Astronauts experience stagnant and reverse blood flow during long periods in space, study finds

A study looking into how zero gravity can affect blood flow in the body showed astronauts staying onboard the International Space Station for long periods of time can face health risks.

Eleven healthy astronauts – nine men and two women – onboard the ISS participated in the study, which was published in the journal JAMA Network Open.

The study looked into how long stays in space could affect blood flow and fluid shifts in the body, particularly in the left jugular vein, which is a main vein that carries blood to the brain.

Of the 11 astronauts who participated in the study, the results showed six of them either experienced stagnant or reversed blood flow. One astronaut had a blood clot.

The authors of the study stated that exposure to zero-gravity leads to chronic blood flow to the head. On Earth, humans spend about two-thirds of the day upright and only one-third on their back, but those position changes cause daily fluid shifts, according to the study.

But when humans are in space for a long period of time, the weightlessness halts that fluid redistribution. It also leads to puffy faces, loss of volume in the legs, increased stroke volume and lower plasma volume.

The participating astronauts were scheduled for long stays on the ISS, with the average length coming to about 210 days. The astronauts had fluid measurements done in three positions: seated, supine and a 15-degree head-down tilt. They held those positions for 45 minutes each.

Data and fluid measurements were taken about 95 days before their space launch and then on days 50 and 150 while onboard the space station. Measurements were taken again about 40 days after landing, according to the study.

Researchers stated that relationships between altered blood flow and spaceflight need to be “further investigated.” They also stated that methods on how to restore “vascular physiology to a state similar to that seen in the upright and supine positions on Earth” should be considered.