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It is well documented that spending long periods in microgravity takes a significant toll on the human body. These effects include muscle atrophy, loss of bone density, and changes to the cardiovascular, endocrine, and nervous systems. However, recent findings indicate that female astronauts face an additional, specific threat: a greater risk of developing blood clots. This discovery underscores a critical gap in current research. To date, the vast majority of studies regarding human health in space have involved male astronauts. As the number of female astronauts continues to grow, it is essential to conduct more research to address these potentially gender-related health risks.
This specific concern motivated a new study designed to examine how microgravity affects blood clotting, with a particular focus on women. The research was conducted by scientists from Simon Fraser University (SFU) and the European Space Agency (ESA), with financial support provided through a grant from the Canadian Space Agency (CSA). The study involved 18 women who participated in a five-day dry immersion test. This experimental setup was used to assess the risk of developing potentially life-threatening blood clots in an environment that simulates space conditions. The results support existing evidence that women are at a greater risk of venous thromboembolism. Furthermore, the study identified hypercoagulability as a potential key mechanism driving this increased risk.
The research was led by a team from SFU's Aerospace Physiology Laboratory. This laboratory routinely collaborates with the Canadian Space Agency and various international space agencies to study the effects of space exploration on astronauts. The research team included experts from the Department of Biomedical Physiology and Kinesiology at SFU, the Medical University of Graz, Antwerp University Hospital, University Medical Center Maribor, the European Astronaut Center Department (EACD), and the Mohammed Bin Rashid University of Medicine and Health Sciences. The study describing their detailed findings appeared in the journal Acta Astronautica.
On Earth, blood clots are often associated with aging, though they can affect younger people as well. Due to Earth's gravity, clots generally form in the leg veins. When they form there, they block blood flow, which leads to pain and swelling. In some severe cases, these clots can travel to the lungs, causing a life-threatening pulmonary embolism, heart attack, or stroke. Fortunately, when clots form in the legs, the person has more time to seek treatment. Doctors can often remove the clot, or the body may break it down on its own. However, in microgravity, the rules change. Blood pools in the head and sometimes in the feet, creating the specific conditions where clots are much more likely to form.
As explained by Blaber in an SFU press release, the location of clots in space creates a unique danger:
We've found that in space, blood clots are more likely to form in the jugular vein. From there, it doesn't have to travel far to reach the lungs or heart, and trigger a serious medical event. Space is not a place where you want these things to happen. Now that they know it can happen, they're looking at it more frequently as part of the standard measures.
The first hints that female astronauts could be at greater risk of clotting emerged in 2020. At that time, a female astronaut aboard the International Space Station (ISS) developed an unexpected clot in her jugular vein. For their current study, the team examined real-time clotting responses in 18 healthy women over a five-day period. They utilized a dry immersion tank, which consisted of a water bath with a waterproof seal. This device kept the volunteers dry while they floated, effectively simulating the sensation of weightlessness without the dangers of actual space. The team then analyzed the participants using rotational thromboelastometry, known as ROTEM. This advanced method measures the rate at which blood clots form and progress.
This rapid blood-testing method assesses coagulation function and identifies the causes of bleeding in real time. The researchers also analyzed the volunteers' blood for menstrual hormones. They found that these hormones had no significant effect on blood coagulation in this specific context. The key findings of the study indicate a complex process: the time it takes for blood clots to form in microgravity is actually longer. However, once the process starts, the clots form much faster. Additionally, once formed, the clots showed greater strength and stability than what is typically observed with patients on Earth.
We know that on Earth, clotting in men and women can vary with age, but we have little information on whether these will be different when in space. In this microgravity environment, we found the female participants took longer for their blood to start clotting. But once that clotting began, it formed faster and was more stable, making it harder to break down.
While the study findings were not alarming after just five days, they do raise serious concerns for astronaut crews that will be far from medical or emergency care. The results indicate that further research is needed to assess potential risks to crews on long-duration missions. This includes crews operating on the Moon as part of NASA's Artemis Program and on future missions to Mars. On these missions, astronauts will spend months in transit, away from immediate help. Blaber and his team are now analyzing and comparing their results to dry immersion studies involving male volunteers to see if there are gender differences in these responses.
These studies will help inform future medical monitoring and treatment procedures needed for longer missions to space. Understanding these mechanisms is critical for the safety of future explorers. In the meantime, space agencies are already taking precautionary measures. Astronauts aboard the ISS are regularly getting jugular vein ultrasounds. This is the same diagnostic treatment that accidentally identified the female astronaut's clot in 2020. By increasing the frequency of these screenings, agencies are ensuring that potential clots are detected early. This proactive approach is essential as humanity prepares to venture further into the cosmos, where medical resources are limited and the consequences of a blocked vessel could be catastrophic.
The transition from short-duration missions in low Earth orbit to long-duration voyages on the Moon and beyond requires a deeper understanding of human physiology. The findings from this study provide a crucial piece of that puzzle. They confirm that women face distinct physiological challenges in space, particularly regarding blood flow and clotting stability. As the space industry diversifies and more women join the ranks of astronauts, the data generated from these studies will become increasingly vital. It ensures that medical protocols are not based on assumptions derived solely from male physiology, but on comprehensive data that reflects the reality of the diverse crews who will explore the solar system. The safety of these crews depends on the precision of this research and the willingness to adapt medical standards to the unique environment of space.