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.

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.

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.

WARNING: 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.

WARNING: 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.
WARNING: 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.

Did you know
» It is high levels of carbon dioxide not low levels of oxygen that urge the diver to breathe.
» There are cases in which the breath was held for 15 minutes.
» With the increase of depth, buoyancy changes from positive through neutral to negative.
» It is more difficult to equalize the pressure in the first ten meters than after 100 meters.