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Lungs underwater

Discussion in 'Basic Scuba Discussions' started by Matvei, Nov 25, 2019.

  1. Matvei

    Matvei Angel Fish

    # of Dives: 50 - 99
    Location: Russia
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    Now I'm reading a book about scuba diving and this book has the following paragraph:
    "Let's say a freediver has 4 l of air in the lungs. At 10 meters the pressure doubles and the volume of air in the lungs squeezed by water is 2 l. At 30 m it is 1 l."

    Also, I googled it and found a lot of pictures like that
    https://i.pinimg.com/originals/9f/90/3a/9f903a464526b710814c12dd0f725b92.jpg
    that show that the volume of lungs is 5 times less at 40 m.

    And, I don't understand why it is true. This law is true for BCD or balloons. But my lungs are not a balloon, they are inside my body, and water presses my rib cage. I can agree that the volume can decrease twice or something like that but not 5 times. (Let's say the volume of my lungs becomes 1 l, but the volume inside my rib cage is almost the same, so something should fill the difference (gas or liquid), what is it?)

    Could someone explain why I'm wrong?
     
  2. BRT

    BRT Giant Squid

    10,952
    6,862
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    The water is pushing in from the outside of your body. You have no muscles to resist it. If you doubt me about the muscle issue try breathing through a tube a 3 feet below water level.
     
    Khrissi and Subcooled like this.
  3. James79

    James79 Regular of the Pub

    # of Dives: 25 - 49
    Location: Lower Alabama
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    To tie in to that, for the freediver at 40m their surface lungfull has been compressed to 1/5th of a lung full. For a scuba diver, the lung full of air at 40m would be provided at 5 ATM, and would be 5 times the air that they could contain at the surface. That is why scuba teaches to never hold your breath.
     
    Snoweman likes this.
  4. Matvei

    Matvei Angel Fish

    # of Dives: 50 - 99
    Location: Russia
    7
    1
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    I understand that it is impossible to breathe through a tube at that depth because the water pressure is very high, but I don't understand how it implies that the volume of lungs gets so small, it just implies that I cannot move my diaphragm and expand my rib cage.
     
  5. James79

    James79 Regular of the Pub

    # of Dives: 25 - 49
    Location: Lower Alabama
    253
    190
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    The lungs capacity isn't shrinking at depth, the air inside is being compressed.
     
  6. PfcAJ

    PfcAJ Orca

    # of Dives: 5,000 - ∞
    Location: St Petersburg, Fl
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    I get what you’re saying.

    So I *think* that what happens is that your whole chest compressed along with the air in your lungs. I would bet that if you took a deep breath, tied a string around your chest, and swam down to 70’, that string would be super loose. On the way up, it would get tighter again.

    I agree with your idea that the decrease in volume won’t be the same as say with a balloon, but I bet it s not far behind.
     
    Matvei likes this.
  7. Matvei

    Matvei Angel Fish

    # of Dives: 50 - 99
    Location: Russia
    7
    1
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    For a scuba-diver the situation is the opposite: I have 5 atm in my lungs to equalize the pressure of water to be able to breathe, and if I hold the breath when ascending, then the difference between pressures will be big, and I could get barotrauma.

    So, is it true if the skin, ribs, and tissues of a freediver were transparent and we could see the lungs of the freediver, then at depth 40 m they would be 1/5 of the size at the surface?
     
  8. Matvei

    Matvei Angel Fish

    # of Dives: 50 - 99
    Location: Russia
    7
    1
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    but the air cannot be compressed without changing the volume, because pV = const.
     
  9. Matvei

    Matvei Angel Fish

    # of Dives: 50 - 99
    Location: Russia
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    This string measures the changing of my body size, not lungs. I agree it would be loose, it is like the difference between just I inhale and exhale (maybe plus a little bit more underwater). But not ~sqrt(5) times (if I approximate my body by a box).
     
    PfcAJ likes this.
  10. Matvei

    Matvei Angel Fish

    # of Dives: 50 - 99
    Location: Russia
    7
    1
    3
    OK, I read wikipeadia
    Lung volumes - Wikipedia
    it turned out that total lung capacity is ~6 l and residual volume (the volume of air remaining in the lungs after a maximal exhalation) is ~1.2 l. I just didn't realize this huge difference. So, the number above for 40 m seems to be true.
    Thank you
     

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