The heart is a muscle, and like any muscle, it shrinks when you ask less of it.
Within days of leaving Earth, an astronaut’s heart begins to lose mass — a response to workload, not a sign of damage, because the constant pull it normally works against has vanished.
The key number comes from a paper published a quarter of a century ago. Perhonen and colleagues, working in Dallas, scanned the hearts of four astronauts before and after a 10-day shuttle mission. On average, the main pumping chamber — the left ventricle — was about 12 percent smaller after landing.
The mechanism is the same one that makes an athlete’s heart grow, run in reverse. Push the heart hard through endurance training or high blood pressure, and it thickens. Ask less of it and it thins.
On Earth, the heart works constantly against gravity, pushing blood upward from the legs to the head. In orbit that uphill effort disappears. Fluids that gravity normally pools in the lower body drift toward the chest and head, and the heart has an easier job. With less to do, the left ventricle gives up mass, sizing itself to the work.
A second thing happens at the same time. The body reads all that fluid gathering in the chest as a sign it has too much blood, so it sheds some. The amount of fluid in the blood drops fast in the first days. A smaller, less-filled heart is partly a structural change and partly a fluid one, and the two are difficult to separate.
A 12 percent average drop over 10 days sounds alarming, but the evidence doesn’t read it as damage. A shrunken heart in orbit is one that has adjusted its size to a lighter workload.
Benjamin Levine, senior author on that paper and on much of the follow-up work, has put it plainly: “The heart gets smaller and shrinks and atrophies, but it doesn’t become weaker — it’s just fine.”
The trouble comes not in orbit but on the way home. A heart that has shrunk, in a body that has shed fluid, is poorly prepared for the sudden return of gravity. That is why astronauts can feel faint when they first stand after landing. As Levine explains, “Space causes a loss of plasma volume that is accentuated by the re-entry process.” Crews are given saline on landing to help refill the system before gravity hits.
After short flights, the rebound is quick. Fluid levels recover over days, and the lost mass returns over days to weeks, much as an athlete’s heart rebuilds after a long break from training.
Longer missions complicate the story. When Levine’s group tracked astronaut Scott Kelly across 340 days on the International Space Station, his left ventricle lost about 0.74 grams per week, even though he exercised six days a week. Levine described the change as modest: “It did shrink a little bit. It did atrophy and it did get a little smaller, but the function remained good.”
A larger study points the other way. A 2023 study of 13 astronauts who spent about five months on the ISS found no clear shrinkage in either chamber. That suggests the exercise routines now standard on the station can largely cancel out the effect. In an earlier six-month study, Levine’s team found that “Exercise keeps the heart size and function intact and fluid fills it in preparation for Earth’s gravity.” These are small samples, and they sit in some tension with the Kelly result, so the picture is still being assembled.
The long haul stays unresolved. A short flight is one shrink-and-rebuild cycle the body handles without apparent difficulty. A trip to Mars would mean months of unloading, then arrival on a planet where the heart must fight gravity again with no medical team nearby, then the return trip and a second landing.
