The problem lies in the internal jugular vein (IJV), a major blood vessel running down the neck from the brain. A study of 11 astronauts who spent time on the International Space Station (ISS) found that six of them had developed stagnant or backwards blood flow in this particular vein, within a period of just 50 days.
One crew member was found to have developed thrombosis, or blockage in the internal jugular vein, the first time that this has been recorded as a result of spaceflight.
According to the team behind the new findings, this issue needs to be investigated before we start sending astronauts on long trips to Mars. It's not yet clear just what the consequences of this kind of thrombosis might be, but the implications could be severe and perhaps even fatal.
"Exposure to a weightless environment during spaceflight results in a chronic headward blood and tissue fluid shift compared with the upright posture on Earth, with unknown consequences to cerebral venous outflow," write the researchers in their published paper.
Down here on Earth of course, gravity takes care of the job of pulling blood down from the head to the rest of the body – it's one of the reasons you'd start feeling very strange if you stood on your hands for an extended period of time.
Up in the microgravity environment of the ISS, it's a different story – and bloodflow issues aren't the only health risks we need to worry about.
"Headward fluid shifts during prolonged weightlessness result in facial puffiness, decreased leg volume, increased stroke volume, and decreased plasma volume," write the researchers.
Medical experts used readings and images gathered on board the ISS to identify the potential issue with the IJV, while the astronaut who developed an occlusive thrombus was treated with anticoagulants for the remainder of the mission (the identities of the astronauts are being kept back for privacy reasons).
More research is needed to work out how big of a problem this actually is, and how we might mitigate against it in future spaceflights; but the high number of astronauts who developed some kind of blood flow issue is worrying.
We already know that time in space can reduce bone density, change the make-up of our gut bacteria, and put a squeeze on our brains. At least we're working to discover these effects before we try to get farther than the Moon, so there's a better chance of developing potential solutions.
"[These] are novel findings that may have significant human health implications for civilian spaceflight as well as future exploration-class missions, such as a mission to Mars," conclude the researchers.
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