elaboration of immunoglobulins that will prevent reinfestation by the same organism. In vitro experiments have shown a dramatic decrease in the proliferation rate of T- lymphocytes, as well as a failure to be activated. The activation of lymphocytes exposed to microgravity is less than 3% of the activation of ground controls. There have not been reports of alterations in B-lymphocyte functions [1,6]. The problems posed by the deactivation of lymphocytes are the risk of severe infections in space and during the immediate post flight period, and the increased risk of developing malignancies, especially in the presence of such facilitating factors as ionizing radiation. 1.5 Changes in Number and Structure of Red Blood Cells Anaemia (reduction of the concentration of haemoglobin) was first noticed in the Gemini missions and confirmed in later missions. It appears around the fourth day after launch and reaches its maximum 30 to 60 days later. The anaemia is very likely to contribute to cardiovascular deconditioning and to the intolerance to a standing position. The cause of this anaemia is still unclear, but it probably involves several mechanisms. The headward redistribution of fluids could be interpreted by receptors in the upper part of the body as an increase in blood volume: the feedback would reduce the production of red blood cells. There could also be increased destruction of red cells facilitated by the changes in their structure. A third hypothesis is a direct action of weightlessness on the organ that produces the precursor of the red cells, the bone marrow. After return to earth, the concentration of haemoglobin comes back to its normal level in two weeks [6], The aggregation of red cells has been studied during haemorheological experiments on the space shuttle ‘Discovery'. The purpose of the study was to determine whether microgravity altered the shape and the aggregation of red cells. It used blood samples taken from donors on earth and studied in vitro. The experiment established that red cells do not change shape under zero gravity and that aggregates can be formed normally. It was a response to a prior study reporting changes in the shape of red cells in astronauts. It might be that, in vivo, other metabolic alterations such as the changes in calcium metabolism are responsible for the modifications of red blood cells [17, 18]. 1.6 Gastro-intestinal Tract The gastro-intestinal tract in space undergoes few modifications, except for the stomach [4]. In microgravity, the gas bubble of the stomach is not necessarily in front of the oesophagal hiatus: when there is an elevation of the intra-stomachal pressure, which would normally result in eructation, the gas exit does not happen, and instead the astronauts experience an acid gastro-oesophagal reflux. An attractive countermeasure to the adverse effect of weightlessness would be the creation of artificial gravity, or the use of partial gravity. It would be necessary to determine the key parameters of the artificial gravity or what fraction of lg is necessary to prevent the occurrence of adverse effects [4]. A suggested method for creating artificial gravity would be to design a spacecraft in the shape of a bola: two cabins with a tether between them. This would provide adjustable gravity inside the cabins, and the tether could allow passage from one to another. However disturbing, the adverse effects of weightlessness are mostly functional and do not put the astronauts lives at risk. It seems that adaptation to weightlessness
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