It causes important changes in body functions, thought  processes, and the maintainable degree of consciousness.  The resultant sluggish condition of mind and body produced  by insufficient oxygen is called hypoxia. There are several scenarios that can result in hypoxia. During  aircraft operations, it is brought about by a decrease in the  pressure of oxygen in the lungs at high altitudes. The air  contains the typical 21 percent of oxygen, but the rate at  which oxygen can be absorbed into the blood depends upon  the oxygen pressure. Greater pressure pushes the oxygen  from the lung alveoli into the bloodstream. As the pressure is  reduced, less oxygen is forced into and absorbed by the blood.  At sea level, oxygen pressure in the lungs is approximately  three pounds per square inch (psi). This is sufficient to  saturate the blood with oxygen and permit the mind and body  to function normally. As altitude is increased, this pressure  decreases. Below 7,000 feet above sea level, the available  oxygen quantity and pressure remain sufficient for saturation  of the blood with oxygen. Above 7,000 feet, however, the  oxygen pressure becomes increasingly insufficient to saturate  the blood. At 10,000 feet mean sea level (MSL), saturation  of the blood with oxygen is only about 90 percent of normal.  Long durations at this altitude can result in headache and  fatigue, both symptoms of hypoxia. At 15,000 feet MSL,  oxygen transfer to the bloodstream drops to 81 percent of  saturation. This typically results in sleepiness, headache,  blue lips and fingernails, and increased pulse and respiration.  Worse yet, vision and judgment become impaired and safe  operation of an aircraft becomes compromised. Higher in the  atmosphere, decreasing pressure causes even less oxygen to  enter the bloodstream; only 68 percent saturation at 22,000  feet MSL. Remaining at 25,000 feet MSL for 5 minutes, where  oxygen transfer to the blood is reduced to approximately 50  percent saturation, causes unconsciousness.  [Figure 16-2] The second most prevalent substance in the atmosphere,  oxygen, is essential for most living processes. Without  oxygen, humans and animals die very rapidly. A reduction  in the normal oxygen supply alters the human condition.  Ozone is a form of oxygen. It contains three oxygen atoms per  molecule, rather than the usual two. Most of the atmosphere’s  ozone is formed by the interaction of oxygen and the sun’s  rays near the top of the stratosphere in an area called the  ozone layer. This is important to living organisms because  ozone filters out most of the sun’s harmful ultraviolet (UV)  radiation. Ozone is also produced by electrical discharges,  such as lightning strikes. It has a faint odor, somewhat like that  of weak chlorine, that may be detected after a thunderstorm.  Auroras and cosmic rays may also produce ozone. Ozone is  of great consequence to living creatures on earth and to the  circulation of the upper atmosphere. The amount of water vapor in the atmosphere is variable but,  even under humid conditions at sea level, it rarely exceeds 5  percent. Water also occurs in the atmosphere as ice crystals.  All forms of water in the atmosphere absorb far more energy  from the sun than do the other gases. Water plays an important  role in the formation of weather. The small quantity of carbon dioxide in the atmosphere is  utilized by plants during photosynthesis. Thus, the food  supply for all animals, including man, depends on it. Carbon  dioxide also helps control breathing in man and other animals. Nitrogen is an inert gas that is not used directly by man for life  processes; however, many compounds containing nitrogen  are essential to all living matter.  Figure 16-1.  The percentage of the various gases that comprise  the atmosphere. Human Respiration and Circulation Oxygen and Hypoxia 16-2 Figure 16-2.  Oxygen pressure in the atmosphere at various altitudes. 0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 3.08 2.57 2.12 1.74 1.42 1.15 0.92 0.76 0.57 Altitude MSL (feet) Oxygen pressure (psi) Oxygen 21% Other gases 1% Nitrogen 78%