Every second, the human body both creates and destroys two million red blood cells. That’s an incredible feat that happens around the clock, refreshing our supply of these oxygen-carrying cells as they wear out. But it’s a balance that has been finely tuned by evolution for our lives on Earth.
What will happen as humans attempt longer missions in space?
Ever since our first astronauts returned home from space, we’ve known about a condition called space anemia. They returned with fewer red blood cells than they had before their missions began, and we didn’t have a good understanding of how or why it happens.
Momentum is starting to build for sending crewed missions to the moon and Mars. As they embark to explore the solar system, they will need better tools to stay healthy while they’re away.
Guy Trudel, rehabilitation physician at The Ottawa Hospital and professor of medicine at the University of Ottawa, led a study to learn more about space anemia. The study was published in Nature Medicine.
When astronauts are in space, the lack of gravity shifts fluids into the upper body. In response, an astronaut rapidly loses about 10 percent of the liquid in their blood vessels.
Without further study, it was assumed that the body responded to this reduction in blood volume by destroying an extra 10 percent of its red blood cells to hit the right concentration. After about 10 days of adjustment, it was thought that normal red blood cell production and destruction rates were restored.
The truth is stranger, because the body actually destroys many more red blood cells than it needs to. The destruction rate increases to 54 percent more than usual, and that rate of around three million cells being destroyed every second is kept up for the entire mission.
This discovery was made by asking 14 astronauts to collect breath samples during six-month missions at the International Space Station. They each exhaled into special canisters, once pre-flight as a baseline, and again in space at five days, 12 days, three months, and just before returning home at six months. All but one of the astronauts also gave a blood sample shortly after arriving back on Earth.
As red blood cells are destroyed, their deep-red pigment — called heme — is also destroyed. And every time a molecule of heme is destroyed, it releases a molecule of carbon monoxide. By measuring the amount of carbon monoxide present in the breath samples, the research team estimated the rate of red blood cell destruction.
Although other processes in the body also produce carbon monoxide, about 85 percent comes from red blood cells being destroyed.
The team believes the body must compensate by producing more red blood cells to make up the shortfall.
This didn’t pose a problem for the crew while in space and still weightless, but five out of 13 blood samples after returning home revealed clinical anemia. Blood volumes start to return to normal upon reintroduction to gravity, diluting the red blood cells the astronauts still have.
In a second study, they also found that longer exposure to spaceflight worsens the resulting anemia; over time the body is less able to keep pace with this extra work.
While astronauts showed eventual recovery from space anemia within three or four months, post-flight breath still showed red blood cell destruction was 30 percent above pre-flight levels even a year later.
The same is likely to happen after landing on other planets or moons, and that could pose a problem for energy and stamina required to carry out physically-demanding mission tasks. This may also have consequences for their nutritional needs.
As we learn more about space anemia, we may find triggers other than changes in blood volume. It may become possible to treat or prevent it once we understand the primary cause.
Preparing for the health consequences of spaceflight will help astronauts stay well as they reach for the moon and beyond.