Many of today's gliders and home built aircraft are
capable of transporting man to high altitudes in near record time, with
the average age of the pilot base at well over 50 years old, a practical
knowledge of physiological human principals and atmospheric physics are
not only desirable, but necessary in order to sustain safe operating
parameters. Therefore, the pilot should have a firm understanding of the
relationships between oxygen, altitude and the body.
The various gases that the atmosphere is made of
consists of about 78% nitrogen, about 21% oxygen and about 1.1% carbon
dioxide and other gases. These three main gases are very important to
the body physiologically. Due to the constant mixing of winds and other
meteorological factors, the percentages of each gas in the atmosphere
are normally constant to about 70,000 ft. throughout a wide range of
temperature and barometric changes.
Nitrogen, present in a high percentage, is responsible
for the major portion of the atmosphere's pressure or weight. Some
nitrogen is dissolved in and is carried by the blood, but this gas does
not enter into chemical combinations as it is carried throughout the
human body. Each time we breathe, the same amount of nitrogen is exhaled
as was inhaled.
Oxygen is a colorless, odorless, tasteless gas, but is
absolutely essential to all life on earth. Each time man breathes,
approximately 21% of that breath is oxygen. In the lungs, a portion of
this oxygen is absorbed into the blood-stream where it is carried to all
parts of the body. It is used to "burn" or oxidize food material and
produce energy transformations in the body.
Man can live for weeks without food and for days
without water, but only a few minutes without oxygen. Because man can
not store oxygen in his body, as he can food and water, he lives a
breath-to-breath existence. He continues to live only as long as he can
continually replenish the oxygen consumed by his metabolic process. Air
is a relatively heavy substance. It weighs 14.7 pounds per square inch
at the earth's surface sea level. That is the pressure created by one
column of air one inch square that is about 100 miles high (the
approximate total thickness of the layer of free air or atmosphere
covering the earth). Because the air pressure is equal from all sides
one does not notice the atmosphere's weight in pressure.
The weight of the atmosphere does not remain the same
from to bottom. In one respect the atmosphere can be viewed as an ocean
where a person finds that the absolute pressure around him increases the
deeper (or closer to the earth's surface) he goes. The composition of
the atmosphere always remains the same, but is more dense at the bottom
or at the surface).
The pilot should recognize that atmospheric pressure
does not diminish at a uniform (linear) rate with altitude. Although the
atmosphere covers the earth to a height of about 100 miles,
three-fourths of the molecular destiny of the atmosphere rests just
below our tallest mountain, Mt. Everest.
At an altitude of 18,000 feet above sea level the
absolute air pressure has decreased by 1/2, to only about 7 psia.
(pounds per square inch absolute). Deep interstellar space would be near
zero psia. In other words 18,000 ft. MSL is half way through the density
of the atmosphere. At 34,000 feet, the pressure has been cut in half
again to a mere 3.5 psia. At 65,000 ft. there is only 1 psia. and only
0.15 at 100,000 ft. Beyond that, the atmosphere is largely a vacuum.
The principal purpose of respiration is to supply the cells of the body
with oxygen and remove the carbon dioxide, a biological waste product,
produced by cellular activities. Three basic processes are involved with
respiration phases. The first process (external) is ventilation, or
breathing, the movement of air between the atmosphere and the lungs. The
second and third processes (internal) involve the exchange of gases
within the body through the blood stream. External respiration is the
exchange of gases between the blood and lungs. Internal respiration is
the exchange of gases between the blood and tissue cells throughout the
The respiration cycle begins with inhalation of air
into the lungs. Inhalation is produced by the contraction of the
diaphragm, the large muscle separating the thoracic and abdominal
Ordinarily, a person breathes 12 to 16 times a minute,
although the rate will be slower when resting and faster when
exercising. The average, quiet, resting man inhales about a pint (400
ml) of air for each breath, or six to eight quarts (8 liters) per
Oxygen used in the body is inhaled through the nose or
mouth, passes through the trachea and bronchial tubes, and is directed
into the lungs where it transfers to the blood. The blood then carries
this oxygen to living cells where energy is obtained by molecular
cellular transfer for all body functions. This energy transfer produces
carbon dioxide (CO2), a biological waste product. As carbon dioxide is
produced, blood then carries it back to the lungs to be released to the
atmosphere through the exhaling respiration phase back through the nose
or mouth. fig xx
Within the lungs, there are millions of tiny air sacs
called alveoli which inflate like tiny balloons. The number of alveoli
in the lungs is estimated to be around 750 million with a surface area
between 700 and 800 square feet, or about the size of a tennis court.
Blood is pumped from the heart through arteries to
microscopic capillaries, or tiny tubes, through which blood is
constantly flowing. The walls of the alveoli have micro-capillaries in
which the oxygen is diffused into the blood. Pressures inside and
outside, (the natural molecular tensions of body fluids and pressure
altitude) play an important factor in the effectiveness of the entire
respiration system. Once these differential pressures are reversed or
placed below a certain point, life-giving gases may not properly
exchange through the lungs or tissues.
