The first thing that changes is where your blood flows
On Earth, your body is constantly fighting blood flowing downward. Pressure sensors in your neck reason: if there's this much up top, then blood volume is normal. The whole system is calibrated for a vertical column of fluid.
Take away gravity - and there is no column. About 2 liters of fluid shift from your legs to your chest and head. Your face swells, your legs get noticeably thinner - astronauts themselves call this "puffy face, bird legs." Your head feels like you have a mild cold: congestion and a nasal voice.
So, how are you doing up there?
My face is swollen, like I haven't slept in a week
And my legs got so thin, you can really see it
Will it last long?
They say I'll get used to it. But the stuffy nose won't go away, even though I don't have a cold
Sensors in your chest and neck detect an excess and conclude: there is too much fluid. Your kidneys start getting rid of it, and over the first few days plasma volume drops by about a tenth. The swelling goes down, but there is less blood in your body than at the start. You'll pay that price on the way back.
Your bones and muscles are left without work
Your skeleton isn't a finished structure - it's a construction site that never stops. Bone is constantly broken down and rebuilt, and the balance is set by load: where there is pressure and pulling, more gets built. In weightlessness, the pressure stops, and the bones in your hips and lower back lose about 1% of their density per month - faster than older people on Earth lose in a year.
The same goes for muscles: the first to give in are the ones that kept you upright - your calves, back, and neck. Without compression, your spine straightens, making you a few centimeters taller, and this is linked to back pain in the first weeks of flight. That's why astronauts exercise for about 2 hours a day on the station - not for fitness, but to give their bodies at least some "pressure."
- HoursFluid shifts upward: your face swells and your legs get thinner
- DaysYour body decides there is too much blood and gets rid of some plasma
- WeeksThe muscles that held your posture shrink: they have no work to do
- MonthsThe bones in your hips and spine noticeably lose density
Why it's hard to stand after returning to Earth
In weightlessness itself, your body manages. Things get rough when gravity switches back on: there is less blood, the vessels in your legs have forgotten how to constrict, and when you try to stand, pressure in your head drops - your vision goes dark. The muscles that held your posture have weakened, and your inner ear is getting a "down" again that it has unlearned how to trust.
There is also something not fully explained: after long flights, some people's vision changes - the shape of the eye and the optic disc. This is linked to the same fluid in the head, but the exact mechanism is still unknown.
This is what your body feels like in the first hours after landing
Why can your vision go dark after returning to Earth?
Why do bones and muscles start weakening in weightlessness?
Sources
- Based on materials from NASA's Human Research Program and reviews of long-duration spaceflight physiology
Next in the “Body in Space” series
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