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 approximately 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 through 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 hemoglobin. Hemoglobin 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.