In this thesis we discuss the changes in human body , gravity, and microgravity. Also we discuss the causes of the multiple systems in the body and the rehabilitation of the astronauts after coming back from the space . As well as how can we treat those astronauts in the most efficient way. Also we focus on how to reduce both long and short term side effects Physiological Redistribution of Fluids and Muscle Mass After Microgravity Exposure.