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Aerospace Medicine is the branch of preventive medicine that is concerned with the physiological and psychological stresses on the human body in flight. The study of effects within the earth's atmosphere is also called aviation medicine; beyond this atmosphere the study of effects is also called space medicine.
Specialists in aviation medicine study the reactions of human beings to the stresses of air travel. They are concerned with the proper screening of candidates for flight training, the maintenance of maximum efficiency among aircrews, and with clinically oriented research into the effects of flight on the body. They also cooperate actively with aeronautical engineers in the development of safe aircraft.
Aviation medicine is rooted in the early 18th-century physiological studies of balloonists, some of whom were also physicians. In 1784, a year after the first balloon flight by the French physicist Jean Pilâtre de Rozier, a Boston physician, John Jeffries, made the first study of upper-air composition from a balloon. The first comprehensive studies of health effects in air flight were made by the French physician Paul Bert, who published his research on the effects of altered air pressure and composition on humans in 1878 under the title La pression barometrique. In 1894 the Viennese physiologist Herman Von Schrötter designed an oxygen mask with which the meteorologist Artur Berson set an altitude record of 9,150 m. With the advent of aircraft, the first standards for military pilots were established in 1912. Significant work in this area was directed by the physician Theodore Lyster, an American pioneer in aviation medicine. Technical advances included the first pressurised suit, designed and worn by the American aviator Wiley Post in 1934, and the first antigravity suit, designed by W. R. Franks in Great Britain in 1942. In an effort to help design better restraint systems for military jet aircraft, the United States flight surgeon John Stapp conducted a series of tests on a rocket-powered sledge, culminating on December 10, 1954, when Colonel Stapp underwent deceleration from a velocity of 286 m/sec in 1.4 sec.
Aviation medicine is concerned primarily with the effects on human beings of high speed and high altitude, and involves the study of such factors as acceleration and deceleration, atmospheric pressure, and decompression. In civil-aviation medicine, an additional concern is passenger airsickness.
In itself, high speed does not produce harmful symptoms. What can be dangerous are high acceleration or deceleration forces; these are expressed as multiples of the earth's gravity at sea level, or gs. In pulling out of a dive, for example, a pilot may be subjected to an inertial force as high as 9 g. If a force of 4 to 6 g is sustained for more than a few seconds, the resulting symptoms range from visual impairment to total blackout. Protection is provided by a specially designed outfit, called an anti-g suit, which supplies pressure to the abdomen and legs, thus counteracting the tendency for blood to accumulate in those areas. Proper support of the head is essential during extreme deceleration in order to avoid swelling of the sinuses and severe headaches. While facing backwards in a seated position, properly supported human test subjects have been able to tolerate a deceleration force of 50 g without severe injury.
A critical consideration in aircraft travel is the continuing physiological requirement for oxygen. The only oxygen stored by the body is that in the bloodstream. Although muscles can function temporarily without oxygen, the buildup of toxic products soon limits activity. Brain and eye tissues are the most sensitive to oxygen deficiency.
The earth's atmosphere, which contains 21 per cent of oxygen by volume, is under a normal sea-level pressure of 760 tons. The barometric pressure up to about 4,575 m is sufficient to sustain human life. Above this altitude the air must be artificially pressurised to meet the respiratory needs of human beings.
High-altitude military aircraft are provided with oxygen equipment, and military personnel are required to use it at all times when participating in flight above 3,050 m. Military craft that can fly above 10,675 m usually also have cockpits under pressure. Positive-pressure breathing equipment is also used in all other aircraft capable of flight above 10,675 m. Full or partial pressure suits with additional oxygen equipment are required in military aircraft capable of flight above 16,775 m.
Commercial carriers provide oxygen systems and pressurised cabins in accordance with civil air regulations. An airliner flying at 6,710 m, for example, must maintain a "cabin altitude" of 1,830 m.
This physiological condition results from a state of acute oxygen deficiency, known medically as hypoxia, at high altitudes. Ascending from the lower atmosphere, called the troposphere, the atmosphere is thin enough at 3,900 m to produce symptoms of hypoxia, or oxygen hunger. At the lower limit of the stratosphere, about 10,675 m, normal inhalation of pure oxygen no longer maintains an adequate saturation of oxygen in the blood.
Hypoxia produces a variety of reactions in the body. Mild intoxication and stimulation of the nervous system are followed by progressive loss of attention and judgement until unconsciousness occurs. Respiration and pulse rate increase, and the systemic oxygen content is reduced. Prolonged lack of oxygen may cause damage to the brain.
