Human Physiology is evolutionarily adapted to be efficient up to about 12,000 feet above sea level (the limit of the physiological efficiency zone). Outside of this zone, physiological compensatory mechanisms may not be able to cope with the stresses of altitude.

Professional Pilots often undergo a series of exercises in high altitude hypobaric (low pressure chambers to simulate the early stages of hypoxia (oxygen depletion in the body). The tests provide evidence of the rapid deterioration of motor skills and critical thinking ability when undertaking flight above 10,000 feet above sea level (ASL) without the use of supplemental oxygen. Hypoxia can also lead to hyperventilation as the body attempts to increase breathing rates.

Altitude-Induced decompression sickness is another common side effect of high altitude  exposure in unpressurized or inadequately pressurized aircraft. Although the percentage of oxygen in the atmosphere remains about 21% (the other 79% of the atmosphere is composed of nitrogen and a small amount of trace gases),  there is a rapid decline in atmospheric pressure with increasing  altitude. Essentially,  the decline in pressure reflects the decrease in the  absolute number  of molecules  present in any given volume of air.

Pressure changes can adversely  affect  the  middle  ear,  sinuses, teeth,  and  gastro-intestinal tract.  Any sinus block (barosinusitis) or occlusions  that inhibit  equalization of  external  pressure with pressure  within the ear  usually results  in severe pain.  In severe cases,  rupture of  the tympanic  membrane may occur. Maxillary sinusitis may produce pain that is improperly perceived as a  toothache.  This is an   example of referred pain. Pain related to  trapped gas in the  tooth itself (barondontalgia) may also occur.

Ear  block  (barotitis  media)  also  causes  loss of   hearing  acuity (the ability to hear sounds across a broad range of pitch and volume). Pilots and passengers may use  the Valsalva Maneuver (closing the mouth and pinching the nose  while attempting to  exhale) to  counteract  the effects  of water  pressure  on the  Eustachian Tubes  and to eliminate pressure problems associated with the middle ear. When subjected to  pressure, the tubes may collapse or fail to open unless pressurized. Eustachian tubes connect the corresponding left and right middle  ears to the back of the  nose and  throat, and function to allow the equalization of pressure  in the middle ear air cavity with the outside (ambient) air pressure. The  degree  of Eustachian Tube  pressurization can be roughly regulated by the intensity of  abdominal,  thoracic, neck, and mouth  muscular contractions used to increase pressure in the closed airway.

Rapid changes in altitude allow trapped gases to cause pain in joints in much the same way - although to a far lesser extent - that the bends causes pain in scuba divers.  Lowered outside atmospheric pressure creates a strong pressure gradient that permits  dissolved nitrogen and other dissolved or trapped gases within the body to attempt to bubble off or leave the blood and tissues in an attempt to move down the concentration gradient toward a region of lower pressure.

Spatial Disorientation Trainers demonstrate the disorientation and loss of balance (Vestibular Disorientation) that can be associated with flight at night, or in clouds - where the pilot loses the horizon as a visual reference frame. Balance and the sense of turning depend upon the ability to discriminate changes in the motion of fluids within the semicircular canals of the ear.  When turns are gradual, the changes become imperceptible because the fluids are moving at a constant velocity. Accordingly, without visual reference, pilots can often enter into steep turns or dives without noticing any changes. Spacial Disorientation Chambers allow pilots to learn to trust their instruments as opposed to their error prone sense of balance when flying in IFR (Instrument Flight Rules) conditions.

In addition to vestibular disorientation, spatial disorientation can also lead to motion sickness. Because of the highly repetitive nature of the active pilot scan of instruments, fatigue is a chronic problem for pilots.  Fatigue combined with low oxygen pressures may induce strong and disorienting visual illusions.

Although not often experienced in general aviation, military pilots operate at high speeds and undertake maneuvers that subject them to high "g" (gravitational) forces. In a vertical climb, the increased g forces (called positive "g" forces because they push down on the body) tend to force blood out of the circle of Willis supplying arterial blood to the brain. The loss of oxygenated blood to the brain eventually causes pilots to lose their field of peripheral vision. Higher forces cause "blackouts" or temporary periods of unconsciousness. Pilots can use special abdominal exercises and "g" suits (essentially adjustable air bladders that can constrict the legs and abdomen) to help maintain blood in the upper half of the body when subjected to positive "g" forces.

In a dive, a pilot experiences increased upward "g" forces (termed negative "g" forces) that force blood into the arterial circle of Willis and cerebral tissue. The pilot tends to experience a red out. Increased arterial pressures in the brain can lead to stroke.

Although pilots have the equipment and physical stamina to sustain many positive "g" forces (routinely as high as five to nine times the normal force of gravity) pilots experience red out at about 2–3 negative "g's." For this reason, maneuvers such as loop, rolls, and turns are designed to minimize pilot exposure to negative "g" forces.

--Earth Science Study Guide--