flights on. Special suits are designed to create a load of about 50% of body weight when in normal earthbound posture; the astronauts are required to perform strenuous exercise for 80 minutes daily; they undergo static compression, for four hours daily at a force of 80% body weight as well as intermittent compression. To stimulate the gravity-sensitive receptors, they make violent impacts to the bottom of the heel and try various periods of standing and ambulation: the most effective countermeasure so far seems to be ambulation for a minimum of four hours per day [5, 13, 6]. During the 19-day Cosmos 936 flight, a population of rats centrifuged to maintain artificially a 1G gravity did not experience any bone resorption; in the group of young growing rats, however, the decrease in bone formation was still present although not as important as in the non-centrifuged group. The short length of the flight and the specific characters of rats' physiology should make one cautious in extrapolating these results to man [14, 15]. According to the published results of different studies, these physical measures seem to have proven more efficient in reducing bone resorption during Soviet than during American flights, for reasons not yet explained. It may be that the Soviet crews had actually a more strenuous exercise programme which they followed rigorously [3]. Pharmaceutical measures have attempted to reduce the decrease in total body calcium by increasing calcium intake and stimulating the hormonal system responsible for calcium retention [3]. The astronauts had a diet with high nutrient intake and supplements of calcium and phosphate, and a hormone, known to reduce bone resorption, calcitonin. This diet had been tested during bedrest simulation of weightlessness, and had not prevented the calcium balance from becoming negative, nor did it prove very efficient during actual flight. A new drug, clodronate, which appears to decrease bone resorption in bed rest simulation may be the most promising biochemical factor thus far. Since there might be long-term adverse effects on the bones' structure, preventive measures have also been proposed: it may be prudent to leave an appropriate interval between two missions when the same astronauts have to participate in both [5, 13]. The muscular system would also be expected to undergo changes, since the efforts required to move in macrogravity are minimal, and the muscles would be expected to atrophy as they do when a member is immobilized in a cast. Observations relative to the muscular system have been conducted during the Skylab flights as well as in the Soyuz-Salyut missions [11]. After a distinct muscular loss during the first month, the balance was held close to zero by a protein intake of 120g/day and vigorous muscular exercise, the equivalent of four hours of ambulation and 10 km of bicycle every day. The muscles however were not fully protected, the average decrease in leg volume was about 10% and the decrease in muscular strength from 10 to 25%. In the post flight period, exercise seems to result in satisfactory muscular recuperation; there is no evidence of any long-term muscular deficiency. 1.4 Alteration of the Immune Response The immune system is responsible, among other functions, for the recognition and elimination of elements foreign to our body, be they bacteria, viruses or degenerative cells. The immune system involves different types of cells, and the cells that seem to play the major role are the lymphocytes. Although this is a field of ongoing discoveries and changes, it is useful to describe the immune response as twofold. The T- lymphocytes are responsible for cellular immunity, the recognition and destruction of foreign elements. The B-lymphocytes are responsible for humoral immunity, the
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