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Mammals deep underwater, water pressure, & implications

  1. Jun 14, 2015 #1
    Large ocean/sea mammals, say a blue whale, are able to withstand large water pressures underwater, and I read that they're able to do so for a number of reasons, one of which is that the air cavities are thought to fill with water. Is it this equalization of pressure that allows whales to deep dive, or is it the absence of air in its cavities? There is a distinct difference between these two ideas, though similar.

    So, say the whale equalizes pressure to survive deep dives. The air cavity is displaced by water. There is an absence of air because there is now water. But conversely, let's say that you have an unmanned submarine that has a vacuum inside equal to zero air pressure or 0 ATM. Would the lack of the presence of air be the primary reason that deep dives are possible, all things constant (e.g. the sub's hull can withstand external water pressures of the deepest dives)? If there is no air inside to be affected by pressure, would a vacuumed object be able to withstand greater water pressures? Or is it something else? Thanks in advance.
     
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  3. Jun 14, 2015 #2

    SteamKing

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    I don't know about whales, but with submarines, you want them to be able to float when on the surface and be able to submerge when desired.

    Submarines on the surface float because their hulls are buoyant, and the submarine's weight is equal to the weight of the amount of water its hull displaces.

    If a ship or other floating body cannot develop enough buoyancy to keep floating, it will sink to the bottom of the ocean.

    What you want for a submarine is to alter its characteristics so that when submerging, its hull is not buoyant enough to float, but at the same time, you want to keep it so that the hull does not want to sink fully to the bottom of the sea. This condition is called 'neutral buoyancy', where the weight of the submarine when it is fully submerged is exactly equal to the weight of the amount of water displaced by the hull. Submarines generally accomplish this result by adding ballast water when on the surface so that they can submerge, and when it is desired tocome back to the surface, this ballast water is pushed out of the ballast tanks, usually by forcing compressed air into the tanks. After the ballast water has been pushed out of the ballast tanks, the submarine hull is lighter than the weight of the water its hull displaces, and the submarine pops back to the surface.

    http://teamuv.org/tag/stability/
     
  4. Jun 14, 2015 #3
    I guess what I'm asking is how do these large mammals survive deep dives? Why aren't they destroyed by the water pressure? Is it because of the water displacing the air, or is it the absence of air preventing "the squeeze" feeling?

    If I had a wristwatch that went deep into the water, and it's rated at the deepest depths, will the air inside of it destroy the watch on the inside, even though the case of the watch is strong? Conversely, if the watch had a vacuum on the inside with the same case, will it be better off, all things equal?
     
  5. Jun 14, 2015 #4
    Not sure what you mean by a "vacuumed object" but the closer the air cavity pressure is to the water depth pressure then the deeper the dive. That's why deep sea divers breathe pressurized air and are only limited to the effect it has on blood chemistry rather than the water pressure itself.

    I think sea animals must be doing a similar thing by squeezing their lungs and/or air cavities to pressurize the air gases in their tissue. I have read that sea turtles caught in nets and brought rapidly to the surface exhibit symptoms of the bends.
     
  6. Jun 14, 2015 #5

    SteamKing

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    Whales and such are adapted to dive a certain distance, but there is a limit below which they cannot survive.

    This article on sperm whales discusses some of the adaptations which allow these creatures to dive deep (however briefly) without injury:

    https://en.wikipedia.org/wiki/Sperm_whale

    There are species of fish which live only at extreme depths above which they cannot survive.
     
  7. Jun 14, 2015 #6

    russ_watters

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    I'm sorry, but your question is based on a false premise: you are assuming that the pressure is something that animals need to "withstand", but it isn't. The difference between your watch or a submarine and an animal is that your watch and a submarine are rigid and therefore must be strong enough to avoid being crushed. Animals are not rigid, they are soft, so there is nothing even trying to "withstand" the pressure. As a result, the pressure inside your body and pressure outside are in an easy equilibirum and no forces are felt.
     
