Microgravity made some astronauts on the International Space Station experience reversed blood flow. NASA

Being in microgravity can have strange effects on the body – now it has emerged that it can make people’s blood flow backwards.

The changes to circulation caused two astronauts to develop small blood clots, which could have been fatal. Fortunately, though, the man and woman affected came to no harm.

The blood changes happened in a vessel called the left internal jugular vein, one of two that normally move blood out of the head when we are lying down. When we are upright, they mostly collapse to stop too much blood from draining out of the head, with our circulation taking a different route through veins with more resistance instead.


On Earth, people have occasionally been spotted with backwards blood flow in the left internal jugular vein if there is a blockage lower down, such as from a tumour growing in their chest.

Microgravity is known to change people’s blood flow, so Karina Marshall-Goebel of KBR, an engineering, procurement and construction company based in Texas, and her colleagues wondered if it would also affect this vein.

They carried out measurements and ultrasound scans of the blood vessel in nine men and two women on the ground before and after their missions on the International Space Station. The astronauts took measurements 50 and 150 days into their flights.

In two of the astronauts, the blood flow was backwards – perhaps because the lack of gravity caused organs in the chest to shift around, pressing on the vein lower down, says Marshall-Goebel. She adds that this vein is predisposed to be blocked based on where it lies in the body.

In another five members of the crew, blood in this vein was more or less stagnant, and in one of these, the scan revealed a clot blocking the vessel. “That was definitely alarming,” says Marshall-Goebel. Blood clots can be fatal if they get carried to the lungs, so the person began taking blood-thinning medicines to break it down.

Because of this surprise finding, the team asked a panel of experts to review all the previous scans and another small clot was spotted in one astronaut who had already returned to Earth.

The team also had the participants test a device on the space station that encases their lower body in a chamber with lower air pressure for an hour to suck more blood into their legs. They found that this improved blood flow in 10 of 17 tests, but worsened it in two.

Marshall-Goebel says the findings may cause female astronauts to reconsider taking the contraceptive pill to suppress their periods while on the space station, as this raises the risk of blood clots.

The two astronauts in this study who had a clot included one man and one woman, although the team aren’t giving any further details to protect their privacy.

Journal reference: JAMA Network Open, DOI: 10.1001/jamanetworkopen.2019.15011

This story was amended as astronauts on the International Space Station experience microgravity