the pressure is on
About 175 years ago
scientists first discovered that the prime purpose of breathing was to
obtain oxygen needed by the body and to get rid of excess carbon
dioxide, a waste product.
The human body is a heat
engine which, like any engine, consumes fuel (the carbohydrates, fats,
and proteins derived from food). This fuel is converted into the energy
we need to live by a burning process called oxidation. As in any other
burning process, a certain amount of oxygen is necessary. When the body
is resting, it consumes approximated one-half pint of oxygen per minute.
When given an added workload (such as walking or running), the body,
like any other machine, will generate more heat and use more oxygen,
perhaps as much as 8 pints per minute.
To extract this oxygen
from the air, the body is equipped with a respiratory system (lungs).
The oxygen is then distributed throughout the body by a circulatory
system (heart, arteries, veins, and capillaries).
Air contains about 20%
oxygen and about 80% nitrogen. At sea level, a healthy man can extract
enough oxygen from the air to maintain his system and continue his
normal activities. Above 8,000 or 9,000 feet, however, problems of
oxygen shortage begin to appear. Because the air is less dense, it
offers less actual oxygen per breath of air inhaled - even though oxygen
and nitrogen are still mixed in the 20:80 ratio. The density of air is
measured by barometric pressure and it is on this principle that your
altimeter is built.
Oxygen is transported
throughout the body in the bloodstream which contains, among other
things, the red blood cells. These cells contain a complex substance
called haemoglobin. Haemoglobin picks up oxygen at the lungs and carries
it to the tissues for use and picks up carbon dioxide at the tissues and
transports it to the lungs for exhalation. Blood can be compared to a
conveyor belt, constantly hauling oxygen in and carbon dioxide out. The
amount of oxygen that can be carried in the blood depends, to a large
extent, upon the pressure that the oxygen gas from the air exerts on the
blood as it passes through the lungs. (Manufacturers of carbonated
drinks take advantage of this pressure principle to dissolve large
amounts of carbon dioxide gas in their beverages).
At 10,000 feet, the
blood of a man who is exposed to outside air can still carry oxygen at
90% of its capacity. At this altitude, the flight performance of a
healthy pilot is impaired only after some time, when he may find himself
a little less dexterous than usual at tuning radios, slower at working
navigational problems, and less able to sustain close concentration. At
14,000 feet, he may become appreciably handicapped - forgetting to
switch tanks, flying off course, or disregarding hazardous situations.
From 18.000 feet and beyond, exposure to environmental air will quickly
cause total collapse and inability to control the aircraft.
This means that if you
choose to fly at high altitudes, you must take along either oxygen or
pressure. You have a choice, then, between pressurizing the cabin of the
aircraft or breathing a mixture with more oxygen in it.