The volume of a given mass of gas is
inversely proportional to the pressure being exerted on it (temperature
remaining steady). For every 10 metres of descent the
pressure increases by one atmosphere (atm). Therefore,
total lung volume during a breath-hold dive at 10 metres
is one-half that at the surface. At 20 metres it is
1/3, at 30 metres it is 1/4 and at 40 metres it is 1/5. On surfacing these figures are reversed.
However when breathing compressed gases as in diving
the mass of gas in the lungs is increased to fill the normal volume. An ascent
from 30 metres to the surface without venting
(exhaling) would cause the gas, in already full lungs, with minimal ability, to
expand further to increase its volume to three times normal with the greatest
change occurring in the last 10 metres where it would
double. This is the key law to explain pressurisation
and depressurisation issues and injuries.
As a diver descends the total pressure of
breathing air increases in accordance with Boyles Law; therefore, the partial
pressures of the individual components of the breathing air are increased
proportionally. As the individual descends deeper under water, nitrogen
dissolves in the blood and is carried to all body tissues until a new
equilibrium is reached. Long before this however Nitrogen at the higher partial
pressures in blood alters the electrical properties of cerebral nerve cell
membranes, causing an anaesthetic effect termed
nitrogen narcosis. For every 15 metres of depth this
is roughly equivalent to one alcoholic drink. At 50 metres
divers may experience alterations in reasoning, memory, response time, and
other problems such as idea fixation, overconfidence, and calculation errors.
During descent the partial pressure and hence the amount of dissolved oxygen
increases. Breathing 100% oxygen at 2.8 atmospheres absolute (1.8 atm or 18 metres) may cause
oxygen toxicity in as little as 30-60 minutes. At 100 metres,
the normal 21% oxygen in compressed air can become toxic, because the partial
pressure of oxygen is approximately equal to 100% at 10 metres.
For these reasons deep divers (usually professional or military, but
increasingly sport divers as well) use specialised
mixtures that replace nitrogen with helium and allow for varying percentages of
oxygen depending on depth. The percentage is small and provides a partial
pressure which supports life and strenuous activity without inducing oxygen
toxicity.
With increasing depth, nitrogen in
compressed air equilibrates through the alveoli of the lungs into the blood and
thence into the tissues. Over time nitrogen dissolves and accumulates initially
in the mainly aqueous tissues or those with a high rate of blood flow e.g. the
brain, and progressively in the lipid or fatty component of tissues. On longer
dives some or all tissues become saturated and will not take up any more
nitrogen. As an individual ascends, there is a lag before saturated tissues
start to release nitrogen back into the blood. It is this delay that creates
problems. When a critical amount of nitrogen is dissolved in the tissues,
reduction of pressure caused by ascending induces the dissolved gases to outgas
and form small but myriad bubbles in tissue cells, tissue spaces and blood.
Ascending too quickly causes the dissolved gases - nitrogen - to return to gas
form more quickly increasing the number and size of the bubbles and while still
in the blood or tissues causing local damage which may be felt as symptoms of
DCI Further reductions in pressure through flying or ascending to a higher
altitude also contribute to bubble formation. The average airline cabin is pressurised only to 8000 feet or 0.8 atm.
If a person flies too soon after diving, this additional decrease in pressure
may be enough to precipitate bubbling or enlarge any bubbles already in
existence. With or without the effects of flying If
the bubbles are in the blood in some divers paradoxical embolisation
may occur through a PFO.