Astronauts' incredible shrinking hearts

3 min
Astronaut Scott Kelly preparing in a Soyuz simulator before spending 340 days in space.

Astronauts' hearts shrink. It is not a metaphor for the fright caused by the sight of the place where, as Carl Sagan said, everyone they know, everyone they have heard of, everyone they love, has lived. No. It is a real, studied, physical effect. During his 340-day stay on the International Space Station, the heart of a NASA astronaut, Scott Kelly, went from 190 to 139 grams, a reduction of 27%, as published in the Circulation journal in an article signed by cardiologist Benjamin Levine, of the University of Texas Southwestern Medical Center.

Kelly, as all astronauts do, did not stay still while he was on the station. He ran on a treadmill, pedaled a stationary bike and exercised the equivalent of lifting weights for 30 to 40 minutes, six days a week. All this activity was not enough to counteract the effect of the absence of gravity, which makes it less difficult to pump blood to send it to the upper body. The reason for this is not that astronauts are so far away that the Earth's gravitational field has weakened. Like any body orbiting another, astronauts are continually falling towards the Earth. And this permanent falling, as happens when two cars move at the same speed and see each other standing still, somehow cancels out the effects of the gravitational field. Einstein already said it: the best way to free yourself from gravity is to jump out of the window.

"It atrophied and shrunk a little bit, but it continued to function properly", the lead author of the paper on Kelly's heart has explained in a statement. "And this is encouraging for long-term space travel, because it shows that even after a year in space, the heart adapts relatively well", Levine added. This phenomenon is an old acquaintance of physicians: reductions of this magnitude, and even more, have also been observed in patients who are forced to rest for long periods of time.

Unknown long-term effects

In addition to Kelly's heart, the researcher has studied the hearts of 13 other astronauts who have spent six months in orbit and has observed that the shrinkage of this organ varies depending on the physical condition of each one. Astronauts who are more physically fit experience more pronounced heart shrinkage than those who are less fit. This means that the difference between doing physical activity on Earth and in space is greater than the difference between doing nothing on Earth and exercising in orbit.

The effects of weightlessness or lower gravity, such as on the Moon or Mars, are relevant to assessing the safety of long-duration space travel such as that which is planned for the next decade, which is intended to take people to Mars. "We do not know the long-term effects of weightlessness", says Marta Sitges, director of the Cardiovascular Institute of the Hospital Clínic de Barcelona and Idibaps researcher. "We know that the heart is a muscle that adapts to each situation and that works with minimal requirements", she explains. "In situations of weightlessness", she continues, "it loses muscle mass and changes shape, but there is not much literature because there are not many people who have gone into space".

A significant loss of heart muscle mass can cause problems such as extreme bradycardia, that is, a marked slowing of the heartbeat that makes the heart unable to distribute oxygen throughout the body, even to its own tissues. This lack of oxygen in the heart muscle can cause the heart to stop beating. "This happens in some people with anorexia", says Sitges, "in whom the heart has low muscle mass and does not receive nutrients, so it ends up stopping". "We don't know if weightlessness can cause extreme situations like this", she says. "As a preventive measure, astronauts do a lot of exercise".

Studies like this can be useful when designing better training programs for astronauts, which, according to the plans of several space agencies, will be increasingly necessary.