There are two outstanding
factors which characterize the free diver – the process of breath-holding
and the changes that occur in the lungs and the chest as a result of the
increased water pressure. |
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Apnea or Holding Your Breath
The apnea is a state in
which one voluntarily or involuntarily “ceases to breathe”. The diver takes
a deep breath and descends under water. While he is submerged, he does
not inhale air from the atmosphere which does not mean that he doesn’t
breathe at all. As a result of physical activity, cooling and raised pressure,
the tow processes of cellular and pulmonary respiration accelerate which
shortens the apnea. The length of the apnea depends not only on an individual’s
lung capacity but also on the speed of CO2
release in the organism. For example, while at rest the apnea lasts 60
seconds but it is only 20 seconds during heavy physical exercise. This
is due to the increased release of carbon dioxide and the reduced amount
of oxygen in the blood.
 Impulse
of Breathing and Critical Line
We inhale oxygen and exhale
carbon dioxide. In a state of apnea (when the breath is held), the release
of CO2 temporarily stops which
results in the accumulation of carbon dioxide in the cells, blood and lungs.
Simultaneously, carbon dioxide starts irritating the respiratory center.
In a particular moment, the irritation becomes so unbearable that the person
is not able to hold his breath anymore. There occurs an irresistible will
to exhale and release the large amount of CO2
called an impulse of breathing, which discontinues the apnea. The
concentration of CO2 in the blood,
which forces the impulse of breathing is called the critical line.
The critical line cannot be strictly determined because of individual differences.
The high level of the critical line might be due to the richer concentration
of O2, better training of the
apnea or simply holding the breath after maximum inhalation.
Usually, a healthy person
has an apnea of one minute. If he, despite his abilities, manages somehow
to overcome the impulse of breathing, the amount of CO2
exceeds the critical line and might cause blackout or suffocation.
Hyperventilation
Certainly, people are not
satisfied with an apnea of one minute and search for other methods to prolong
their stay and distance themselves from the impulse of breathing. In fact,
this is done by hyperventilation – the practice of excessive breathing.
There are two ways of hyperventilation:
-
Using atmospheric air: First,
take about 10 deep breaths. This way, the usual level of CO2
in
the blood is artificially reduced so that the diver can have longer apnea
(there will be larger space for CO2
to accumulate). The inhalations should be quick, periodical and not exceeding
12–15 because a very low level of CO2
is abnormal and might cause disturbances such as dizziness, nausea or convulsions.
Hyperventilation can prolong the apnea on land for up to 1.5–2 minutes
and the training of hyperventilation – up to 2–2.5 minutes.
After
hyperventilation, do slow and easy movements under water. This will reduce
quick oxygen consumption and avoid the rapid increase of carbon dioxide.
-
Using pure oxygen: After the
regular hyperventilation, take several breaths of pure oxygen or start
hyperventilation with pure oxygen from the very beginning. This type may
prolong the apnea for about 7–8 minutes. Even records of 15 minutes were
set with careful practice of this kind of hyperventilation.
Pathology of the Apnea
Carbon Dioxide Paralysis
of the Respiratory Center
This pathological condition
is due to an overcoming the impulse of breathing and thus crossing the
critical line.
Lack of Oxygen
Hyperventilation hides potential
danger if straight after that the diver does tiring physical activity.
Swimming or moving actively under water increases the release of oxygen
which adds up to quick exhaustion of oxygen in the blood. At the same time,
vigorous hyperventilation has led to a very low level of CO2
to prolong the apnea. In this case, the diver loses consciousness under
water before he has any need to breathe. He cannot feel the decrease of
oxygen in his blood. Besides, the low concentration of CO2,
caused by hyperventilation, still has not reached the critical line of
CO2 and has not sent any signals
to the respiratory center to discontinue the apnea. Such cases of drowning
are common among trained divers.
Avoid
intensive physical work under water after hyperventilation.
Raised Partial Pressure
of Oxygen
Example: A well-trained
spearfisherman, engrossed in chasing fish, spends a minute at a depth of
25–30 meters without feeling any need of oxygen and without any impulse
of breathing. Convinced that he has not used up his oxygen yet, he is ascending
when suddenly, he feels a strong necessity to breathe which he cannot resist.
Two or three meters before he reaches the surface, he drowns.
Explanation: The raised
partial pressure of oxygen creates a false feeling of well-being. At the
same time, CO2 accumulates slower
without signaling the dangerously-decreasing oxygen concentration in blood.
During ascent, the partial pressure of oxygen is suddenly reduced, robbing
the diver of oxygen to breathe. Carbon dioxide itself quickly enters the
blood and expands causing irresistible impulse of breathing. Since he has
no oxygen supply, the diver drowns.
 Effects
of Pressure on the Lungs and Chest
Changes in the Chest
According toBoyle’s
law, chest starts to contract with the increase of depth.
With its anatomical peculiarities, the chest resembles a spring which reacts
to the changes of ambient pressure: during descent, it contracts and reduces
the volume of the pulmonary air; during ascent, it expands and enlarges
this volume.
Cupping-Glass Effect
Once the chest contractions
reach the volume of residual air, further, no matter what the pressure
is, the chest cannot shrink any more. Due to the differences in pressure,
the cupping-glass effect occurs. As a result, a large amount of blood enters
the lungs which might lead to a rupture of the heart muscle. If the cupping-glass
effect is insignificant, bronchopneumonia or small hemorrhages
in the alveoli may take place. Actually, this is observed in deep dives.
Another situation of cupping-glass
effect is when the diver is at a depth of 1.8–3 meters and breathes atmospheric
air through a long tube. At such depth, chest muscles cannot overcome water
resistance.
A Diver’s Abilities
The safe depth for the breath-hold
diver depends on the larger total pulmonary volume and the smaller volume
of residual air. 15 meters is considered a safe depth. Every additional
meter below 20 is connected with risks.
Do
not swim below 15 meters with your breath held!
Of course, there are fantastical
records in free diving below 100 meters. Divers such asJacques
Mayol, Francisco Ferreras
and Loui Leferme excel with perfect physique, huge total pulmonary volume,
insignificant residual air, increased resistance towards carbon dioxide
and an ability to slow down their cardiac activity.
Changes in Buoyancy
According toArchimedes’
principle, a diver with air-filled chest and a mask has
positive buoyancy and makes efforts in order to descend. His chest gradually
shrinks with the increase of depth and buoyancy becomes neutral at 6 to
7 meters. Further descent causes the chest to contract more and after some
time, the diver acquires negative buoyancy.
These changes of buoyancy
are used by good free divers to save their energies. During ascent, they
swim actively back to 6–7 meters where they let water take them to the
surface. |
