Astronauts Lose Up to 22% of Their Blood In Space

Gravity is not just a force, it's also a signal — a signal that tells the body how to act. 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. 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. 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, saying the body has too much blood. Within two to three days of weightlessness, astronauts can lose as much as 22% of their blood volume as a result of that errant message. This change affects the heart, too. If you have less blood, your heart doesn't need to pump as hard, so it's going to atrophy. 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. Astronauts get thirsty when they come back because their body says, "You don't have enough blood in your blood vessels," and that causes the messengers to say, "drink more." Also, the body doesn’t urinate as much. Normally, within a month or so, everything gets back to normal.