Orthostatic stress is necessary to maintain the dynamic range of cardiovascular control in space

In the upright position, gravity fills the low pressure systems of human ci rculation with blood and interstitial fluid in the sections below the diaph ragm. Without gravity one pressure component in the vessels disappears and the relationship between hydrostatic pressure and oncotic pressure, which r egulates fluid passage across the capillary endothelium in the terminal vas cular bed, shifts constantly. The visible consequences of this are a puffy face and "bird" legs. The plasma volume shrinks in space and the range of c ardiovascular control is reduced. When they stand up for the first time aft er landing, 30-50% of astronauts suffer from orthostatic intolerance. It re mains unclear whether microgravity impairs cardiovascular reflexes, or whet her it is the altered volume status that causes the cardiovascular instabil ity following space flight. Lower body negative pressure was used in severa l space missions to stimulate the cardiovascular reflexes before, during an d after a space flight. The results show that cardiovascular reflexes are m aintained in microgravity. However, the astronauts' volume status changed i n space, towards a volume-retracted state, as measurements of fluid-regulat ing hormones have shown. It can be hypothesized that the control of circula tion and body fluid homeostasis in humans is adapted to their upright postu re in the Earth's gravitational field. Autonomic control regulates fluid di stribution to maintain the blood pressure in that posture, which most of us have to cope with for two-thirds of the day. A determined amount of inters titial volume is necessary to maintain the dynamic range of cardiovascular control in the upright posture; otherwise orthostatic intolerance may occur more often.