So one of our readers asked us this question the other day: Does the weightlessness in space have an affect on blood circulation in astronauts? For example, if you (in relation to the earth) hung upside down would you feel the blood rush to your head like you do on Earth? Does the Earth’s gravitational pull still effect you? Perhaps just less?
The condition of being weightless in orbit is technically called microgravity. (Astronauts in orbit around the earth are still well within the earth’s gravitational field, but they feel weightless because they’re freefalling at the same rate as their vehicle.) Anything in orbit is falling to earth, but is also moving sideways so fast that the earth “curves away” due to its spherical shape.
Without weight to define the direction of “down,” astronauts can’t tell which way is “down” without looking out the window. So they can’t hang in microgravity, upside-down or otherwise, because there is no up or down in space.
The effects of microgravity on blood circulation are somewhere between standing upright and hanging upside down on earth. In an earthbound person, gravity causes blood to pool in the lowermost parts of the body. In standing and sitting positions, blood accumulates in the veins of the leg. In orbit, this no longer happens. An astronaut’s face will puff up a bit, since its tissues and blood are no longer pulled downward. Also, in orbit, the heart doesn’t have to push blood against gravity: weightlessness is even easier on the heart than lying down flat on a bed. This reduction in the heart’s workload causes astronauts’ hearts to lose muscle mass.
More about the circulatory system in orbit.
Astronauts eyesight also gets worse because more blood in the head equals more pressure on the eyes.
Due to these circulatory effects, when astronauts return to Earth, they frequently experience orthostatic hypotension, an inability to properly manage the blood flow in the body, resulting in lightheadedness and fainting – especially upon standing (you might experience something similar when you stand up too fast). It’s also common in the elderly, pregnant women, and can be caused by medication or autonomic nervous system disorders. But it’s consistently common in astronauts returning to Earth because their time in space has made them more susceptible to it.
So being in space essentially de-conditions parts of the body to the point that it has trouble handling proper blood flow in Earth’s gravity at ground level. As that article points out, some cosmonauts would have to be carried away from their landings in stretchers.
“Fun” fact. James Irwin, one of the Apollo astronauts on Apollo 15 had a heart condition show up while they were getting ready to leave the moon, and NASA’s team of doctors concluded that sitting in the capsule on the way back from the moon in microgravity was about the best thing anyway:
“Dr. Charles Berry stated to Chris Kraft, deputy director of the Manned Spacecraft Center (MSC) at the time: “It’s serious, [i]f he were on Earth. I’d have him in ICU being treated for a heart attack.” But Berry concluded that since Endeavour’s cabin atmosphere was 100% oxygen when in space, Irwin was in the best of circumstances. Specifically, “In truth,…he’s in an ICU. He’s getting one hundred percent oxygen, he’s being continuously monitored, and best of all, he’s in zero g. Whatever strain his heart is under, well, we can’t do better than zero g.””