You know the feeling of blood rushing to your head when youÕre hanging upside down? That happens because the liquids in your body are responding to gravity. About two-thirds of the human body is water. Just like water sloshing in a bottle or balloon, the liquids in our body also move around.

Normally, gravity helps a personÕs cardiovascular system move blood through the body, particularly to our legs. In the microgravity of space, body fluids are shifted upward to cavities in a personÕs head and torso. When in orbit, astronauts get puffy faces, and they feel as if they have a head cold because their sinuses are ÒstuffedÓ with liquid. Their legs shrink in size. This effect is called Òbird legs.Ó

Because of all the extra liquid pressure in their upper bodies, astronauts donÕt get thirsty as often as we do on Earth. They drink less and go to the bathroom more. Their bodies lose liquid, which means that their hearts have less fluid to pump through the body. As a result, their hearts donÕt have to work as hard. If the astronauts donÕt get regular cardiovascular exercise, their hearts will become weak.

An astronautÕs muscles and bones donÕt have to work as hard, either, since they arenÕt pushing against gravity. When they arenÕt being used, a personÕs muscles and bones begin to shrink. The bones lose calcium and density while the muscles start to waste away. This isnÕt a big problem on short trips into space. However, on longer flights astronauts must exercise for at least 30 minutes a day to make sure their bones and muscles donÕt deteriorate too much.