When the first astronauts returned from space in the 1960s, their health assessments revealed something unexpected. Besides significant loss of muscle and bone mass, they were anemic — meaning their blood contained a lower-than-normal amount of red blood cells.

This phenomenon, known as “space anemia,” seems to be an unavoidable part of space travel. It happens to everyone who ventures beyond Earth’s atmosphere. But decades later, scientists still don’t fully understand the process.

What Is Space Anemia?

For years, scientists thought space anemia was a temporary adjustment, part of the shift in bodily fluids that astronauts experience as they transition to microgravity, and therefore not much of a problem.

Recent research, however, suggests otherwise: A paper published in Nature Medicine in 2022 found that space anemia is the result of a dramatic and persistent spike in red blood cell destruction. Whereas on Earth our bodies break down 2 million cells every second, in space they ramp up to 3 million per second, a 50 percent increase.

“It’s happening for the entire time you’re in space,” says Guy Trudel, a professor of medicine at the University of Ottawa and the paper’s lead author.

Notably, the condition does improve throughout post-flight rehabilitation. And astronauts have dealt with it just fine up to now, but most trips to outer space, like those for individuals on the International Space Station, last only six months. It’s unclear how human physiology would fare on the multi-year voyages of the future — not to mention permanent colonies on the moon or Mars.

“It brings up additional risks when we’re talking about longer missions,” Trudel adds.

Read More: Why and How Do Astronauts Get Sick in Space?

Anemic in Space

To clarify, astronauts actually aren’t anemic while in space. They have roughly 10-15 percent fewer red blood cells, but they also have less fluid in their blood, keeping the relative concentrations about the same. It’s only when they get home and replenish their fluid to earthly levels that the shortage of cells becomes apparent.

Even if they aren’t technically anemic, however, the fact remains that in space their bodies are working harder to compensate for the surge in red blood cell destruction, or hemolysis.

Life among the stars is easier in some ways, of course. Given the lack of gravity, Trudel explains, it requires comparably little effort to perform tasks than on Earth. “You can carry huge loads by just pushing them with a finger,” he says. “You don’t need to build bones, you don’t need to build muscles, and you’re functioning with way lesser needs for oxygen.”

It’s oxygen, after all, that your red blood cells distribute throughout the body; if we don’t need as much of it in space, maybe we can also get by with fewer cells. In that case, space anemia would mainly be a problem for astronauts returning to Earth, where gravity demands more of them.

On the other hand, some data suggests that anemia does keep getting worse as missions get longer. If it turns out the body can’t regenerate enough red blood cells to make up for increased hemolysis indefinitely, Trudel says, “anemia may even reach levels that would prevent an astronaut from being able to carry out their mission.”

In such scenarios, anemia could impact not only an astronaut’s capacity for physical work, but also their cognition. And in the event of a bleed, they might not be able to balance the lost blood with new cells.

Read More: How Scientists Create Oxygen for Astronauts on Prolonged Space Missions

Red Blood Cells in Space

What’s causing the rise in hemolysis among astronauts is an open question. One possibility is that red blood cells deform under microgravity, similar to how water becomes spherical. Their specific shape — known as biconcave disks, which look like donuts that haven’t been punched all the way through — allows them to squeeze through our narrow capillaries. The slightest change could prevent passage, at which point the useless cells would be destroyed.

Alternatively, it could be a problem with red blood cell filtration, which occurs at the spleen. Normally this system keeps our blood healthy by removing only aged and degraded cells. But if the spleen functions differently in space, it could be targeting them all indiscriminately.

The next step for researchers is to locate the site of the extra hemolysis (as well as the subset of red blood cells being disproportionately destroyed), which could illuminate the underlying cause and perhaps point to solutions. “Finding where it’s happening will be a key to the mechanism,” Trudel says. “This is what we’re after.”

Read More: 10 Ways Space Changes the Body

Potential Impacts on Space Tourism

Whatever the hazards of space anemia, astronauts — who are among the fittest people on Earth — are probably the least of our worries. As Trudel notes, “they’re athletes. They’re young and active, [and] they don’t have any other diseases.” Indeed, their hardiness could account for the lack of anemia-related complications thus far.

But as space tourism becomes more accessible, the demographic could expand to include people with heart problems, lung problems, and other conditions that may compound the effect of space anemia. Plus, there are more than a thousand types of hemoglobin (the oxygen-bearing proteins in our red blood cells), some of which might respond to microgravity in different ways.

Ideally, as scientists get a better grasp on the mechanisms behind space anemia, they’ll be able to pinpoint which populations are at greatest risk and use that information to screen would-be spacefarers.

“Still lots to learn,” Trudel says, “but, certainly, the red blood cells are vital. If you don’t have them, you’re not surviving extreme environments or landing on other worlds.”

Read More: What Would a Trip to Mars Look Like For a Tourist?

Article Sources

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Cody Cottier is a contributing writer at Discover who loves exploring big questions about the universe and our home planet, the nature of consciousness, the ethical implications of science and more. He holds a bachelor’s degree in journalism and media production from Washington State University.