The blood and water are fluids within the body. These fluids move from the lowest of the body to the highest. The brain thinks that there are several other fluids. It will tell the body to create less. Once the astronauts return to Earth, they do not have enough fluids in their systems. It takes their bodies some days to create a lot of blood and water. The astronauts got to rest so that their bodies have time to create new blood and water. If they do not, they will feel terribly weak. They may even faint!