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Gravity Hurts (so Good)
Strange things can happen to the human body when people venture into space -- and the familiar pull of gravity vanishes.
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August 2, 2001
Gravity hurts: you can feel it hoisting a loaded backpack or pushing a bike up a hill. But lack of gravity hurts, too: when astronauts return from long-term stints in space, they sometimes need to be carried away in stretchers.
Gravity is not just a force, it's also a signal -- a signal that tells the body how to act. For one thing, it tells muscles and bones how strong they must be. In zero-G, muscles atrophy quickly, because the body perceives it does not need them. The muscles used to fight gravity -- like those in the calves and spine, which maintain posture -- can lose around 20 per cent of their mass if you don't use them. Muscle mass can vanish at a rate as high as 5% a week. 
Astronaut Bill Shepherd prepares for a long stay on the International Space Station with muscle-building exercises on Earth.
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For bones, the loss can be even more extreme. Bones in space atrophy at a rate of about 1% a month, and models suggest that the total loss could reach 40 to 60 per cent.
Blood feels gravity, too. On Earth, blood pools in the feet. When people stand, the blood pressure in their feet can be high -- about 200 mmHg (millimeters of mercury). In the brain, though, it's only 60 to 80 mmHg. In space, where the familiar pull of gravity is missing, the head-to-toe gradient vanishes. Blood pressure equalizes and becomes about 100 mmHg throughout the body. That's why astronauts can look odd: their faces, filled with fluid, puff up, and their legs, which can lose about a liter of fluid each, thin out.
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Cheers,
Dan
Gravity Hurts (so Good) . . .