 The Ideal Gas Law

# Gas Laws in Aerospace Physiology

There exist a huge number of Laws in Physics & Chemistry.  However, there are only a few that are really important in aerospace medicine.  Most of these are the basic Gas Laws.

Let’s review some of the most important ones.

• Ideal Gas Law  PV = nRT

The ideal gas law is the equation of state of a hypothetical ideal gas. It is a good approximation to the behaviour of many gases under many conditions, yet it has many limitations.  It is actually not ‘ideal’ for studying gases relevant to human physiology.

At a constant temperature, the volume of gas is inversely proportional to its pressure.  Basically, as you ascend in altitude (or to the surface if diving), gas expands to a greater volume due to decreased pressure exerted on it.  This is important for both flyers and divers to consider as gas in the middle ear, sinuses, GI tract or other places can expand and cause tissue injury.  Read more about the condition of Trapped Gases.  Water vapor, or wet gas, behaves slightly differently.  This is important since most gases in the human body are humidified.

• Dalton’s Law PT = P1 + P2 + … + Pn

The total pressure of a gas mixture is equal to the sum of the partial pressures of each gas.  At sea level this sum should add up to 760 mm Hg. The independent pressure of each gas is called the partial pressure of that gas.  The pressure of water vapor is also not usually considered in this calculation and varies from 0% to 6.2% at sea level.  For the flyer, this law demonstrates how increasing altitude results in proportional decreases in the partial pressure of the gas without changing the percentage concentration of that particular gas.  It is this lowering of the partial pressure of oxygen that causes the medical condition known as Hypoxia at altitude.  Additionally, the subsequent decrease of nitrogen’s partial pressure at altitude leads to decreased solubility of the gas.  This increases the risk of developing decompression sickness (DCS).

The amount of gas in solution varies directly with the partial pressure of that same gas over the solution. In other words, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid.  If the partial pressure of the gas decreases, so does the solubility of the gas in the liquid and bubbles of this gas will form.  This is what happens to carbon dioxide when a soda bottle is opened and the high pressure of the closed container rapidly decreases.  If pressure is rapidly to pressurized cockpit, a similar phenomenon occurs, but with nitrogen bubbles in the vascular system.  This can cause the medical condition knowns as Decompression Sickness.

• Charles’ Law  V1/V2 = T1/T2

This is essentially Boyles’ Law with a different variable held constant.  In Boyles’ version, temperature is held constant whereas Charles assumed a constant pressure.  This law states that gas expands when heated if pressure is kept constant and the discovery of this principle led directly to advances in lighter than air flight (hot air balloons).  Since temperature in the human body is fairly constant, this equation is not as useful for human physiology.  Interestingly, this Law led Lord Kelvin to discover the concept of ‘Absolute Zero’.

• The Speed of Sound = 761 mph (340 m/s) at sea level.

The speed of sound is a function of the square root of temperature (usually expressed in Kelvin).  This means that it changes depending on the temperature of the ambient environment.  The speed of an object divided by the speed of sound in the fluid is called the Mach number. Objects moving at speeds greater than Mach 1 are traveling at supersonic speeds (faster than sound).  Interestingly enough, sound travels faster thru liquids and porous solids than air.  Everyone knows what happens when an aircraft breaks the sound barrier – SONIC BOOM!!!  Nothing remarkable happens to a human being physiologically at supersonic speeds, but it’s still interesting to consider.

GAS LAWS MADE SIMPLE. CORNY BUT KIND OF CLEVER.