Some pilots have developed a misconception about the FAR covering the use of supplemental oxygen. They ignore this sensible and realistic guideline because of their false belief.
These pilots, because they live at higher elevations, feel they have become acclimated. This, they reason, allows them to go to higher altitude, without supplemental oxygen, than the sea-level dwelling pilot.
This is true, so some extent, because the person living at a higher altitude has more red blood cells for oxygen transport.
A person living at 6,000-feet elevation may get by flying an additional 1,000 to 2,000 feet above the FAR guidelines, but his tolerance is not extended an additional 6,000 feet.
Even this additional 1,000- or 2,000-feet altitude can be suspect. There are many variables–such as physical shape, fatigue, smoking habits, business pressures, food consumption, and the like–that alone or in combination, can raise the physiologic altitude of the body.
Also, the body reacts to density altitude, not indicated altitude.
The atmosphere contains two problem areas for aviators.
First, the atmosphere is composed of about 21-percent oxygen.
Dry air contains 79.02-percent nitrogen, 20.95-percent oxygen, 0.03-percent carbon dioxide and included in the nitrogen are small amount of rare gases–argon, neon, helium, krypton, hydrogen, xenon, and radon–that apparently have no physiological significance on use mere mortals.
The second problem is that half the atmosphere by weight is located below 18,000-feet MSL.
The decreased partial pressure of oxygen encountered at increasing altitude can quickly lead to incapacitation or death.
The lethal effects of acute altitude hypoxia cannot be underestimated. Deaths have occurred at altitude between 17,000 and 20,000 feet.
Even hypoxic episodes that lead to mental confusion may result ultimately in the loss of the airplane because of the mental disorientation during or after the episode.
The main purpose of our respiratory process is to supply the lungs and the blood and tissues with adequate oxygen, and to eliminate that carbon dioxide that is generated by the metabolism of the body tissues.
Hemoglobin is the oxygen carrying agent of the blood. Oxygen must diffuse from a gaseous state to a dissolved state to combine with the hemoglobin.
The oxygen diffuses across the alveolar membrane, through the interstitial fluid and capillary endothelium. Within this capillary, the dissolved oxygen diffuses through the plasma, the red blood cell membrane, and the intracellular fluid within the red cell to combine with the hemoglobin.
The solubility of a gas, and its partial pressure, greatly influences its diffusion characteristics. Carbon dioxide is about 25 times more soluble than oxygen in pulmonary tissues and fluids and its capacity for diffusion is about 20 times greater than oxygen.
PARTIAL PRESSURE OF OXYGEN
The quantities of a gas at various altitudes, expressed in percentages of the atmosphere, has little significance. Percentage represents the relative volume of a gas and not its molecular concentration. Molecular concentration, or partial pressure, determines the availability of the gas to the body.
The partial pressure of a gas, in a mixture of gases not interacting with one another, is equal to that pressure that the particular gas would exert if it alone occupied the space taken up by the mixture (Dalton's Law of Partial Pressure).
The total pressure of a mixture of gases is the sum of the pressure of the individual gases composing the mixture.
The body requires hemoglobin saturations of 87-97 percent and arterial oxygen at 60-100mm Hg (millimeters of mercury) in order to function normally. Below this level the body is hypoxic.
The standard pressure at sea level is 760mm Hg. Since oxygen comprises about 21 percent of the air, we would expect the dry air oxygen partial pressure in the lungs to be 159.6mm Hg (760 times 21 percent), but through physiologic processes, the partial pressure of oxygen in the arterial blood is normally about 100mm Hg.
Air inhaled into the lungs enters small air sacs (alveolus) where the exchange of oxygen and carbon dioxide occurs. When the partial pressure of the oxygen is higher than it is in the blood, oxygen molecules are picked up by the hemoglobin molecules. This hemoglobin saturation is approximately 97 percent at sea level.
The atmospheric pressure decrease at 10,000-foot altitude causes 523mm Hg ambient air pressure resulting in 87 percent hemoglobin saturation and 61mm Hg arterial oxygen.
At 15,000 feet (429mm Hg) the hemoglobin saturation is 80 percent (we need 87-97 percent for normal functioning), and arterial oxygen is 44mm Hg (the body requires 60-100mm Hg.).
Hypoxia, sometimes called mountain sickness or altitude sickness, is simply a lack of oxygen at the tissue level of the body due to a decreased partial pressure of oxygen in the inspired air. It's advanced stage produces euphoria, a false sense of well-being, that renders a person incapable of understanding that anything is wrong. Hypoxia may lead to death.
Like an airplane, the human body has a service ceiling. Use supplemental oxygen to avoid exceeding your service ceiling.