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.