The human heart may be able to adapt to prolonged weightlessness better than previously thought. That is the conclusion reached by researchers who monitored 13 astronauts who spent about six months aboard the International Space Station. After returning to Earth, the study participants tolerated well a test simulating the effects of Martian gravity.

The results of the study, published in the journal Circulation, provide encouraging data for planning crewed expeditions to Mars. However, the findings are based on a small group of participants and on simulated conditions, rather than on testing how astronauts would actually feel on the Red Planet itself.

Why weightlessness changes how the heart works

On Earth, the heart has to pump blood while accounting for gravity, and the muscles have to support the body’s weight. In weightlessness, the strain on the cardiovascular system and the musculoskeletal system decreases. Over time, this can lead to a loss of muscle mass and a reduction in heart muscle mass.

Returning to gravity creates a different problem. Under the influence of gravity, blood shifts to the lower part of the body, which can cause blood pressure to drop sharply. This condition is called orthostatic hypotension. It can cause dizziness and weakness when astronauts try to stand after returning to Earth.

For future expeditions to Mars, it is important to understand whether astronauts will be able to function normally after several months in space. Gravity on the planet’s surface is about 38% of Earth’s, so circulation there will work differently than it does in weightlessness.

Daily exercise helps preserve health

The study involved scientists from Northwestern Medicine, the University of Texas Southwestern Medical Center, and McGovern Medical School at UTHealth Houston.

During the missions, researchers regularly assessed the astronauts’ heart function using echocardiography, an ultrasound examination that makes it possible to study the heart’s structure and measure how efficiently it works. Ultrasound recordings were made before the flight, during the stay on the ISS, and after the return. On the station, the astronauts performed the examinations on one another every few weeks, transmitting the recordings back to Earth.

To counteract the effects of weightlessness, the participants also exercised every day. Under the NASA program, they trained for up to two hours a day: about 30–45 minutes were devoted to aerobic exercise and 60–75 minutes to strength training.

Such workouts help support the cardiovascular system and preserve muscle mass and physical strength. According to the study’s senior author, James Thomas, a cardiologist and professor at Northwestern University Feinberg School of Medicine, physical activity is especially important in conditions where the heart and muscles have to work less than usual.

At first the indicators worsened, but then recovered

Ultrasound examinations showed that during the first two weeks after the start of the spaceflight, heart function really did change. Scientists recorded reduced filling of the heart with blood between contractions, a decrease in stroke volume — the amount of blood the heart ejects in a single beat — and a worsening of some other indicators of cardiovascular function.

However, these changes did not persist throughout the mission. As the body adapted, the indicators gradually improved and approached the values recorded before launch.

This observation suggests that the cardiovascular system is capable of adapting to prolonged weightlessness. Regular exercise likely plays an important role in maintaining its performance, although the study itself does not allow researchers to determine separately the contribution of physical activity to each of the observed effects.

How scientists simulated Mars gravity

The most important part of the study was the test conducted after the astronauts returned to Earth. Scientists wanted to find out how their cardiovascular systems would cope with the change in blood distribution that the crew is expected to face when transitioning from weightlessness to the gravity of another planet.

To do this, the participants were placed on a special surface and tilted upward by 22 degrees relative to the horizontal position. This body position causes blood to shift to the lower part of the body and makes it possible to simulate some effects of Martian gravity.

The result was encouraging: after about six months on the ISS, the astronauts’ hearts handled this test no worse than they had before the flight under normal standing conditions on Earth.

As James Thomas noted, the findings give reason to believe that humans will be able to endure a long flight to Mars, reach the planet, and work on its surface.

Nevertheless, the results do not guarantee the absence of problems during an actual landing. The test only simulated certain physiological effects of Martian gravity, while in a real expedition the body would also be affected by other factors, including the duration of the journey and the conditions on the surface.

The study provides important information about how the human heart adapts to prolonged weightlessness and the subsequent return of gravitational load. Together with data on the benefits of exercise, these results help scientists assess the risks of future crewed missions to Mars.