are very minor. The veins, on the other hand, have to assure the return of the blood from the extremities to the right cardiac cavities. Venous return from the lower parts of the body is normally antagonized by hydrostatic pressure. Under normal conditions, the elasticity of the veinous wall and of the surrounding muscles compensate for the hydrostatic gradient. Major changes occur in the venous circulation, with an overload of the right cardiac cavities and an elevation of the central venous pressure. The elevation of the venous pressure in the upper parts of the body is responsible for the sensation of fullness of the head. The veins of the lower extremities seem to partly lose their elasticity: in the absence of adverse hydrostatic gradient, the veins' wall no longer have to support the blood flow towards the right cavities [9]. The cardio-vascular changes measured in the post flight period are a decrease in exercise capacity, an intolerance to standing position, a loss of body fluids and body mass. A decrease in heart size has been found, as well as suggestive evidence of a decrease in left ventricular mass in the most recent studies, and still needs to be confirmed by further study [1,3]. Countermeasures to prevent cardiac deconditioning have been proposed. They include physical measures such as performing strenuous exercise, or wearing special garments in which a pump maintains a negative pressure around the lower parts of the body and restores normal blood flow and blood pressure [5, 6]. During the last days of a flight, the astronauts consume substantial amounts of salted solutions in order to create fluid retention and reduce intolerance to orthostatism after landing [1, 6]. It is interesting to speculate whether the cardiovascular changes that accompany the headward shift of fluid represent an appropriate adaptation to microgravity, and therefore should not be counteracted in flights of long duration [6]. 1.3 Changes in Phospho-calcic Metabolism and in the Musculoskeletal System The musculoskeletal effects of weightlessness appear after a few days of exposure. In a weightless environment, no muscular effect is needed and no pressure is imposed on the skeleton when moving in space. As a result, the astronauts undergo a progressive loss of muscular and skeletal mass [11, 12, 13, 14, 15]. To understand the mechanism of this loss, it is necessary to discuss the structure and physiology of the skeleton. The skeleton is formed of bones, some of which are weight-bearing, e.g. in normal circumstances they support the weight of the body. At the microscopic level, bones are formed of phospho-calic crystals that are deposited on a protein matrix. Bones undergo an ongoing remodeling, which removes ‘ancient' phospho-calcic crystals and replaces them with new ones, and which renews the protein network on which these crystals are deposited. Two systems regulate the remodeling activity and maintain a constant bone mass: • a hormonal system which involves vitamin D, parathyroid hormone and calcitonin as the major actors; • gravity, probably through cellular receptors sensitive to pressure, that provide the feedback to ensure the adaptation of the skeleton to the body's weight. Three ways to determine, at a given point in time, the status of the bones, are: the integrated measure of urinary and fecal calcium, which gives a good approximation of the overall bone loss; the urinary hydroxyproline, which reflects the turnover of the protein matrix of the bones; and the mineral density of bones measured by photon
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