Gravity pulls downward which supplies blood to the lower limbs. Microgravity pushes the blood to the upper portions of the body. The effects of microgravity isn't a big deal until astronauts travel back to Earth. Gravity pulls back the blood pressure down, but all the blood will transfer down to the astronauts' legs. This will cause other parts of the body to not have the blood they need to support the functioning of the astronauts' upper portion of their bodies.