Chapter 3 - The Physiology of Diving and Nitrox
The Physiology of Diving and Nitrox
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to its solubility in lipid (fatty) tissues. Because helium
has minimal narcotic potency, it is used as a diluting
gas in trimix when diving to depths where nitrogen narcosis
becomes a serious concern or incapacitating.
The effects of narcosis can be measured at shallower
depths, but they become more pronounced when the
partial pressure of nitrogen is approaching four atmospheres,
which is about 40 msw (132 fsw) when breathing
air. Symptoms increase with increasing depth. Other
factors also affect the degree of narcosis. Anxiety and
stress, fatigue, cold, hard work, high carbon dioxide levels
in the body, and alcohol have all been shown to enhance
narcosis. On the other hand, positive motivation seems to
reduce its effects. Some divers report acclimatization following
repeated exposures, but studies have shown that
any adaptation is largely subjective.
Narcosis itself is not the danger, but the impaired
judgment, loss of orientation, and reduction in problem
solving capabilities are, and the “narked” diver is
at increased risk. It may become difficult for a diver to
monitor time, depth, and air supply, remember the dive
plan, or concentrate on the task at hand. Perhaps the
most insidious thing about nitrogen narcosis is that divers
may not be aware that they are impaired.
At first glance, you might assume that you should
be less subject to nitrogen narcosis when diving with
nitrox because you have replaced some of the nitrogen
with oxygen. However, there is no hard or definitive evidence
to support this, and it is safer to assume that there
is no appreciable benefit to breathing nitrox.
As you will remember from your entry-level scuba
course, nitrogen narcosis is easily reversible. An ascent to
a shallower depth is all that is required. The symptoms
disappear as you ascend. It could also be that the stress
or anxiety that you are experiencing at depth is as much
due to psychological factors of being outside your personal
“comfort envelope.” In this case, the obvious solution
is also an ascent to a shallower, more comfortable depth.
Decompression Sickness
You learned about the basics of decompression sickness
(DCS or “the bends”)–what causes it, its signs and
symptoms, how to avoid it, and how it is treated–in your
Depth
Dive Dive Dive
Time
Represents tissue nitrogen levels
FIGURE 3-1: TISSUE NITROGEN LEVELS INCREASE DURING DIVES
AND DECREASE DURING SURFACE INTERVALS
entry-level scuba course. A brief review will be useful,
and we can now use some of the concepts you learned in
earlier chapters of this book. Again, if you want to learn
more about decompression theory, the NAUI Master
Scuba Diver course will greatly increase you knowledge
and understanding.
During a dive, our bodies are exposed to increased
pressure. While we are underwater, the increased partial
pressure of the nitrogen in the air (or gas) we are
breathing forces additional nitrogen into solution in the
tissues of our body (Henry’s Law). Passing out of the
lungs through the walls of the alveoli, dissolved nitrogen
enters the blood and is carried to all parts of our body.
There, the pressure gradient between the nitrogen dissolved
in our blood and the nitrogen in the surrounding
tissues causes dissolved nitrogen to move into the tissues.
The greater the pressure (depth) and the longer we are
submerged, the more nitrogen will dissolve in our bodies
until eventually the gas tension (partial pressure) of the
dissolved nitrogen in our tissues reaches equilibrium
with the partial pressure of the nitrogen in our breathing
mixture at that depth. This may take more or less
time depending on many factors, including the type of
tissue and the circulation to it. Because nitrogen is metabolically
inert, it simply remains in our tissues while
we are at depth.
When we ascend properly from a dive, the reverse
occurs. The partial pressure of nitrogen in our lungs is
now reduced (to 0.79 atmosphere at the surface), and
what is now excess dissolved nitrogen migrates from the
areas of higher nitrogen tension (tissues) and diffuses