normal tendency to accumulate in the lower parts of the body. The headward shift from the lower extremities represents 1.5 to 2 litres of fluids and is responsible for the appearance of the astronauts in space, typically with a puffy face and skinny legs. For the receptors located in the upper body, this redistribution is an apparent increase in pressure and volume. It triggers a humoral response to increase the excretion of water and electrolytes and restore an apparently ‘normal' balance [3, 4]. Studies performed during and after Soyuz and Salyut flights, as well as on Gemini- 7 and Skylab have shown that the electrolytes' excretion reaches its peak by the end of the first month, with no further elevation. For the rest of the flight, the astronauts will live with a stable but lower total volume of fluids. This difference of 1.5 to 2 litres is dramatic, since under baseline conditions the body fluids are a tightly regulated system where a variation of 500 ml triggers compensation mechanisms. The major ill-effect of dehydration is the diminution of cardiac output which translates clinically to intolerance to orthostatism, shortness of breath and a reduced ability to perform exercise. In extreme cases of dehydration, patients die of cardiac shock. For the astronauts, a particularly dangerous situation may occur during reentry when the crew members encounter abnormally high G loads [1]. The most recent explanations for fluid excretion involve the newly described atrial natriuretic factor, a substance released by the right atrium and able to induce a reduction in blood pressure and an increase in diuresis [3, 4], This early loss of fluid and electrolytes has not always been observed: astronauts may voluntarily reduce their water intake before launch, and subsequently maintain or increase this negative balance because of nausea and motion sickness. In that case, the fluid loss has, in a way, occurred beforehand [4], Post-flight measurements have shown a severe dehydration of 9-10% of the body weight, which partly explains the post-flight intolerance to standing. The effects of the fluid redistribution and of the lack of muscular exercise on the cardio-vascular system still need to be studied more extensively. The cardiac studies performed during the Salyut-7 flight have used a specially designed bidimensional echography system. It allowed study of the volume of the cardiac cavities, the cardiac output and the thickness of the myocardial muscle. This test has shown an increase in the cardiac rate during the entire flight duration and the first post flight week. There is an elevation of cardiac output, resulting from the elevation of both cardiac rate and ejection fraction: a 40% increase was measured at the fourth day of flight. Both phenomena probably result from the increased input in the right cavities, due to the upward fluid redistribution. The risk of prolonged tachycardia is heart failure; the cardiac deconditioning observed after landing is a minor form of it, which probably has no long-term consequence thanks to the good physical training of the astronauts. However, there is a confounding of the effects of cardiovascular deconditioning from a state of high athletic training with those of primary adaptation to microgravity. Changes in cardiovascular status occur more rapidly and dramatically in athletically trained persons like the astronauts than in sedentary persons: therefore the acute alterations in the astronauts' cardiovascular system may represent mostly the shift from a high level of physical training to a lesser one [3, 6]. The arterial and venous systems have also been studied [3, 9]. The arteries, which assure the circulation of blood from the left cardiac cavities to the periphery do not seem to undergo much change. There do not seem to be any noticeable variations of the flow in the carotid arteries (which are responsible for the oxygen supply to the brain) during flight. The changes observed in the arterial circulation of the lower body
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