, etc.) and cause the lungs volume to enlarge. The air pressure inside the lungs decreases and the pressure differential (P_air_outside – P_air_lungs ) forces air into our lungs. During exhalation, the opposite happens: muscles relax, the lungs volume decreases and the air pressure in the lungs becomes slightly larger than the ambient air outside. Correct?

    Now let’s go underwater. The water pressure increase linearly with depth d and P_water> P_air = 1 atm. For example, at a depth d = 2m, the water pressure is
    P_water = 1.2 atm. The lungs are connected to the outside ambient air via a tube that is 2m long. Why is it difficult to breath at that depth with a tube, i.e. why can we not inhale and exhale the same way we normally do using the tube? Because the muscles in the body are unable to contract to cause the lung expansion. To contract, they need to overcome the outside pressure which, if too high, cannot be overcome. At a depth of 2 m the diver's body feels a compressive force 17.6 lbs on every square inch.
    So that explains why it is impossible to inhale at large depths in water even using a long snorkeling tube: our muscle cannot contract by winning the external force due to the water. But let's say we managed to inhale some air. What would be the problem with exhaling?

    Scuba tanks:
    As far as using the scuba tanks, the supplied pressure must be the same as the water pressure at that depth: When the pressure of air in the lungs are sufficient higher than the ambient (water) pressure at the chest, it is very easy to sustain potential fatal injury to the lungs from over expansion (search for pulmonary barotrauma).
    Does that mean that inhaling would be possible but we would be inhaling air that forces the lungs to over expand which is not good?
     
  16. Aug 3, 2016 #15

    jbriggs444

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    No problem exhaling. Just relax and your lungs will be squeezed empty.

    Please learn to use the quote function. Highlight the section of text you wish to reply to and click "quote". Then "insert quotes" into your response. I've editted Filip Larsen's quote into yours using that technique.
    It's not about inhaling as such. One would not refer to having air forced into one's lungs as "inhaling", for instance. Like sticking an air compressor hose in one's mouth and turning it on. The more plausible problematic behavior is not exhaling when surfacing. As Filip said, do not hold your breath.

    Either way, it is about having air in your lungs at higher than ambient pressure regardless of how that situation came about.
     
  17. Aug 4, 2016 #16

    Filip Larsen

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    Yes, even a slight pressure difference equates to large forces that muscle and tissue then has to maintain. In metric units (its too early in the day for me for Imperial units) my napkin seems to indicate that if we model the lungs as a 5 liter sphere, then at 2 m of water depth it would roughly have to withstand 300 kg of compression force due to pressure difference with surface air pressure.

    Its a bit hypothetical, but I would guess that avoiding lung collapse and fatal injury to the lungs due to over-compression would be an issue that would be almost impossible to avoid. You don't even need to inhale for that to happen, just by exposing your lungs to large pressure difference like that will be fatal.

    Yes. Based on my diving training alone (and not any medical knowledge), I suspect the specific type of damage to your lungs might be different whether it is a case of over- or under-expansion, but either case has the potential to be fatal. As an aside, note that under-expansion with lung collapse as a consequence may even occur when you are free diving if you dive deep enough.

    Searching the net for maximum respiratory pressure difference, [1] seems to indicate that normal humans can be expected to be able to maintain some kind of respiration cycle only up to around 3% pressure difference, which corresponds to around 30 cm of water.

    [1] http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1501025/
     
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