Computer systems analysis of the cardiovascular mechanisms of reentry orthostasis in astronauts.

Reentry orthostasis secondary to a prolonged exposure to microgravity is a common problem among astronauts. However, the physiologic mechanisms are poorly understood due to the many control systems involved. In this study an advanced computer model of cardiovascular functioning was employed in a systems analysis approach to clarify the relative importance of some of the adaptive physiologic processes engaged when humans return from space. After simulation of the conditions of zero gravity for one month, the model predicted that the change in capacitance of the lower extremity veins resulting from a loss of external fluid forces in the dehydrated extracellular compartment was the dominant mechanism associated with reentry orthostasis. This condition appears accentuated in women due to their inherent lower center of gravity and proportionately larger mass in the lower extremities.