Forces produced by high acceleration overdistend one part and compress another part of the lungs. Blood flow diminishes in some parts of the lungs and increases in others. Fluid leaks from the blood into the tissues and into the air sacs in parts of the lungs. These effects cause difficulty in breathing, low arterial oxygen saturation, and impaired consciousness during high sustained acceleration and, to a lesser extent, after its cessation. They must be considered when selecting the best gas to be breathed, since a high partial pressure of oxygen is favorable for consciousness, but a low inert-gas concentration during acceleration is unfavorable for rapid lung recovery afterward.
PHYSIOLOGICAL PROBLEMS
A study of the manned space flights and laboratory observations to date suggests that during long periods of weightlessness, some physiological difficulties may arise which may produce serious effects on human performance. Although recent experience gives no grounds for expecting insuperable difficulties, neither the quantity nor quality of the available observations permits the conclusion that long-term exposure to weightlessness will not have serious consequences. The critical role to be played by the astronaut demands that every effort be made to identify in advance those phenomena which may affect performance, and to study their qualitative and quantitative relationships so that proper precautions can be taken.
Lawton ([ref.197]), in reviewing the literature on prolonged weightlessness, found few instances in which physiological function was truly gravity dependent. He stated that the physiological systems likely to be most affected by weightlessness were the musculoskeletal system, the cardiovascular system, and the equilibrium senses. Subsequent experience proved this to be the case. McCally and Lawton ([ref.198]) analyzed the data from experiments since 1961 and concluded that much more basic laboratory work is necessary. Studies using immobilization, immersion, and cabin-confinement techniques were recommended approaches toward simulating weightlessness.
Much of the difficulty in obtaining precise information of anticipated problems arises from a lack of knowledge of normal mammalian physiology. Many of these deficiencies can be remedied in the laboratory. In space-flight development, however, two distinct investigational approaches can be adopted. The first of these may be characterized as empirical and incremental; that is, the capabilities of the astronaut are explored in successive flights involving relatively modest increases in difficulty or severity of the environmental conditions. In this way it is hoped to ascertain the human limitations without running too great a risk. The second approach can be described as fundamental: determining by a series of controlled experiments the effects of exposure to space-flight conditions upon comparative mammalian physiology, with emphasis on man. A fundamental understanding of the observed effects would be sought so that predictions for new situations and possible ways to control them could be made with confidence.
It is not possible now to predict for flights of 30 days or more—
- The effects of sudden reimposition of reentry accelerations and terrestrial gravity
- Changes in body fluid distribution and composition
- The effects of violent physical effort on respiratory and cardiovascular systems in prolonged weightlessness
- Central nervous system functions, especially coordination, skilled motor performance, judgment, and sleep-wakefulness cycles
NASA has emphasized that planning for manned space programs involves a systematic extension from physiological observations in animals to man, and finally the establishment of man as part of the man-vehicle system design. Moreover, these studies require the evaluation of central nervous, cardiovascular, respiratory, gastrointestinal, and other systems as a matrix in mutual interdependence. There is particular interest in the effects of weightlessness on flights exceeding 30 days.
Mammalian flights of about 30 days also merit attention, including the development of the life-support systems which must precede such a program. Development of facilities for biological experiments may well be an important requirement for studies in anticipation of manned flights of longer duration than Apollo. Unless the biological satellite programs of the type mentioned above are successful in providing the necessary data, a manned orbiting laboratory may also be important in studies of shorter range.