Depressurization
Depressurization can arise as a result of a
mechanical failure of the pressure controller , or if, in an unlikely
event of a terrorist sabotage, ... an in- flight explosion.
Very simply, the effect of a rapid
depressurization (more likely from an in-flight explosion) will cause
your ears to 'pop' out. If you don't grab and don the oxygen mask
immediately, you will, depending on the aircraft altitude, lose
consciousness very rapidly as a result of hypoxia.
Please read on if you want to be very
knowledgeable in this topic....
Physiology of Respiration
To understand the effect of hypoxia, you
need to have a clear knowledge on the physiology of respiration.
The human body needs a constant supply of
oxygen to break down food and produce energy. The body is unable
to store oxygen and nervous tissue is very sensitive to deficiency of
oxygen. The brain uses 20 percent of total resting oxygen
consumption. Stopping its blood supply and subjecting it to anoxia
(no oxygen) produces unconsciousness in as short as 10 seconds.
Brain cell damage will occur after 4 minutes.
Carriage of Oxygen in the Body
Gases are exchanged between the body and the
air in the lungs. The lungs are composed of numerous tiny blind
ended sacs - the alveoli.
Oxygen passes from the alveolar air into the
bloodstream and is carried by the red blood
cells to the tissues. Carbon dioxide is
carried from the tissues to the lungs where it is breathed out.
This exchange of gases depends on a pressure
gradient in the lungs and the blood stream.
Atmospheric pressure at sea level is 760 mm
Hg or 14.7 psi (pounds per square inch). Air is a mixture of two
main gases, oxygen (20%) and Nitrogen (80%). The pressure of each
constituent in a mixture of gases is called its partial
pressure. In a mixture of gases, the
sum of the partial pressures is equal to the total pressure.
Each constituent in
a mixture of gases, contributes to the total pressure in proportion to
its volume contribution. Therefore, the partial pressure of oxygen
in dry air is 160 mm Hg (20% of 760 mm Hg). A partial pressure of 160 mm
Hg is more than adequate for normal body requirements.
The volume of the lungs is large compared
with the amount of air taken in with each breath so that air exchange at
each breath does not nearly empty and fill the lungs. Since oxygen is
constantly being removed from the gas mixture in the alveoli and the
carbon dioxide added to it through the alveolar walls, it follows that
the composition of the alveolar air is different from that of the
atmospheric air.
The composition of
dry
air at sea level is approximately:
Oxygen
14.5 %
Nitrogen
80.0 %
Carbon
dioxide 5.5 %
Before working out the partial
pressure of the constituents of the alveolar gas mixture, a further
factor must be taken into consideration. Alveolar air is always
completely saturated with water vapor at body temperature and this
vapor exerts its own partial pressure.
Since the partial pressure of
oxygen and carbon dioxide in the blood on the other side of the alveolar
wall are different, there is rapid diffusion of these gases across the
wall, so that almost instantaneous equilibrium is reached and the blood
leaving the alveoli has the same partial pressures of oxygen and carbon
dioxide as the alveolar air.
If the pressure of the oxygen
in the lungs decreases, that is when rapid
depressurization occurs, less oxygen will
diffuse across the alveoli into the blood stream and less oxygen will
reach the tissue. If cabin altitude is increased to 47,000 feet,
even breathing 100 % oxygen, oxygen will diffuse out of the blood stream
into the lungs and unconsciousness will be
lost within 10 to 20 seconds.
