If humans ever become an interplanetary species, we must face the many perils that come with traveling through deep space over extended periods of time. We know that long exposure to microgravity changes brain structure, affects vision, and causes muscle and bone loss. According to a new study, we’ll have to add blood flowing in reverse inside the upper body to the list.

Deep space missions could lead to dangerous blood clots

Researchers at NASA’s Johnson Space Center analyzed periodic ultrasound scans performed on 11 astronauts who lived for six months inside the International Space Station (ISS).

The study, which began years ago, was prompted by reports that nearly two-thirds of astronauts experience blurry vision and impaired eyesight. These effects would linger even after the astronauts had returned to Earth.

So, the team led by Michael Stenger, who is the manager of the Cardiovascular and Vision Laboratory at NASA’s Johnson Space Center in Houston, decided to investigate. They were very surprised to find that astronauts’ bodily fluids struggled to drain normally.

While they were stationed in microgravity, the astronauts’ internal jugular vein became engorged, causing blood to flow in reverse or even stagnate.

“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,” the authors wrote in their new study, which was published in the journal JAMA Network Open.

This all explains why astronauts sometimes get puffy faces — gravity simply doesn’t tug on the blood cells in the upper body anymore.

One of the most concerning consequences is the potential formation of blood clots. If a clot formed in the jugular vein travels to the lungs, it can cause a pulmonary embolism, which is very dangerous — especially in space, where medical attention is limited.

These findings have major implications for long-duration space missions, like the planned manned launches to Mars, which would require eight months at least.

There is still much we do not know about how the human body responds to deep space. Many of the effects reported thus far are discouraging. But our actions must match our ambitions, so we just have to develop proper counter-measures. For instance, advanced shielding might counter some of the effects of radiation in space while some drugs could improve blood circulation.