Because of the reduction of barometric pressure at altitudes above 9,150 m, the body tissues can no longer retain atmospheric nitrogen in solution. As a result, liberated gas bubbles, as well as ruptured fat cells, may enter the circulatory system and form obstructions, or emboli, in the blood vessels. This condition, known medically as aeroembolism and popularly as the bends, leads to confusion, paralysis, or neurocirculatory collapse. The most characteristic symptoms of the bends are pain in the large joints resulting from pressure of the gas on tendons and nerves, together with spasm of the blood vessels. Preflight inhalation of pure oxygen to eliminate nitrogen from the system has proved valuable as a preventive measure. Rapid decompression, resulting from accidental failure at high altitudes of the pressure within the cabin, causes major damage to the heart and other organs by the ram effect of gases formed in the body cavities.
Airsickness is produced by a disturbance of the labyrinthine mechanism of the inner ear, although psychogenic factors such as apprehension can also play a part. Motion sickness can be prevented by taking drugs containing scopolamine or some antihistamines before flying.
As transport planes became faster, pilots and passengers were able to travel across many time zones in less than a day. The resulting disturbance in the body's "biological clock" or circadian ("about a day") rhythm can produce disorientation and reduce concentration and efficiency. This condition is popularly known as jet lag. While troublesome to passengers, the problem is more acute for pilots, who may have to fly another assignment in a short time. Concern has been expressed about the possible effect of this situation on air safety, although no air accident has yet been clearly identified as jet-lag-induced.
Specialists in space medicine - a discipline also known as bioastronautics - study the human factors involved in flight outside the atmosphere. Most of the potential dangers in space travel (such as acceleration and deceleration forces, the need for an artificial atmosphere, and noise and vibration) are similar to those encountered in atmospheric flight, and can be compensated for in similar ways. Space-medicine scientists, however, must consider two additional problems-weightlessness and the increased radiation outside the atmosphere.
The first information about human performance during space travel was gathered in Germany in the 1940s under the direction of Hubertus Strughold. Both the United States and the former Union of Soviet Socialist Republics (USSR) conducted rocket tests with animals from 1948. In 1957 the USSR put a dog into the earth's orbit, and the United States used a monkey for tests in 1958. The tests suggested that few biological dangers existed in space flight. This was confirmed when human space flight began on April 12, 1961, with the launching of the Soviet cosmonaut Yury Gagarin into orbit.
The United States followed with the Mercury-Redstone suborbital flights, and then the orbital Mercury and Gemini flights, the Apollo moon landings, the experimental orbital vehicle Skylab, and Space Shuttle flights. Then, in the 1980s, when Soviet cosmonauts began setting records for time spent in the gravity-free or "microgravity" environment, the effects of long-term weightlessness began to be viewed as a serious medical problem.
Few serious biological effects were noted during the early years of space flight. Even the 21-day quarantine of astronauts returning from the Apollo moon mission was subsequently abandoned, because no infectious agents were identified. The body functions that were monitored (often with specially designed miniature instruments) included heart rate, pulse, body temperature, blood pressure, respiration, speech and mental alertness, and brain waves. Few changes occurred. Changes in hormones and in concentrations of salts in the blood did take place, but these were not detrimental. Eating in weightlessness was accomplished by packaging food in containers that could be squeezed directly into the mouth, and special systems were designed for collection of fluid and solid wastes. The lack of a natural time cycle in space was compensated for by keeping the astronauts' schedules synchronised with earth time.
Psychological changes were anticipated because of the close confinement of a few individuals in a small space with limited activity. Few psychological problems were noted, however, perhaps because the astronauts were chosen for their emotional stability and high motivation, and because they were assigned enough tasks to keep them almost constantly busy. Irradiation was also found to have little effect. Short orbital flights produced exposures about equal to one medical X-ray - about the same as suborbital flight. The crew on the longer Skylab flight sustained many times this dose. Space flights are planned to avoid periods when solar flares are expected to occur, as these can emit dangerous levels of gamma radiation.
However, although it was assumed that gravity is necessary for normal growth, the magnitude of physiological changes induced by extended periods in a microgravity environment came as something of a surprise. Serious medical problems, including loss of bone matter and muscle strength, were observed to result from long-term weightlessness, as during the 237-day mission of three cosmonauts aboard a Salyut space station in 1984. Moreover, atrophy of certain muscles, particularly those of the heart, was seen to be especially dangerous because of its effect on the functioning of the entire cardiovascular system. The blood itself was found to be affected, with a measurable decrease in the number of oxygen-carrying red blood cells.
On a seven-day Challenger Space Shuttle mission in 1985, these effects were studied in an experiment using 24 rats and 2 monkeys. Post-flight examination revealed not only the expected loss of bone and muscle strength but a decrease in release of growth hormone as well.
These findings are taken into consideration now whenever plans are made for crewed space flight. The busy work schedules of astronauts in space are designed to include regular exercise periods, thereby maintaining muscle tone. And plans for the operation of permanently crewed space stations now include provisions for changing crews on a regular basis, so as not to subject astronauts to weightlessness for indefinite periods of time.
Now try the exercises: Exercise a, Exercise b, Exercise c, Exercise d.
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