Basic physics as related to free diving and shallow water blackout
To free dive safely an elementary understanding of physics and the effects of pressure is needed. At the basic level to avoid shallow water blackout and in more advanced training to avoid lung squeeze and other pressure related injuries.
When we take a lung full of air at the surface we are breathing at one bar of pressure i.e. 1 ata, one atmosphere, that’s to say that the air we breathe is pressurized by the 10,000 metres of air above us and we use this level of pressurisation as our basic measure of pressure.
Pressure effects on volume of air.
Ten metres depth of saltwater is exerts the same pressure as one atmosphere of air so at a depth of 20 meter there is 2ata of pressure. At 20 metres there is 3 ata of pressure and so on. As air is highly compressible gas when the pressure is increased the volume of the gas will decrease in direct relation to the increase in pressure. This does not mean there is less gas; simply that it occupies a smaller volume in space. In free diving terms if we dive with a full lungful of air of 6 litres, when we reach 10 metres depth the volume shrinks to 3 litres and at 20 it will 2 litres and 30 it will 1.5litres and so on. The amount of air in our lungs in unaffected, only the volume is reduced.
What this means to the free-diver.
The reducing volume of air as we go down affects our dive in many ways, it means that airspaces need more air added to them to maintain the volume, i.e. the ears and mask need to be equalised and obviously it means the deeper we go the less air we have to equalise, tying our depth limit to an ability to equalise.
It also has another less obvious but very important effect. In simple terms the increased pressure of the air in the lungs allows the gases in the air in the alveoli to behave as if there were more of them, i.e. their partial pressure increases. This is also why scuba divers get affected by nitrogen and air cannot be used below 66 metres due to oxygen toxicity. This increase in Partial pressure also affects the free diver in a very real way as it allows the free diver to absorb O2 from the lungs at depth that he would not be able to absorb at the surface. In somewhat simplified terms; at depth the volume of air decreases, increasing the density of gases within the air. This increased density allows O2 to cross the membrane wall into the blood more easily. We can imagine it like this, the lungs shrink and the air in them is squeezed in such a way that gas exchange across the membrane wall is made easier.
Shallow water blackout and how to avoid
All this is fine, except when the breath hold diver spends too long below, while hunting a fish for example. The body still has enough 02 to function at depth because of the increased density of the air, raising the partial pressure of the lung O2, but as the diver ascends the pressure decreases and as he nears the surface the lungs expand and suddenly there is not enough partial pressure/density of O2 to allow it to cross the membrane wall. This can result in a sudden drop in accessible O2 for the body causing unconsciousness to occur, sometimes preceded by loss of motor control (samba). This can happen while getting close to the surface but normally happens on the surface when the diver exhales to take his first breath. This is the most common cause of blackout, the diver exhaling deeply on the surface after overstaying their limit at depth. To avoid this we always use recovery breaths after coming to surface i.e. we only exhale 50 percent of our air then quickly inhale as much as we can, then again exhale half of the air, repeating in this way a few times. In the way we avoid dropping the pressure in our lungs too quickly, which could, in the case of the diver being too close to their limit, induce samba or even blackout.
The cause of shallow water blackout is the change in pressure after the diver uses excessive amounts of O2 at depth. To avoid this we always dive within our limits, we always avoid overexertion particularly in the ascent and we always use recovery breathing upon surfacing.