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Chapter 13 — 2
Because blood passing through the lungs immediately equilibrates with any change in
inspired N
2
partial pressure, blood is the fastest tissue of all.
Other tissues such as ligaments, tendons and fat, with a relatively small blood flow, have a
relatively slow N
2
uptake. These tissues are termed
"slow tissues"
. Between the two are
tissues of intermediate blood flow such as muscle. Some organs, such as the spinal cord, have
both fast and slow tissue components. The rate of uptake of N
2
in a tissue is
exponential
i.e.
it varies depending on the amount of gas already taken up by the tissue. As the tissue takes on
gas, the uptake slows because the partial pressure gradient decreases.
The filling of a scuba cylinder is an example of an exponential process. When an empty
cylinder is connected to a high-pressure source, the cylinder initially fills quickly, but the
flow slows as the pressure in the cylinder increases and approaches that of the gas source.
The uptake of gas in any tissue is initially rapid but slows with time. Accordingly, it may take
a long time for a tissue to become fully saturated with gas, but fast tissues become saturated
sooner than slow tissues.
Since the exponential uptake takes a long time to reach completion, even if it starts rapidly,
the concept of
tissue "half times"
is used to compare tissues. The half time is the time taken
for a tissue to reach half its saturation level. A fast tissue may have a half time as little as a
few minutes, while a slow tissue may have a half time of some hours.
GAS ELIMINATION
N
2
is eliminated in a reverse of the uptake process. As the diver ascends there is a reduction
in the partial pressure of N
2
in the air he breathes, allowing blood to release N
2
into the lungs.
The decrease in the blood level of N
2
causes N
2
to diffuse into the blood from the tissues. Fast
tissues naturally unload N
2
quicker than slow tissues.
Theoretically, tissues should lose N
2
exponentially, and most decompression tables are
calculated on this assumption. While large amounts of N
2
are lost initially, the process slows
with time and it may take 24 hours or longer for all the N
2
taken up during a dive to be
released. Diving again during the time of N
2
elimination will mean that the diver will start his
second dive with a
N
2
retention
in some tissues. Adjustments are provided in the
decompression schedule to allow for this and are incorporated as the
repetitive dive tables.
If there is diminished circulation to a tissue during decompression, gas elimination will be
reduced and thus bubble formation will be more likely.
In practice, even during routine conservative dives, bubbles of N
2
frequently form in the
blood and tissues, interfering with N
2
elimination. It has been estimated that as much as 5%
of N
2
taken up by the body after some dives is transformed into bubbles on decompression.
These are often termed "
silent bubbles
" since they usually do not produce any symptoms.
They do however have a profound and unpredictable influence on the decompression
requirements for repetitive diving, because it takes much longer to eliminate gas bubbles in
tissues than it does gas in solution.