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To investigate this theory, Marshall-Goebel and her team targeted a jugular vein on the left side of the neck, which delivers blood from the head to the heart. The study’s astronaut subjects included nine men and two women. (The study did not disclose their identities.) Before the astronauts launched, researchers measured blood flow in their jugular vein in seated, supine, and tilted positions. The readings looked normal. The researchers had the astronauts repeat the ultrasounds during their missions on the ISS.

Scans showed that blood flow in the vein stalled in five of the 11 astronauts. “Sometimes it was sloshing back and forth a bit, but there was no net-forward movement,” Marshall-Goebel says. Seeing stagnant blood flow in this kind of vein is rare, she says; the condition usually occurs in the legs, such as when people sit still for hours on a plane. The finding was concerning. Stagnant blood, whether it’s in the neck or in the legs, can clot. Blood clots can dissolve on their own or with the help of anticoagulants, but the blockages can also cause serious problems, such as lung damage.

In two astronauts, blood in the vessel actually started moving in the opposite direction, from the heart toward the head, which is “extremely abnormal” for this vein, according to Marshall-Goebel. The researchers think the blood switched directions because of a blockage somewhere downstream. The phenomenon has been reported in non-astronauts with tumors or masses that forced blood to find a different path to the heart.

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“It’s almost like a detour, when you’re in your car and you sometimes have to go down the wrong street to get where you need to go,” Marshall-Goebel says. Perhaps something similar was happening in the astronauts; like their bodily fluids, the organs in their torsos had shifted upward and blocked off certain veins. Marshall-Goebel says she was rather impressed by this—the body, plunged into an environment unlike anything it had experienced before, found a small way to adapt.

The researchers had astronauts spend some time inside a special suit that Russian cosmonauts use to prepare for their return to Earth. The suit, which looks like a pair of puffy pants, uses suction to simulate gravity and draw some fluids back toward the lower body. Blood flow improved in some astronauts, but not in others.

The researchers attribute the effects they observed to the space environment. All the astronauts were considered to be in good health before they launched. And when they came home, the conditions vanished in nearly all of them. When the researchers analyzed the data, they found that a second astronaut may have developed a blood clot no one had seen while they were in orbit. But no one experienced any health troubles. “None of the crew members actually had any negative clinical outcomes,” Marshall-Goebel says.