Breath-hold diving. Part 3: The Science Bit!

By Dr Rob Schneider
In the third part of this series on breath-hold diving, Dr Schneider discusses the science behind breath hold diving and the implications thereof for divers.
Part one of this series introduced us to the concept and overview of breath-hold diving, while the second installment detailed the equipment available for use. In this part, we will endeavour to avail ourselves of the physics and the implications thereof concerning our bodies during breath-hold diving.
Implications
Humans were designed to be terrestrial, air (gas) breathing organisms. Thus, the body’s anatomical structures and physiological mechanisms are well suited to this role. The aquatic or marine environment represents a different set of rules in terms of the physics that apply and how that application would affect our bodies during breath-hold diving. Therefore, when we enter this logically hostile environment, there are certain changes or modifications to the laws of physics that govern our atmospheric existence that need due consideration.
Naturally, when one uses the word physics, about 95% of people develop acute transient deafness and the other 5% push their glasses higher up their noses and develop a maniacal gleam in their eyes. Do not be alarmed.
This discussion is not intended to induce post-traumatic flashbacks of highschool science exams, but merely to gently remind us that order exists and is functional. In order to ease the laborious passage through the quagmire that is physics, only that which is relevant will be shared in a hopefully simple manner.

Pressure and the Gas Spaces
There are several gas laws that apply during the course of breath-hold diving and they are important because they relate changes in the gas spaces of the diver, namely the lungs, middle ears, sinuses and other gas spaces. Now let us look at these gas laws:
The general gas law
The general gas law or equation is formed by the combination of three gas laws showing the relationship between the pressure, volume and temperature for a fixed mass of gas.
The three laws are:

• Boyle’s law: The volume of a given mass of gas is inversely proportional toits pressure, if the temperature remains constant.
• Charles’ law: The volume of a given mass of gas is directly proportional toits absolute temperature, if the pressure remains constant.
• Gay-Lussac’s law: The pressure exerted by an ideal gas is directlyproportional to the absolute temperature, if the volume remains constant.

Dalton’s law
The pressure of a mixture of gases is simply the sum of the partial pressures of the individual components.
Henry’s law
At constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.
Boyle’s law has the following implications for breath-hold divers:

• As we breath-hold dive, the surrounding pressure increases and for every10 m descended in seawater, the pressure increases by one atmosphere.
•  Gases in our body, as in the lungs, sinuses and ears, will be affected by the increase in pressure. These gases will decrease in volume as we descend and the surrounding pressure increases. This decrease in volume is not in a linear fashion but in an exponential fashion, otherwise lung volumes would be zero at 20 m of seawater (see the table below).
• We need to compensate for the decrease in gas volume in order to avoid damaging tissues in our bodies. We use equalisation techniques to do this,but note that the lungs cannot be equalised.
• A simple way to observe this law in action is to take an empty plastic bottle, seal it and take it to depth. Deformation of the bottle will be noted at depth as the volume of air decreases inside the bottle due to increased pressure, with the bottle returning to its original form at the surface.

Pressure and the Non-Gas Spaces
What about the effects of pressure on the rest of our body? Contrary to popular belief, like the image of Scrat becoming progressively squashed as hedives after an acorn in the film Ice Age 4, our bodies will not be crushed as we dive deeper and the pressure increases. Our bodies are made up of gas, solid and liquid phases with liquids making up the majority. The effects of pressure on gases have been described above, but what are the effects on the other two phases of this increased hydrostatic pressure? Overall, the hydrostatic pressure is distributed evenly throughout our body tissues thus preventing us from being crushed like an egg in a vice. The reason for this is because our body tissues generally behave like fluids or liquids and therefore Pascal’s law (or the principle of transmission of fluid-pressure) applies.
Pascal’s law
A change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid. In truth, though, the hydrostatic pressure does exert some effects on the breath-hold diver as we will notice in the next part of the series.
Thermal Notions
In the air environment, heat is exchanged between our bodies and the atmosphere in four ways. These are convection, radiation, conduction and evaporation, with the former two being the major pathways. In water, there is no evaporation and radiation becomes negligible, so convection and conduction become the major pathways. This gains abrupt significance in breath-hold diving due to two conditions. Firstly, the thermal conductivityof water is about 23 times that of air and secondly, the specific heat per unit volume of water is about 3 500 times that of air. Skin temperature falls faster and further on immersion in water compared to air. Furthermore, core body temperature drops two to five times faster in cold water than it does in air of the same temperature. In a nutshell, we lose heat much faster from naked skin in water than in air.
Buoyancy
Archimedes’ principle
Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.
Thanks to Archimedes, we understand the ostensibly logical concept of what buoyancy is. Our land-based existence does not require us to grapple with this force but the minute we enter the water it makes itself well known. In breath-hold diving terms, coming to grips with buoyancy and how to ameliorate it for one’s gain is of significant importance.
Sound and Light
The end is in sight. Speaking of that, our sight is affected when immersed in water due to the refraction of light waves as they enter and traverse the watery medium. The result is blurred vision. We can correct this by using a
mask and creating a gas space in front of our eyes but, due to the refraction, it should be noted that objects will appear closer and larger than they really are. At least this gives the spear fisherman a scientific rationale for their tall tales.
Sound waves move faster and further in water than in air. The implication is that we can hear sounds underwater but our ability to differentiate where they emanate from is exceedingly poor in the watery environment.
At long last the science bit is over. For those of you who braved through it, well done! Catch part four in this series where we return to the classroom and indulge in the more amicable setting of biology as we prise open the oyster of physiology and search for the pearls.
References

• Brubakk Alf O. & Neumann Tom S. Bennett and Elliott’s Physiology and Medicineof Diving. 5th edition.
• Edmonds Carl, Lowry Christopher, PennefatherJohn & Walker Robyn. Diving and Subaquatic Medicine. 4th edition.
• www.britishfreediving.org
• www.freediving.co.za
• www.freedive.net
• .www.skin-dive.com
• www.deeperblue.com
• www.freedivers.net
• www.pureapnea.com