Abstract:
A trip to Mars may not challenge the human heart as much as scientists once feared. A study shows that astronauts adapt well to months of weightlessness. The research, published today in Circulation, provides encouraging evidence that humans can withstand space travel to Mars and function normally in its weaker gravity.
In the study, scientists monitored the heart conditions of 13 astronauts who spent about six months aboard the International Space Station. The results showed that although spaceflight initially affects heart function, astronauts gradually adapt to the environment and that daily high-intensity exercise plays an important role in maintaining cardiovascular health.
Since both weightlessness and return to a gravity environment will bring challenges to the human body, the National Aeronautics and Space Administration (NASA) is worried about whether the heart and circulatory system of astronauts can adapt to the microgravity environment of space on Mars, where the gravity is only about 38% of the Earth.
On Earth, the heart needs to continue to pump blood against gravity, and the muscles need to support body weight; in space, these loads are greatly reduced, and over time, the muscles and even the heart will atrophy. But after returning to the gravity environment, the blood will be pulled to the lower body. After landing, astronauts often experience symptoms of dizziness, dark eyes, and even a sudden drop in blood pressure. This is called orthostatic hypotension. However, the astronauts in the study generally adapted well to their new gravity environment, perhaps thanks to the intense exercise program they continued throughout the mission.
As part of NASA's "Exercise Countermeasures" program, astronauts exercise for up to two hours a day. The training includes 30 to 45 minutes of endurance training to strengthen the cardiovascular system and 60 to 75 minutes of resistance training to maintain muscle mass and strength. James Thomas, author of the paper and from Northwestern University Feinberg School of Medicine, explained: "In a weightless environment, you can slide over and say hello to your companions with just a tap of your foot. Due to the reduced amount of exercise, the mass of the muscles and heart will decrease. To combat this, space exercise is crucial."
To understand what happens to the heart during long-duration spaceflight, researchers collected echocardiograms before, during and after each astronaut's mission. Ultrasound scans use sound waves to generate dynamic images of the heart, allowing medical staff to examine the heart's structure and measure its pumping efficiency. Every few weeks, astronauts on the International Space Station scan each other's hearts and transmit the records back to Earth. The researchers monitored changes in heart function throughout the study. Thomas conducted the study in collaboration with Benjamin Levine of the University of Texas Southwestern Medical Center and Michael Bungo of the McGovern Medical School of the University of Texas Health Science Center at Houston.
Scan results showed that the astronaut's heart initially had difficulty adapting to the space environment. During the first two weeks in space, researchers observed decreases in cardiac filling volume, stroke volume, and several other measures of cardiovascular function. Cardiac filling volume refers to the amount of blood entering the heart between heartbeats, while stroke volume describes the amount of blood pumped out by the heart with each contraction. Both measures dropped early in the mission, indicating that the astronauts' hearts were temporarily pumping less blood with each beat.
But as astronauts stay in space for longer periods of time, their cardiovascular systems will gradually adapt. Cardiac filling volume and stroke volume gradually improved, eventually approaching prelaunch levels. These results suggest that the heart may be more adaptable to long-term weightlessness than earlier measurements suggested, especially if astronauts maintain regular physical activity.
The researchers also wanted to answer a question critical to future missions to Mars: What happens when an astronaut who has spent months in weightlessness suddenly returns to gravity? Although the gravity on the surface of Mars is much weaker than that on Earth, the way astronauts' hearts work will still change after months of floating in space. To recreate such effects, researchers conducted simulated Martian gravity tests after astronauts returned to Earth.
During the test, each astronaut tilted upward 22 degrees from a lying position to allow blood to flow to the lower body to simulate the impact of Mars' gravity on the circulatory system. The test results were encouraging: After spending about six months in space, the astronauts' hearts performed just as well in simulated Martian gravity as they did when they were upright on Earth before the mission. This suggests that astronauts who maintain cardiovascular fitness during spaceflight may be able to adapt to Mars' weaker gravity without experiencing major cardiovascular problems.
Although the study has a small sample size and uses simulations rather than real Mars landing scenarios, it still provides valuable evidence for future mission planning. "Based on our findings, it is now reasonable to believe that humans can fly to Mars, successfully land on Mars, and carry out work on the surface of Mars." Thomas said. (Hou Huijing)
Related paper information:
https://doi.org/10.1161/CIRCULATIONAHA.125.078665
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