Air that we breathe consists almost completely of Oxygen (21%) and Nitrogen (79%). The composition of the diver’s breathing gas is the same when his cylinders are filled with compressed air – that’s the case with the majority of diving activities. But what changes during diving is the PRESSURE exercised on the diver, depending on the depth he is at each instance (pressure increases by 1 Atmosphere Absolut every 10 meters of sea water). Pressure change and the rate of this change is the main determinant of effects of diving on human physiology. Understanding how gases respond to pressure changes is crucial and offers the explanation for diving induced conditions. It is also the basic science behind the effective management of these conditions, as well as prevention so as to establish safe diving rules and habits. Let’s take a closer look at the peculiarities of the undersea environment, starting with the gas laws.
Pressure (symbol: P) is the ratio of force to the area over which that force is distributed. Pressure is the main environmental change on human while diving and is the sum of: a. the pressure exerted by water/sea (Hydrostatic Pressure – the weight of the water), b. the pressure exerted by earth’s atmosphere on water/sea surface (Atmospheric Pressure – the weight of atmosphere’s mass). Main units of measurement for pressure are: Atm (ATA) – Psi – mmHg – Torr – msw – fsw – bar – Pascal. Atmospheric Pressure at sea level is considered fixed, equal to 760mmHg or 1Atm. On the other hand, Hydrostatic Pressure increases in proportion to depth by 1 ATA every 10,08 meters of sea water (msw) or 33,07 feet of sea water (fsw). Simplified: 1 ATA is added every 10msw or 33fsw.
Pressure and its variations influence human physiology to such an extent that we consider undersea environment as a high-pressure (hyperbaric) environment, depending on the depth and governed by the well-known gas laws. Gases under pressure store increased energy amounts. On the other hand, small changes in the percentage proportion of gases are maximized when environmental pressure increases and resulting physiological effects vary greatly. For better understanding of the effects of pressure variations, knowledge of these laws is necessary
Ideal Gas Law or General Gas Law: It shows the relationship between pressure-volume-temperature for a fixed mass of gas. P x V = n x R x T , where >
P = pressure, V = volume, n = number of moles in the gas, R = universal gas constant, Τ = temprature (°Κ), όπου °Κ = 273+°C.
The General Gas Law encloses the following gas laws when gas mass is constant (n and R are constant)
Boyle’s law: the volume of a given mass of gas is inversely proportional to its pressure, if the temperature remains constant
P1 xV1 = P2xV2 ή V2 = (P1xV1) / P2
In diving practice, since every 10m of descent total pressure increases by 1 ATA, at the depth of 10m volume of gases is halved comparing to the volume at surface (V1/2). Descent from 20 to 30 meters leads to volume change from V1/3 to V1/4. It appears that greater volume changes while diving occur at the beginning of descent and the end of ascent
Charles’s law: for a gas at constant pressure, the volume is directly proportional to its absolute temperature
Ρ1/Τ1 = Ρ2/Τ2 ή Ρ2 = Ρ1 x Τ2/Τ1.
Partial pressure of gases
In a mixture of gases, each gas has a partial pressure equal to the fraction of the particular gas times the total pressure of the gas mixture. It is the hypothetical pressure of that gas if it alone occupied the volume of the mixture at the same temperature. The total pressure of an ideal gas mixture is the sum of the partial pressures of each individual gas in the mixture
PGAS= FGASxPB , where:
FGAS : fraction
PB: total pressure = (D+10)/10 ,
and so, partial pressure of a gas in a breathing gas mixture, at the depth of D meters is equal to : PGAS= FGASx(D+10)/10.
Example : How much is Ο2 partial pressure (Po2) in air breathing at the depth of 30 meters?
*Fo2 = 0,21
Po2 = 0,21 x (30+10)/10 = 0,84 ATA
In conclusion, partial pressure of Ο2 and Ν2 increase when breathing air as the diving gas in proportion to the depth of diving (the same is true with any other gas present in the breathing gas eg helium in heliox). It should be emphasized that percentage proportion (fraction) of each gas doesn’t increase. Increase of surrounding pressure (depth) alone results in increase of the amount of gas getting in touch with the human body, as explained by the gas laws
Dalton’s law: the total pressure of a mixture of gases is the sum of the partial pressures of the individual gases
Pascal’s law (principle of transmission of fluid-pressure): pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid-filled spaces, such that the pressure variations remain the same. That explains why the diver’s body does not smash when going deeper. Human body (except some air-filled spaces) is a complex of vascularised, watery tissues – consists of more than 60% water