The Pilot's Oxygen Needs & Availability
The amount of oxygen consumed by the body during the respiratory cycle
depends primarily upon the degree of physical or mental activity of the
individual. A person walking at a brisk pace will consume about four
times as much oxygen than at rest. In the course of an average day, a
normal adult male will consume about 35 cu. ft. of oxygen or 2.5 lb.
This is approximately equivalent to the weight of solid food consumed
daily. An oxygen supply which might be adequate for a person at rest
would be inadequate for the same individual while piloting an aircraft
under severe weather conditions or under mental stress.
It should be noted that since only 21% of the
atmosphere inhaled is oxygen, added to the fact that we only benefit
from only 1/5 to 1/8 of the total volume of oxygen inhaled per breath,
one can see that the actual volume of air used per day can be 80 to 90
Lbs. This number can be quite a bit higher by the respiration quality
factor that individual has, i.e. asthma, physical damage and age.
Oxygen becomes more difficult for your body to obtain
with altitude because the air becomes less dense, and the total
(absolute) air pressure decreases compromising your primary (lungs) and
secondary (bloodstream) respiratory systems the ability to transport and
exchange oxygen throughout the body, even though the percentage of
oxygen (21%) remains constant with respect to the atmosphere. As
altitude is increased and the pressure of oxygen is reduced, the amount
of oxygen transferred in the lungs alveoli is reduced which results in a
decrease in the percentage of oxygen saturation in the blood. This
causes a deficiency of oxygen throughout the body, and, for this reason,
supplemental oxygen is required if the body is to receive adequate
oxygen for proper mental and physical functions.
In a relatively simplistic term "oxygen saturation" is
defined as the percentage of available oxygen-carrying hemoglobin that
are carrying oxygen in your tissue and/or bloodstream. Another
simplistic, but fair, example would be if a given volume of blood has
100 hemoglobin cells and 95 of them are carrying oxygen, then the oxygen
saturation level is 95%.
The total effect on an oxygen-deprived individual is
the result of both altitude and amount of time exposed. Every cell in
the body is affected by the lack of oxygen, but the primary effects are
on the brain and the body's nervous system. Above 10,000 ft.
deterioration of physical and mental performance is a progressive
condition. This degenerative condition becomes more severe with
increased altitude or prolonged exposure. A person who is flying at
10,000 ft. for 5 hours can be equally affected as a person who went to
16,000 ft. for only one hour.
Oxygen pressure is about 1/5 that of actual
atmospheric pressure. Therefore, at a pressure altitude of 10,000 ft.,
for a standard day i.e. 70° F @ 29.92 In. Hg., the absolute pressure
would be about 10 psia. while the working pressure for oxygen would only
be 2.0 psia. It's no wonder why of all our critical life-support organs
our lungs are the largest for their function.
Some of the most common indications (symptoms) of
1. An increased breathing rate
2. Lightheadedness or dizzy sensation
3. Tingling or a warm sensation
4. Cold chills and/or cold extremities
5. Sweating and increased heart rate
6. Reduced color vision and visual field
7. Sleepiness, insomnia and/or nervousness
8. Blue coloring of skin, fingernails and lips
9. Behavior change, giddiness, belligerence, cockiness, anxiousness or
Subtle hypoxic effects begin at 5,000 ft.,
particularly noticeable at night. In the average individual, night
vision will be blurred and narrowed. Also, dark adaptation will be
compromised. At 8,000 ft., night vision is reduced as much as 24%
without supplemental oxygen. Some of the effects of hypoxia will be
noticed during the daylight at these altitudes without supplemental
oxygen during long flights, i.e. 3 to 5 hours.
At 10,000 ft. the oxygen pressure in the atmosphere is
about 10 psia. Accounting for the dilution effect of water vapor and
carbon dioxide in the alveoli, this is not enough to deliver a normal
(or less than needed) supply of oxygen into the lungs. This mild
deficiency is ordinarily of no great consequence. However, flying at an
altitude of about 10,000 ft. (not taking density altitude into account)
for 3 to 5 hours will more likely than not cause fatigue in which the
pilot may have compromised performance once he enters his destination.
Since the beginning of powered flight, pilots have reported experiencing
difficulty in concentrating, reasoning, judging, solving problems and
making precise adjustments of aircraft controls under prolonged flight
conditions at altitudes as low as 8,000 ft. MSL.
Commercial aviation pilots are required to be on
supplemental oxygen for flights lasting 30 minutes or more at 10,000 ft.
