Regulation and distribution of body fluid during a 6-day head-down tilt study in a randomized cross-over design

Head down tilt (-6 degrees HDT) examinations are commonly used simulation models for various microgravity induced changes in body functions. Body fluid distribution (by means of dye dilution and two independent multifrequency impedance techniques), water- and sodium-handling, and the plasma/serum concentrations of fluid balance related hormones have been determined in a randomized, controlled, cross-over study in 8 healthy test subjects. The comparison of responses to HDT and an upright control position with respective experiences from space shows some similarities but also various discrepancies between the terrestrial simulation and real microgravity.