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    Re: Sperm whale buoyancy.
    From: George Huxtable
    Date: 2007 Mar 25, 18:24 +0100

    This discussion of sperm whale buoyancy is, admittedly, way off-topic
    for a navigation list, but it seems to have aroused a bit of interest
    (without really answering my original questions), so I will prolong
    it, with apologies to others.
    
    Lu Abel wrote-
    |
    | From what I remember about the "bends," it comes when an excess of
    | nitrogen (due to breathing pressurized air) bubbles out of the blood
    at
    | normal atmospheric pressure.   It seems to me that this should not
    | affect free-divers (or sperm whales) since their air intake has been
    at
    | atmospheric pressure and not artificially higher pressures.
    
    I don't pretend to be any sort of expert on diving. I've never even
    had a yen to try an aqualung, nor even a snorkel. But the Gas Laws
    apply everywhere.
    
    I agree with Lu that the problem that faces deep divers is that under
    high pressure, Nitrogen gas dissolves into the blood, and then as the
    pressure gets released on ascending, comes out as bubbles.
    
    But I think that the danger relates only indirectly to the pressure at
    which the air entered the body.
    
    As I understand it, a pearl diver, prior to going as deep as possible,
    expands his chest far more than any of us would be able to without
    special training. This is for two reasons-
    
    1. It gives him more air to work with during the dive, so prolonging
    it.
    
    2. More importantly, as he descends and the pressure increases, the
    air in his lungs takes up less volume. Because the human frame is
    flexible, it withstands only a small difference between exterior
    pressure and interior pressure. So increases in sea pressure, due to
    increasing depth, result in more pressure, and hence less volume, for
    the air in the lungs. The rib-cage therefore has to shrink, but
    there's only a limited extent to which it can before something has to
    give; presumably more for a trained pearl-diver than for us. Not at
    all like a sperm whale, where the rib cage is designed to collapse to
    nothing. Therefore, the number of atmospheres pressure that a
    pearl-diver can stand is given by the expanded volume of his chest,
    divided by the collapsed volume. And each atmosphere corresponds to a
    depth of about 30 feet.
    
    So there's a basic limit to depth that's determined by human
    physiology, and that seems to be less than the depths at which
    nitrogen narcosis becomes a problem. Another factor is that a pearl
    diver is limited in his time-below, and the dissolving of Nitrogen is
    a gradual process.
    
    Gary LaPook tells us that the greatest depth achieved in free diving
    is about 400 feet, or about 13 atmospheres, which would correspond to
    a factor of about 13 in volume between lung-full and lung-empty. My
    guess is that for most of us that factor would be about 2.
    
    It's a different matter for an aqualung diver. As his lungs start to
    compress, he can top up their volume by taking in extra air from his
    pressurtised bottle. Now, there's no tendency to compress his rib
    cage. That allows him to go much deeper. And that fresh air supply
    provides oxygen for breathing, and allows him to expel carbon dioxide,
    so he can spend a long time down there. Which takes him into the
    pressure/duration conditions in which Nitrogen narcosis is a severe
    danger when he ascends. (He can get much deeper by substituting Helium
    for the Nitrogen, as that doesn't dissolve.) Similar arguments apply
    to a man working in a diving bell or in a pressurised tunnel.
    
    A sperm whale has no access to an aqualung, and can't add additional
    air as it dives. However, it can completely flatten its chest cavity,
    which allows it to descend to depths where Nitrogen narcosis would be
    a danger. Lyall Watson's "Whales of the World" explains how a sperm
    whale gets round that problem. He explains there are breathing
    passages in its enormous head of sufficient volume that as the rib
    cage collapses any air is compressed into those spaces, and displaced
    from the lung area altogether. The membranes that allow transfer of
    gas to blood are all in the lungs, so once that displacement has
    happened, at a certain depth, Nitrogen will not dissolve. (Presumably,
    that means also that from that moment, the whale gets no benefit from
    any Oxygen in the air it has taken down, which is another reason why a
    sperm-whale would derive little advantage from a deep breath in, just
    prior to diving.)
    
    That mechanism for avoiding the "bends" seems plausible, but there's a
    question occurs to me that isn't answered. As explained by Watson,
    those air receptacles, being in the head, are many feet below the lung
    area, when the whale is diving, almost vertically, with its head down.
    So how does the whale ensure that the remaining bubble of air,
    shrinking as it compresses, doesn't float upwards into the lungs, but
    instead stays down at the head-end? That must be achieved by some sort
    of muscular control of the chest cavity to provide the necessary
    several-pounds-per-square-inch of pressure difference.
    
    George.
    
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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