The human body has several air-filled spaces or cavities, such as the lungs, sinuses, and ears, which can be affected with changes in pressure and temperature. Therefore, when looking at the physiology of diving, there are several gas laws that are important to consider.
Charles' Law
Charles' Law States:
At a constant pressure, the volume of a gas varies directly with
absolute temperature. For any gas at a constant volume, the pressure
of a gas varies directly with absolute temperature.
Translation: As the
temperature of a gas decreases, the volume of the gas will also
decrease.
This law can be illustrated by taking a helium balloon outside in the winter. The balloon will shrink visibly in the cold outside air. The quantity of gas within the balloon has not changed, only the volume of space that the gas molecules take up has changed. This law is important to us as divers because the temperature of the water at depth can differ significantly from the water at the surface.
Henry's Law
Henry's law states:
The amount of any given gas that will dissolve in a liquid at
a given temperature is a function of the partial pressure of the
gas that is in contact with the liquid and the solubility coefficient
of the gas in the particular liquid.
Translation: Because the human body is made up to a large extent by water, more gas will dissolve into the blood and body tissues as depth increases until saturation is reached.
An example of this law can be seen by quickly opening a bottle of carbonated soda. When we open the bottle, pressure is releases suddenly, causing the gas to come out of solution and producing the bubbles we see. This is similar to what happens when we ascend from a dive too quickly. The gases come out of solution, and without enough time to remove them through the process of breathing, this rapid release will cause bubbles to form.
Boyle's Law
Boyle's law states:
At constant temperature, the volume of a gas varies inversely
with absolute pressure.
Translation: As the pressure decreases, the volume increases.
Taking a balloon to the bottom of a pool can illustrate this law. At the bottom of the pool the balloon will be smaller than it was at the surface. Upon ascent to the surface, the balloon will return to its original size. Young children also experience this law when they accidentally release their helium balloon into the air. As the balloon rises, the pressure decreases and the balloon grows. Eventually, the balloon will pop because the volume exceeds the balloon's ability to stretch.
Dalton's Law
Dalton's law states:
The total pressure exerted by a mixture of gases is equal to the
sum of the pressures that would be exerted by each of the gases
if it alone were present and occupied the total volume.
Translation: If a container at one atmosphere of pressure contained only one gas, that gas would have a partial pressure of 1 atmosphere. However, if the container contained three gases, together the gases would have a partial pressure of 1 atmosphere and each alone would have a partial pressure relative to its percentage of the whole.
Using the above example, if the container was filled with air, we would observe the following:
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| Nitrogen |
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| Oxygen |
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| Carbon Dioxide |
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| Other |
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Total |
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Air is made up mostly of nitrogen and oxygen. Therefore, these two gases have the greatest partial pressures. Due to nitrogen's large percentage, there are health concerns related to nitrogen's increased partial pressure with increased depth. These can be studied further in Diving injuries.