The Standard Atmosphere
The ICAO Standard atmosphere is defined as follows:
• MSL temperature of +15°C.
• MSL pressure of 1013.25 hPa (760 mm Hg).
• MSL density of 1225 g/m3
• A lapse rate of 1.98°C/1000 ft (6.5°/km) up to 36 090 ft (11 km) thereafter the temperature remains constant at -56.5°C up to 65 617 ft (20 km).
Oxygen and Respiration
3
3Oxygen and Respiration
The altitudes in the standard atmosphere that pressure will be ¼, ½ and ¾ of MSL pressure is approximately:
¼ MSL - 36 000 ft ½ MSL - 18 000 ft ¾ MSL - 8000 ft
Note : Atmospheric pressure decreases at a faster rate at low altitudes than at higher altitudes
Composition of the Atmosphere
The atmosphere is made up of:
21.0% oxygen 78.0% nitrogen 0.93% argon
0.03% carbon dioxide 0.04% rare gases
These volume percentages for each of the gasses remain constant to about 70 000 ft - well within the altitudes at which conventional aircraft operate. For the pilot oxygen is the most important of these gases.
Humidity and Relative Humidity - Definitions
Absolute Humidity. The weight of water vapour in unit volume of air which is usually expressed in g/m³.
Relative Humidity. The amount of water vapour present in a volume of air divided by the maximum amount of water vapour which that volume could hold at that temperature expressed as a percentage.
A Summary of the Gas Laws
BOYLE’S LAW states that:
“Providing the temperature is constant the volume of gas is inversely proportional to its pressure”. (Otic and gastrointestinal tract barotrauma, aerodontalgia).
Expressed mathematically:
P1 P2
V2 V1
=
where P1 = initial pressure P2 = final pressure V1 = initial volume V2 = final volume
3
Oxygen and Respiration3
Oxygen and Respiration
DALTON’S LAW states that:
“The total pressure of the gas mixture is equal to the sum of its partial pressure”.
(Hypoxia and night vision).
Expressed mathematically: Pt = P1 + P2 + P3 ... Pn Where: Pt = total pressure of the mixture
P1, P2 ... Pn = partial pressure of each of the constituent gases HENRY’S LAW states that:
“At equilibrium the amount of gas dissolved in a liquid is proportional to the gas pressure”.
(Decompression sickness and “bends”).
FICK’S LAW states that:
“The rate of gas transfer is proportional to the area of the tissue and the difference between the partial pressures of the gas on the two sides and inversely proportional to the thickness of the tissue”. (Diffusion of gas at the lungs and cells).
CHARLES’ LAW states that:
“The volume of a fixed mass of gas held at a constant pressure varies directly with the absolute temperature”.
Expressed mathematically: V1 T1 (t1 + 273) V2 T= 2 (t= 2 + 273) Where: V1 = initial volume
V2 = final volume
T1 = initial absolute temperature = initial temperature t1°C + 273 T2 = final absolute temperature = final temperature t2°C + 273 THE COMBINED GAS LAW states that:
“The product of the pressure and the volume of a quantity of gas divided by its absolute temperature is a constant”.
Expressed mathematically: PV T =K
Partial Pressure. Looking closer at Dalton’s Law with regards to the atmosphere, it is well- known that the total pressure decreases as altitude increases. As the proportion of oxygen remains constant it follows that the partial pressure of oxygen must also reduce. In dealing with the pressures at various altitudes instead of hectopascals/millibars used in other subjects such as Meteorology or Instruments, the unit of measurement is the millimetre of mercury (mm Hg). At sea level the standard pressure is 760 mm Hg. As oxygen is 21% of the total then the partial pressure of oxygen is twenty one hundredths of 760 - 160 mm Hg.
Oxygen and Respiration
3
3Oxygen and Respiration
Humans operate best at sea level but they are perfectly capable of operating at higher altitudes where the partial pressure of oxygen is lower. People who live permanently at high altitudes can adapt to the reduced amount of oxygen by producing extra red blood cells to enable more oxygen to be carried. Healthy people without these extra cells can function normally up to about 10 000 -12 000 ft provided no strenuous exercise is undertaken.
As altitude increases the overall pressure decreases as does the partial pressures of the various gases in the atmosphere.
The partial pressure of oxygen in the air is not, however, the governing factor. The reason being that the body takes its oxygen from the alveoli of the lungs where the partial pressure is less. The body produces carbon dioxide and water vapour which is passed into the alveoli.
As the total pressure both inside and outside the lungs remains the same then the partial pressure of oxygen must reduce. The table following shows the partial pressures of the various gases in the atmosphere and in the alveoli at various altitudes.
AT SEA LEVEL
Partial Pressures (mm Hg)
Constituents Oxygen Nitrogen Water Vapour Carbon Dioxide
Atmospheric Air 160 (21%) 600 - -
Alveolar Air 103 (14%) 570 47 40 (5.3%)
AT 10 000 FEET
Alveolar Air 55 381 47 40
As a partial pressure of 55 mm Hg is considered the minimum for normal operations, then above cabin heights of above 10 000 ft oxygen needs to be added to the pilot’s air supply.
The oxygen added is sufficient to maintain an alveolar partial pressure of 103 mm Hg which is equivalent to breathing air at sea level.
At lower levels, less oxygen needs to be added and as the altitude increases more oxygen is added. A stage will be reached when one hundred per cent oxygen is required to maintain the 103 mm Hg partial pressure (the equivalent to breathing air at sea level). This stage is reached at:
33 700 ft
This does not, however, limit us to flying only to 33 700 ft when breathing 100% oxygen. We can continue to operate normally with alveolar partial pressure of 55 mm Hg. (equivalent to breathing air at 10 000 ft). This partial pressure is reached at:
40 000 ft
Above this level, 100% oxygen must be supplied at an increased pressure (pressure breathing)
3
Oxygen and Respiration3