Chapter

3

Oxygen and Respiration

Oxygen Intake			37
Composition of the Standard Atmosphere - Humidity, Gas Laws and Partial Pressure			39
Thresholds of Oxygen Requirements Summary			43
Hypoxic Hypoxia . . . . . . . . . . . . . . . . . . . . . . . . . .	. .		43
Hypoxic Hypoxia Symptoms . . . . . . . . . . . . . . . . . . . . . .	. .		43
Stages/Zones of Hypoxia			44
Factors Determining the Severity of and the Susceptibility to Hypoxic Hypoxia . . . .	.		. 45
Anaemic Hypoxia . . . . . . . . . . . . . . . . . . . . . . . . . .	. .		46
Time of Useful Consciousness (TUC)			47
Times of Useful Consciousness at Various Altitudes . . . . . . . . . . . . . .	.		. 47
Effective Performance Time (EPT) . . . . . . . . . . . . . . . . . . . .	. .		47
Hyperventilation . . . . . . . . . . . . . . . . . . . . . . . . . .	. .		47
Symptoms of Hyperventilation			48
Hypoxia or Hyperventilation? . . . . . . . . . . . . . . . . . . . . . .	.		. 48
Cabin Pressurization . . . . . . . . . . . . . . . . . . . . . . . . .	.	.	49
Cabin Decompression			49
Decompression Sickness (DCS)			49
DCS in Flight and Treatment . . . . . . . . . . . . . . . . . . . . . .	. .		50
Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	.	.	52
Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	.		. 56

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Oxygen and Respiration 3

Oxygen Intake

We have seen from the previous chapter that oxygen is required by all the cells and tissues of the body. Certain cells are much more sensitive to a lack of oxygen than others. Brain cells for example will die if they are deprived of oxygen for as little as two minutes. The oxygen required by the body is obtained from the air we breathe. Whereas the brain only constitutes approximately 2% of body weight, it consumes 20% of the total required oxygen for the normal functioning of the body. Nitrogen is also dissolved into the blood to a small extent but plays no part in the bodily processes. However the importance of this nitrogen content and its role in decompression sickness (DCS) is discussed in this chapter.

The level of carbon dioxide in the bloodstream has been referred to in the previous chapter. It is this that triggers the brain to increase or decrease breathing. The higher the carbon dioxide level the more the brain is stimulated to increase breathing and thus increase the oxygen content. This, in turn, reduces the carbon dioxide content. Once the brain senses that the level is normal, the breathing rate is reduced. Certain cells in the brain also detect shortage of oxygen in the blood and will again trigger an increase in respiration.

Air is drawn into the lungs during inspiration, when the intercostal muscles between the ribs acting in unison with the diaphragm increase the volume of the chest cavity thereby reducing the internal pressure. Expiration is the reverse process, achieved in normal breathing by relaxation of the above muscles. This mechanism is sometimes referred to as external respiration. Under normal conditions, external respiration is a subconscious process that occurs at a rate of 12 to 20 breaths/minute, averaging 16 breaths/minute.

Normal breathing is a purely automatic process. In some diseases such as poliomyelitis the automatic system fails and an artificial respirator is required to maintain respiration.

Oxygen and Respiration 3

Figure 3.1

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Air entering the nose (where it is warmed, moistened and filtered) and mouth passes into the trachea, which is a tube reinforced with cartilage rings. The trachea divides into the left and right bronchi which take the air to the two lungs. Within the lungs the airways become progressively smaller until they end in tiny sacs, the alveoli. These sacs are very small but the normal lung contains thousands of them giving a total area of some hundreds of square metres.

The walls of the alveoli are very thin and are covered by fine capillaries which themselves have only a thin wall. Oxygen from the alveoli diffuses into the blood and carbon dioxide and water pass into the lungs to be exhaled in expiration. Effective gas exchange only takes place between the alveoli and the capillaries; the walls of the larger passages in the lung are too thick to allow the diffusion. Figure 3.2 shows the main divisions of the respiratory system.

PulmonaryVolumes and Capacities

Pulmonary means “of the lungs”. It is required that you are familiar with the following definitions and capacities:

•Tidal Volume is the volume of air inhaled and exhaled with each normal breath. It amounts to about 500 ml in the normal male adult.

•Inspiratory Reserve Volume is the extra volume of air that can be inhaled over and beyond the normal tidal volume.

•It amounts to about 3000 ml in the normal male adult.

•Expiratory Reserve Volume is the amount of air that can be still exhaled by forceful expiration after the end of the normal tidal expiration. It amounts to about 1100 ml in the normal male adult.

•Residual Volume is the volume of air remaining in the lungs even after the most forceful expiration. It amounts to about 1200 ml in the normal male adult.

Note: All pulmonary volumes and capacities are about 20% - 25% less in the female.

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Figure 3.2 Air passages in the lungs

Composition of the Standard Atmosphere - Humidity, Gas Laws and

Partial Pressure

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).

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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		=	V2
		P			V
	2		1
where P1 = initial pressure	P2		= final pressure
V1 = initial volume	V2 = final volume

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Oxygen and Respiration 3

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:

V

T

(t

+ 273)

1

=

1

=

1

V

T

(t

+ 273)

2

2

2

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	= K
		T

Partial Pressure. Looking closer at Dalton’s Law with regards to the atmosphere, it is wellknown 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

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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) but this is more relevant to military crews who fly at high altitudes. Pressure breathing for long periods is tiring and it requires practice to perfect the technique.

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