At 07:34 AM 4/1/98 +0800, you wrote:
>My understanding is that Decompression Sickness can only be contracted by
>diving with compressed air at depth and ascending too fast for the stored
>nitrogen to escape from the body without forming large bubbles that
>interfere with body function. The nitrogen concentration increases in a
>diver's body with depth and time as s/he continues to take in compressed air
>delivered by a regulator at the atmospheric pressure the diver is at. At
>less than 10m (2 ATA = 2 Atmospheres Absolute = total ambient pressure at
>that depth) a diver cannot contract decompression sickness (unless maybe
>they shoot to the surface). Unless penguins have a tank of compressed air
>we don't know about, the thought of such a hazard probably never entered
>their minds.
Absolutely true. With a breath-hold dive you cannot absorb more than a
lung full of nitrogen and therefore the body tissues cannot be
super-saturated.
>I am not so confident about Nitrogen Narcosis. The symptoms manifest
>themselves on the nervous system after a particular pressure is reached,
>thus increasing the partial pressure of nitrogen to 80% of the ATA reached.
>The symptoms get worse with depth, but not with continued exposure after the
>initial onset of symptoms. All divers are 'narked' to a degree when they
>reach 30m (4 ATA). However as the onset of symptoms is not immediate the
>small delay of a few minutes suggests to me that they may need to be
>breathing compressed air (ie have access to more than just a single breath
>of air) to contract symptoms. Notably humans can build up a resistance to
>nitrogen narcosis with continued dives and the affect of narcosis varies
>enormously with species (in test chambers, mostly on mammals).
The difference between mammals and birds is the anatomy of the airsacs. A
free-hold human cannot get narked because the lung air volume is
insufficient to saturate the brain with nitrogen. On SCUBA however, the
much increased ambient pressure forces more nitrogen into the blood stream
and brain. With penguins however, the airsac reserve of 80% nitrogen
theoretecially would become almost totally compressed and thus most of the
extra nitrogen gas (and oxygen) should be absorbed by the lungs. The
airsacs thus could act like a mini-internal SCUBA unit. My guess, as you
correctly pointed out is that the penguins brain has evolved to cope with
any increased nitrogen tension.
>Now the real question to my way of thinking is "how do these birds avoid
>Pulmonary Baratrauma of Descent"? As a diver descends on a breath hold
>(free)dive, fluids gather in the lungs equalising the pressure inside the
>lung with the ambient pressure. My understanding is that when the diver
>reaches 40m (5 ATA)(yes people do free dive this deep and deeper) his/her
>lung tissues break down with the overload of fluid. How do penguins over
>come this???
All I can say is that avian lungs are very different from mammals. They
don't have alveoli and they are relatively rigid. Also, they lack
lymphatics - fluid is mainly recovered by the osmotic (oncotic) action of
the blood supply.
P.S. I think the record for a human free-dive is over 150 m !!!!
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Shane Raidal BVSc PhD MACVSc
Lecturer in Veterinary Pathology
Veterinary and Biomedical Sciences
Murdoch University phone: +61 8 9360 2418
Perth,WA, 6150 fax: +61 8 9310 4144
Australia
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