  8. Jun 14, 2015 #7
    A skeleton is not very soft.
     
  9. Jun 14, 2015 #8

    russ_watters

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    No, but unless a bone contains water-tight voids, it doesn't have to support itself from being crushed.
     
  10. Jun 14, 2015 #9
    What about tissue containing absorbed gases?
     
  11. Jun 14, 2015 #10

    russ_watters

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    Absorbed gases are dissolved, but even if they weren't - such as in your lungs - again, there is nothing to withstand. A balloon doesn't break when you hold it underwater, it just gets a little smaller; because it isn't rigid.

    Pressure is extremely powerful. If the human body were built in a way to withstand some pressure, everyday activities like swimming in a shallow pool would be painful or fatal. A car ride up a mountain would cause us to explode.
     
  12. Jun 14, 2015 #11
    So, using the watch example, having a vacuum sealed case is worst than not having a vacuum at all (e.g. 0 ATM vs 1 ATM, respectively, on the inside of the case)?
     
  13. Jun 14, 2015 #12
    But an animal isn't a balloon. It is tissue attached to a rigid skeleton.
     
  14. Jun 14, 2015 #13

    russ_watters

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    The tissue is not attached to the skeleton in a way that causes the skeleton to support pressure inside the tissues. Grab your forearm and squeeze. Are the bones in your arm keeping your forearm's shape against the pressure you are applying? Perhaps if we had exoskeletons like insects, they would (again, they'd have to be air/water tight).
     
  15. Jun 14, 2015 #14

    russ_watters

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    BTW, the only structure in your body that contains a significant amount of air that can be compressed is your ribcage. But it is open at the bottom, so when you go underwater, your lower torso gets squeezed-up into your ribcage like a tube of toothpaste. That's why when diving, people always look absurdly skinny:

    woman-free-diving-snorkeling-coral-reef-28373283.jpg

    Now, there are also some bone and supported soft tissue structures in your head that contain air (sinus cavity, ears), and when swimming underwater it is important to equalize the pressure. If you have a cold and the passages are blocked, even swimming down a relatively shallow depth (10') can be extremely painful.
     
  16. Jun 14, 2015 #15
    So, using the watch example, having a vacuum sealed case is worst than not having a vacuum at all (e.g. 0 ATM vs 1 ATM, respectively, on the inside of the case)?
     
  17. Jun 14, 2015 #16

    jbriggs444

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    Indeed. Having a vacuum inside the watch means that just sitting on the table it is under stress -- it is subject to an unbalanced inward force. With one atmosphere inside, that one atmosphere of internal pressure balances one atmosphere of external pressure and the watch is under no stress as it sits on the table.

    If you were to dive with the air-filled watch, you could (ideally) manage about 30 feet deeper because of the 1 atmosphere of internal pressure. If you made a hole and allowed the watch to fill with water then you could take it much deeper without crushing. It would not be under stress because the internal and external pressures would balance. [However, salt water is not good for watch insides, so this is not often done]
     
  18. Jun 14, 2015 #17
    The whale skeleton doesn't have to resist the water pressure but the tissues and organs do. If you squeezed your arm 15psi (which is 1 atm) you would be in a world of hurt if not permanent damage.

    If you want to model an animal as a balloon it would have to be very flexible. For a diving depth of say 3,000 ft I estimate the pressure at around 1,300psi. Assuming Boyle's Law is valid then the volume would have to compress to around 1% of the original volume. I am skeptical that this happens.
     
  19. Jun 14, 2015 #18

    jbriggs444

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    The atmosphere is pressing on your arm with 15psi at this very moment and is not causing you any harm at all.

    Boyle's Law applies to ideal gasses. Neither people nor seawater are ideal gasses.
     
  20. Jun 14, 2015 #19
    That's because you're breathing in gas at this pressure which fills your cavities and is adsorbed into your tissues at this same pressure.

    But the air inside your cavities and adsorbed in your tissues probably can be. We were after all talking about modeling a sea animal as a balloon.
     
    Last edited: Jun 14, 2015
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