DECOMPRESSION SICKNESS CAUSES |
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The air we breathe is mostly a mixture of two gases, nitrogen (78%)
and oxygen (21%). Unlike oxygen, nitrogen is a biologically inert
gas, meaning that it is not metabolized (converted into other
substances) by the body. For this reason, most of the nitrogen we
inhale is expelled when we exhale, but some is dissolved into the
blood and other tissues. During a dive, however, the lungs take in
more nitrogen than usual. This happens because the surrounding water
pressure is greater than the air pressure at sea level (twice as
great at 33 ft (10 m), for instance). As the water pressure
increases, so does the pressure of the nitrogen in the compressed
air inhaled by the diver. Because increased pressure causes an
increase in gas density, the diver takes in more nitrogen with each
breath than he or she would at sea level. But instead of being
exhaled, the extra nitrogen safely dissolves into the tissues, where
it remains until the diver begins his or her return to the surface (under
some circumstances the extra nitrogen can cause nitrogen narcosis,
but that condition is distinct from Decompression Sickness). On the way up,
decompression occurs (in other words, the water pressure drops), and
with the change in pressure the extra nitrogen gradually diffuses
out of the tissues and is delivered by the bloodstream to the lungs,
which expel it from the body. But if the diver surfaces too quickly,
potentially dangerous nitrogen bubbles can form in the tissues and
cause Decompression Sickness. These bubbles can compress nerves, obstruct arteries,
veins, and lymphatic vessels, and trigger harmful chemical reactions
in the blood. The precise reasons for bubble formation remain
unclear.
How much extra nitrogen enters the tissues varies with the dive's
depth and duration. Dive tables prepared by the U.S. Navy and other
organizations specify how long most divers can safely remain at a
particular depth. If the dive table limits are exceeded, the diver
must pause on the way up to allow the nitrogen to diffuse into the
bloodstream without forming bubbles; these pauses are called
decompression stops, and are carefully calibrated. Decompression Sickness can occur,
however, even when a diver obeys safe-diving rules. In such cases,
the predisposing factors include fatigue, obesity, dehydration,
hypothermia, and recent alcohol use. As well, people who fly or
travel to high-altitude locations without letting 12–24 hours pass
after their last dive are at risk for Decompression Sickness, because their bodies
undergo further decompression. This is true even when flying in
commercial aircraft. Many travelers are unaware that to save money
on fuel the cabin pressure in commercial aircraft is set much lower
than the pressure at sea level. At 30,000 ft (9,144 m), for instance,
cabin pressure is usually equivalent to the pressure at 7,000–8,000
ft (2,133–2,438 m) above sea level, a safe setting for everyone but
recent divers. Exactly how long a diver should wait before flying or
traveling to a high-altitude location depends on how much diving he
or she has done and other considerations. If there is uncertainty
about the appropriate waiting period, the sensible course of action
is to let the full 24 hours pass. |
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| DECOMPRESSION SICKNESS RELATED ITEMS |
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