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Sometimes when a fish is reeled up to the surface
it will appear inflated,
with its eyes bulging out of their sockets
and its stomach projecting out of its mouth,
as if its been blown up like a balloon.
This type of bodily damage, caused by rapid changes in pressure,
is called barotrauma.
Under the sea, pressure increases by 14.7 pounds per square inch
for every 33 foot increase in depth.
So, take the yelloweye rockfish,
which can live as deep as 1800 feet,
where there's over 800 pounds of pressure on every square inch.
That's equivalent to the weight of a polar bear balancing on a quarter.
Now, Boyle's gas law states
that the volume of a gas is inversely related to pressure.
So, any air-filled spaces, like a rockfish's swim bladder,
or human lungs,
will compress as they descend deeper
and expand as they ascend.
After a fish bites a fisherman's hook and is quickly reeled up to the surface,
the air in its swim bladder begins to expand.
Its rapid expansion actually forces the fish's stomach out of its mouth,
while the increased internal pressure pushes its eyes out of their sockets,
a condition called exophthalmia.
Sometimes rockfish eyes will even have a crystallized appearance
from corneal emphysemas,
little gas bubbles that build up inside the cornea.
Thankfully, a scuba diver doesn't have a closed swim bladder to worry about.
A diver can regulate pressure in her lungs by breathing out as she ascends,
but must be wary of other laws of physics that are at play under the sea.
Henry's law states that the amount of a gas that dissolves in a liquid
is proportional to its partial pressure.
The air a diver breathes is 78% nitrogen.
At a higher pressure under the sea,
the nitrogen from the air in a scuba tank
diffuses into a diver's tissues in greater concentrations than it would on land.
If the diver ascends too quickly,
this built up nitrogen can come out of solution
and form microbubbles in her tissues, blood and joints,
causing decompression sickness, aka the bends.
This is similar to the fizz of carbon dioxide coming out of your soda.
Gas comes out of solution when the pressure's released.
But for a diver, the bubbles cause severe pain
and sometimes even death.
Divers avoid falling victim to the bends by rising slowly
and taking breaks along the way, called decompression stops,
so the gas has time to diffuse back out of their tissues
and to be released through their breath.
Just as a diver needs decompression,
for a fish to recover, it needs recompression,
which can be accomplished by putting it back in the sea.
But that doesn't mean that fish should just be tossed overboard.
An inflated body will float
and get scooped up by a hungry sea lion or pecked at by seagulls.
There's a common myth
that piercing its stomach with a needle will let air escape,
allowing the fish to swim back down on its own.
But that is one balloon that shouldn't be popped.
To return a fish properly to its habitat,
fisherman can use a descending device instead
to lower it on a fishing line and release it at the right depth.
As it heads home and recompression reduces gas volume,
its eyes can return to their sockets and heal,
and its stomach can move back into place.
This fish will live to see another day,
once more free to swim, eat, reproduce and replenish the population.


【TED-Ed】The effects of underwater pressure on the body - Neosha S Kashef

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稲葉白兎 2015 年 4 月 14 日 に公開
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