Body volume changes during simulated weightlessness: an overview

It has been hypothesized that many of the adaptive processes that occur in man during exposure to space involve the rapid headward shift of tissue fluids, blood, and other fluids during the first few hours of spaceflight. A total of 53 men and women between the ages of 22 and 55 years were tested during four bed rest studies lasting between 4 h and 7 d to define the segmental volume changes that took place during simulated weightlessness. Impedance plethysmography can be used to measure baseline resistance (Ro) changes of the calf, thigh, total leg, pelvic, torso, and arm segments before, during, and after each bed rest exposure. Records of Ro were analyzed to determine the fluid volume and volume change of each segment during each test sequence.