is no evidence that the rate of bone calcium loss diminishes or terminates during weightless exposure lasting as long as 3 months. Much research on the problem of space flight osteoporosis remains to be done. At present the countermeasures used include the provision of adequate bone replacement salts in the diet along with muscular exercise in an attempt to simulate the forces on bones and muscles normally provided by gravity. Exercise appears to be beneficial in moderating the loss of muscle strength and mass. NEUROSENSORY DISTURBANCE: For reasons as yet unexplained, the sensory systems affected by pressure, acceleration, position, and visual patterning appear to lose the fine tuning of their normal integrated interplay. Sensory confusion occurs below the level of consciousness and the conflict of incoming information may give way to dysfunction with resulting space motion sickness. The disturbance appears early in the exposure to null-gravity. Stomach awareness, malaise, and vomiting can occur. The symptoms appear to wane as exposure continues, and in the Skylab experience, the problems of orientation and "space motion sickness" disappeared after a few days. The most effective countermeasures presently available are drugs which dull the acuteness of the symptoms without impairing performance. Much more research must be done to understand the processes involved in this malady and to develop more effective countermeasures. OTHER EFFECTS: There have been measurable changes in the total mass of circulating blood cells, in several parameters of body fluid composition and regulation, and in the endocrine and immune systems. These latter changes are presently considered to be adaptive and not likely to interfere with the individual's ability to live and work productively in space. An interesting relation can be drawn between the rate of onset of effects during exposure to weightlessness and the rate of recovery upon return to Earth. The rapid onset of cardiovascular and neurological disturbances is mirrored in rapid recovery. Disturbances in bone and muscle appear less rapidly, reflecting the relative sluggishness of these biochemical changes at the cellular level. However, the observed changes all appear to operate within the predictable range of physiological performance. All changes observed have been shown to be reversible. Although much more research has yet to be done on the long-term effects of space flight on man, there is no compelling evidence to discourage continued lengthening of the nul1- gravity exposures or increasing the complexity of tasks to be accomplished in space.
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