The microgravity environment causes significant changes to cardiovascular haemodynamics. Due to the absence of gravity, a redistribution of body fluids occurs and can result in significant short-term effects as well as long-term adaptations. On return to the Earth, astronauts with prolonged microgravity exposure, have significant orthostatic intolerance due to the adaptations that occur. In this project, a lumped parameter computer model of the cardiovascular system will be used to explore the effect of microgravity on the cardiovascular system. In particular, the rapid redistribution of body fluids which occurs during gravity transitions will studied for models of the cardiovascular system that represent short duration and long duration spaceflight.