Orthostasis and transcapillary fluid shifts.
Postural blood volume changes aggravate the regulation of arterial blood pressure and perfusion vis-a-vis the hydrostatic effects of orthostasis, ie, blood pooling below the hydrostatic indifferent points and reduced cardiac preload.  Corresponding problems surface with extended passive standing, particularly in highly trained, dehydrated, or otherwise compromised subjects, or after long-lasting immobilization, as with space flight.
Hinghofer-Szalkay
H G
HG
Institute for Adaptive and Spaceflight Physiology, Austrian Society for Aerospace Medicine (ASM), Graz, Austria.
D001798:Blood Proteins / Q000378:metabolism; D001810:Blood Volume / Q000502:physiology; D002196:Capillaries / Q000502:physiology; D018495:Fluid Shifts / Q000502:physiology*; D006801:Humans; D007024:Hypotension, Orthostatic / Q000503:physiopathology*; D008833:Microcirculation / Q000502:physiology; D010953:Plasma Volume / Q000502:physiology*; D011187:Posture / Q000502:physiology*
D001798:Blood Proteins