Physiologic mechanisms effecting circulatory and body fluid losses in weightlessness as shown by mathematical...

The mechanisms causing large body water losses in weightlessness are not clear. It has long been considered that a central volume expansion drives the physiologic adaptation to a reduced total blood volume, with normal blood composition eventually regained. However, inflight venous pressure measures suggest that central volume expansion in weightlessness may be very transient, or that considerable cardiovascular adaptation to fluid shifts occurs on the ground while astronauts wait in the semi-supine pre-launch position. If a central volume stimulus does not persist, other mechanisms must drive the adaptation of circulation to a reduced blood volume and account for body fluid losses. Recent results from the SLS-1 mission suggest that body fluid volumes do not simply decline to new equilibria but that they decrease to a low point, then undergo some recovery. Similar "under-shoots" of body fluid volumes have also been shown in computer simulations, providing confidence in the validity of the model. The purpose of this study was to examine the mechanisms which could explain the loss of body fluids in weightlessness and how a cardiovascular preadaptation countermeasure we previously tested ameliorated body fluid losses. It is assumed that the physiology of head down tilt (HDT) provides a reasonably accurate analog of weightless exposure.