At 15,000 ft. drowsiness, headaches, weariness, fatigue and a false
sense of well-being will most likely be experienced in 1 to 2 hours
without oxygen. Most important and less evident to the individual is the
psychological impairment which could cause judgment errors, poor
coordination and difficulty in performing simple, let alone, important
At 20,000 ft. the absolute pressure altitude drops to
6.75 psia. and the oxygen pressure drops to 1.38 psia. This is less than
half that at sea level. Oxygen saturation of the blood drops to 62 to
64% at this pressure altitude. Unconscious collapse and/or convultions
will result within 10 to 15 minutes of exposure. Death is not uncommon
as a result of complications acquired from long or quickly changing
exposures to low partial pressures (high altitudes) without supplemental
oxygen or pressurized cabins.
At a pressure altitude of 34,000 ft. the lungs are
compromised so much in the ability to transfer gases to the blood and
air that the oxygen saturation level will drop to only 30%. Total
unconsciousness will result in 3 to 4 minutes. At this point a person
breathing 100% oxygen would not benefit from the supply because
pressures in water vapor and tissues will be the same as the absolute
pressure of oxygen (0.76 psia) where nearly nothing is transferred. One
would need to be using a full pressure-demand-type oxygen mask.
It is true that susceptibility to hypoxia varies from
person to person, and there are some who can tolerate altitudes well
above 10,000 ft. without suffering from the effects. It is equally true
that there are persons who develop hypoxic effects below 10,000 ft. As a
general rule, individuals who do not exercise regularly or who are not
in good physical condition will suffer from the effects of hypoxia more
readily. It is also true that even with tip-top shape athletes the
effects of hypoxia are still the same as a person who is in good
physical condition, but they simply have the ability to tolerate the
effects much better.
Individuals who have recently over-indulged in
alcohol, who are moderate to heavy smokers, or who take certain drugs
will be considerably more susceptible to the effects of hypoxia.
Susceptibility to the effects of hypoxia can also vary in the same
person from day to day or from morning to evening.
High altitude acclimation can be an improving factor
at moderate altitudes, i.e. 10,000 to 15,000 ft., however, once again,
at high altitudes the laws of physics prevail and even the most
acclimated will still suffer the effects of hypoxia from the exposure.
While not all of the known symptoms listed occur in
each individual, any given person will develop the same symptoms in the
same order for each time he becomes hypoxic. For this reason, a person,
having once experienced hypoxia is usually better prepared to recognize
the onset of hypoxic symptoms the next time around. One can participate
in a controlled hypoxic awareness program through an altitude chamber
that is offered by many commercial and/or university flight medical
Because hypoxia affects the central nervous system,
the general effects of hypoxia are almost identical to alcoholic
intoxication. A typical individual suffering from hypoxia, induced by
exposure between 15,000 and 20,000 ft. will be comparable to an
individual who has consumed five to six ounces of whiskey. The most
hazardous feature of hypoxia, as it is encountered with aviation, is its
gradual and rather insidious onset. Its production of a false sense of
well-being called euphoria is particularly dangerous. Since hypoxia
obscures the victim's ability and desire to be critical of himself, he
generally does not recognize the symptoms even when they are very
obvious to others. The hypoxic individual commonly believes things are
progressively getting better as he nears total collapse. There are some
false indicators of a hypoxic condition which should be considered. The
"blueness" color test of the finger nails has been suggested by some as
a guide to determine the degree of hypoxia, but this approach is usually
invalid because any hypoxic individual should consider himself an
unreliable observer that has all the appearances to himself of operating
effectively. Almost all of the symptoms of hypoxia are useless for
self-diagnosis, but have proven to be a life-saver from the standpoint
of an unaffected observer.
So remember, don that oxygen system before the
effects of hypoxia can manifest themselves. This will help you to arrive
at your destination safely!
- Breathing Station - Includes flow
control system (EDS or constant flow), cannula and face mask but
does not include a regulator or oxygen cylinder.
- Breathing System: A complete oxygen
system. It includes an oxygen cylinder, regulator, tubing,
connectors, flow control device (constant flow or pulse-demand), nasal
cannula and mask.
- Cannula: Two small tubes which feed
oxygen directly into the nose.
- Constant Flow System: The oxygen flows
at a continuous rate which is adjusted manually to a pressure and flow
needed by humans at various altitudes.
- Cylinder: The container for the
oxygen. It generally includes an on/off valve.
- Delivery System: A complete oxygen
system with everything except the oxygen cylinder. It includes
a regulator, tubing, flow control device (constant flow or
pulse-demand), nasal cannula and mask
- EDS: Electronic (Pulse Demand)
Delivery System - A small pulse of oxygen is delivered at the
beginning of every inhalation. This method is much more efficient
than constant flow oxygen systems. EDS systems require as little as
1/8 the amount of oxygen and 1/4 the weight and volume of conventional
- Oxymizer Oxygen-Conserving Nasal
Cannula: Includes an oxygen-conserving reservoir. For use with
constant flow systems only - not for use with the EDS system.
- Regulator: The reducing regulator
takes the oxygen pressure from the cylinder and regulates it down to a
manageable pressure for the delivery device. Many types of regulators
are found in aviation oxygen systems. Some include a pressure gauge.
Regulations in the USA