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Greetings.

After following the "why do whales NOT get bent" thread for a couple of
weeks, I finally got around to looking in my messy archives of
information and found the answer.
Mr. David Denison published a book on physiology of oxygen and other
gases and I photocopied a particularly interesting section a couple of
years ago.  I don't have the exact title of the book, but from the
photocopy I will quote the exact sections explaining the reason. 
Anything in quotes is from the book, all else is yours truly.

"All marine mammals, ie., toothed whales, baleen whales, true seals,
manatees, dugongs, walruses, and sea otters, regardless of their diverse
evolutionary origins, dive much deeper, for much longer, and much more
frequently, without coming to harm.  They can do this because their lungs
differ from our own."

Someone posted the actual reason here a little while ago, but I thought
it might be interesting to see a more detailed answer to our quest.


"All terrestrial mammals, even beavers, hippopotamuses, and river otters,
have lungs of the same sort, with floppy peripheral airways.  On
compression, these collapse before the alveoli they serve are empty,
trapping compressed air in contact with pulmonary capillary blood."

"By contrast, all marine mammals, including the sea otter that evolved
most recently, have peripheral air passages which are reinforced with
cartilage or muscle all the way to the mouth of the alveoli.  When these
lungs are compressed during a breath-hold dive, alveolar gas is displaced
into the airways away from pulmonary blood.  This striking example of
convergent evolution protects these creatures from oxygen toxicity,
nitrogen narcosis, and the bends, but does not explain why they can stay
underwater for so long."

In a question/answer section at the back of this chapter, this discussion
is explored as follows:

Question:  "Can I ask you about these seals.  First of all do they trap
gas and then lose it via collateral ventilation, or do they just get
straight atelectasis?"

Denison: "They do not trap gas at all.  If you take a seal or dolphin lung
and press on it gently it will empty completely.  And we attribute this to
the reinforced structure of the airways.  When they dive they usually dive
in expiration, they have ribs that are modified, having only one
articulation on the spine, they have more false ribs, they have fewer ribs
altogether and the chest is much more compliant.  A blue whale has only
four ribs and if you watch them underwater you can see them blow out
before they begin to dive.  They just do not need it.  The remarkable
thing, I think, is the re-expansion of the lungs that occurs when they
come to the surface.  But a very beautiful demonstration of the rigid
nature of their airways is given by the flow-volume curve, that is
something Cohen has shown in a pet seal which he called Houdini."
 [snip here]

Question: "Is the reason why diving mammals do not develop decompression
signs because they are able to shift all the air from the air exchanging
zone into the dead space, I mean the rigid, non gas exchanging area?"

Denison:  "Exactly so.  And if you measure the arterial nitrogen tension
in a seal during a dive to 300 M we would expect the nitrogen tension to
rise to about 27 atmospheres, something like that.  In fact it rises to
about 3 atmospheres, and then falls continuously during the dive, even
though the seal is at 30 atmospheres.  It just requires a pressure of 3
atmospheres to close the airways and thereafter the nitrogen in the blood
is leaking away into the tissues.  When the seal comes to the surface it
stays at that pressure until it takes its first breath".


If necessary, I could go back to my old workplace and try and find the
book if anybody is interested.  Looks like Christina's idea of putting a
measuring device on a "whale" "and see what happens" was used on the seal
Houdini.

Included in the list of references for this article is the following:
Denison, D.M and Kooyman, G.L. (1973), The STructure and Function of the
small airways in Pinniped and sea otter lungs.  from Respiratory
Physiology, 17: 1-10

Does this help?

...Brian...

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