Aviation physiology deals with the physiological challenges encountered by pilots and passengers when subjected to the environment and stresses of flight.
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.
Military pilots undergo a series of exercises in high altitude simulating 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 pilots undertake flight above 10,000 feet above sea level 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 gastrointestinal 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 losses 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. Spatial disorientation chambers allows 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.
See also Aerodynamics; Atmospheric composition and structure