Gravity Hurts

Strange things can happen to the human body when people venture into space - and the familiar pull of gravity vanishes.

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.

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 (millimetres 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 equalises 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 litre of fluid each, thin out.

But that shift in blood pressure also sends a signal. Our bodies expect a blood pressure gradient. Higher blood pressure in the head raises an alarm: The body has too much blood! Within two to three days of weightlessness, astronauts can lose as much as 22 percent of their blood volume as a result of that errant message. This change affects the heart, too. "If you have less blood," explains Dr. Victor Schneider, research medical officer for NASA headquarters, "then your heart doesn't need to pump as hard. It's going to atrophy."

The question is, do such losses matter?

Perhaps not if you plan to stay in space forever. But eventually astronauts return to Earth - and the human body has to readjust to the relentless pull of gravity. Most space adaptations appear to be reversible, but the rebuilding process is not necessarily an easy one.