Graham’s Law states:


"An area of high gaseous pressure will exert force towards an area of low gaseous pressure", causing gases to move back and forth across a permeable membrane (such as the air sacs). The higher partial pressure of oxygen now diffuses through the air sac wall and into the blood. At the same time this is happening, the high pressure of carbon dioxide in the blood will cause some of the carbon dioxide to diffuse into the lung thereby eliminate through exhalation.


The transport of O2 in the blood is dependent on the Partial Pressure of Oxygen (pO2). Carbon dioxide, with a higher partial pressure in the alveoli is also forced into the lungs to be eliminated.


Oxygen Saturation


The amount of oxygen in the bloodstream is an important window to the respiratory and circulatory health of an individual. Measurement of O2 level in the bloodstream is accomplished using “pulse oximetry,” which is the non-invasive measurement of the color of the red blood cells carrying oxygen throughout the body.


Levels of cardiopulmonary health, physical condition, illness, diet, and age, all play an important role in circulatory and respiratory health and oxygen saturation levels. Generally, a healthy individual, at sea level, or their acclimatized altitude (altitude at which they are accustomed, i.e., where they live), should produce a pulse oximetry reading of between 95 - 100%. HOWEVER, many “normal” people have readings in the low 90% area. While the healthy, sea level readings may be “normal”, inflight readings will probably be less than “normal” due to the decreased cabin pressure (cabin altitude). This normal decrease in oxygen saturation in flight may exacerbate other medical problems, resulting in a medical emergency.


Many factors can and will lower oxygen saturation, such as:


• Common Cold / Congestion • Apnea • Insufficient breathing • Fatigue • Asthma • Drugs


• Tobacco • Alcohol • Illness • Shallow Breathing • Stress • Pollution


Because they decrease your ability to carry and use the available oxygen, these same factors will affect (decrease) your Times of Useful Consciousness (TUC) in a decompression.


11 High Altitude Awareness Training


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