spacestation infirmary digital rendition

Red Alert in Microgravity: The Complexities of Bleeding in Space

Last Updated: March 25, 2024

Blood, in general, has a tendency to send people into panic mode or at least controlled concern if they are trained for such situations. But what about blood in space? How does the essential liquid react in space? What are the procedures for a bleeding astronaut? How will we handle the threat of bodily injury on future human space exploration missions? 

Let’s dive into bleeding in space!

Table of Contents

The effect of microgravity on blood and the body

Every time we stand up, gravity pulls blood to parts that are below the heart. If you let your hand hang down for a bit, veins will appear bigger, but if you raise your arm above your head, the veins seem to disappear as our veins and the blood flowing within them adapt to our body’s movement.

In space, there’s no gravity to pull the blood to the lower extremities which causes blood to accumulate in the chest and head, creating puffy faces and even bulging neck blood vessels, especially for the first few weeks until the body adjusts to the new environment. Red cell mass and the total amount of circulating blood decrease, causing the phenomenon known as space anemia, and blood vessels don’t constrict and dilate as well. This also causes astronauts to feel faint and dizzy due to the lack of blood flow to and from the brain, which obviously causes concern. Blood clots, reverse blood flow, and blood pressure concerns can also occur in space and after they return to Earth as their bodies readjust to gravity.

Since the early 2000s, NASA and other space agencies have been studying this and other medical effects of microgravity by monitoring astronauts on the International Space Station and experimenting with different options to mitigate these and other adverse effects of microgravity on the body. Astronauts also continue undergoing medical tests after they return to Earth to help better understand long-term effects of microgravity on the human body.

But what about when that blood… leaves the body? Before we get into the details of blood specifically, let’s discuss liquids in space. Due to microgravity, liquids need to be tightly contained on the space station. While spills are a concern everywhere, they don’t behave the same way in microgravity. Liquids clump together in spheres and other blobs due to surface tension, at least when they are under control. But in the case of a splatter, the impact of gravity is reduced meaning it can splatter more than on Earth. The spilled liquid hovers, just like everything else in microgravity. 

Airflow is very important in the space station to keep a clean environment so the circulation system moves the liquid on the air currents, meaning it can travel a lot more than it would here on Earth.

first aid kits on spacestation digital rendition

Blood and traumatic injury in space

Traumatic injury is one of the biggest worries about astronaut safety and health because of the massive impact it could have on the mission, the obvious amplified concerns of being injured in an isolated environment, and the fact that it’s a concern that we know the least about in part because it’s never happened.

Throughout the history of human space exploration, no astronaut has ever had a major injury or needed surgery in space. There have been medical concerns (including a blood clot scare in 2019 that required anticoagulant injections) and even disasters such as ones that caused the deaths of astronauts, but no major injury or surgery where first aid had to be performed in space. 

Yet, it’s something that will eventually happen, especially as human space exploration continues and astronauts’ missions take them further and further away from Earth (for a crew of six on a mission to Mars, one major emergency is all but guaranteed based on the 0.06 chance per person-year on a space mission from a 2002 ESA report).

The concerns of blood in space

We talked briefly about how liquids behave in space in the intro, but now let’s delve into the various concerns of blood in space in particular.


While any liquid causes concerns of mess, blood is a special concern for various biohazard reasons. In space, blood would form into a droplet or mist and float around, causing it to “splatter” even more than on Earth. Since gravity can’t keep it flowing down, there can be an unexpected increase in the amount of gushing and splattering.  The ensuing mess can cause a number of issues such as obstructing vision, infiltrating equipment, and contaminating the air supply.


Due to the microgravity, the blood can pool into a dome or bubble around a wound/ incision until the surface tension can no longer hold it and it pops, spreading little bubbles out. This can make it difficult to understand the actual trauma because the view is obstructed.

Risk of bleeding out

Due to the various issues of microgravity including the fact that you have less overall volume of blood in space and the spurting from the lack of gravity, bleeding more than 100 milliliters per minute essentially means you’re at the point of no return.

Internal bleeding concerns

If the bleeding is internal, it can cause other problems as it would pool in empty spaces in the body, hindering the body’s ability to heal itself since it wouldn’t drain out of the tissues like normal. In fact, you need external pressure to treat internal bleeding, which is almost impossible in the environment of space.

In the vacuum of space (outside the space station): You should be in a space suit. If you are without a space suit, there are other factors going on and you won’t survive for more than a few minutes anyway. For the sake of argument, if something snagged and ripped a spacesuit and injured the astronaut, the blood would simultaneously freeze and boil (since the warm water in the blood would vaporize due to lack of pressure and then almost immediately freeze once the vapor cools, going from gas straight to solid). If pressure was applied, it would freeze onto the skin and stop the bleeding. However, at this point, the astronaut is likely dead considering what happens if you take your helmet off in space.

Getting help

Communication with Earth can be delayed or even impossible if there was an accident on the station, making it harder to receive expert opinion and treat the injury. 

General health/ hygiene, etc. concerns

Your organs rearrange and change shape in space, making treatment harder, and your immune system does not work as efficiently as it does on Earth. Keeping the space hygienic despite debris from skin grafts, bodily waste, blood, and more is also a concern. Even administering drugs can be complicated in space, such as bubbles remaining in IV solutions as opposed to floating to the top.

Procedures/ Supplies/ Innovations

All astronauts are trained on basic first aid for the space station, with usually one member having a bit more training as the designated medical officer. Crews primarily learn how to stabilize and restrain an injured astronaut and call ground control to consult a flight surgeon for further aid. Medical gear on the ISS includes:

  • Over-the-counter and more serious drugs and EpiPens
  • Basic first aid supplies such as anti-coagulant bandages
  • AED (Automatic External Defibrillator for correcting irregular heart beats like heart attacks)
  • Equipment to administer IV fluids
  • Diagnostic equipment like blood pressure cuffs and an ultrasound device

In general, the procedure is the same as on Earth: 

  • Stop the bleeding
  • Stitches, surgery, or a tourniquet may be necessary
  • Seek further aid/ advice from professionals
  • Clean up

In general, the standard operating procedure if an astronaut on the ISS experiences trauma and hemorrhaging (excessive bleeding), is to schedule a rapid return to Earth.

red liquid floating in microgravity

Recent and Future Innovations/ Plans

NASA has always been concerned about the health and safety of astronauts, but especially so as further human space exploration away than the ISS is planned as those environments make injuries significantly more complicated. Research has already produced some remarkable innovations to help astronauts stay safe on their journey and continues to do so.

To address the issue of blood bubbling up in microgravity, a research team led by Carnegie Mellon biomedical engineer James Antaki tested a solution to a bleeding wound which would utilize a bubble filled with a liquid like saline to enable laparoscopic surgery (think tiny, even robotic, instruments with tiny incisions).

Various research experiments are being conducted to help answer bleeding/ trauma in space and other long-term spaceflight questions. Similar to how astronauts train for zero-g,  “vomit comet” flights of weightlessness provide the opportunity for researchers to perform intubations, open and close wounds, repair blood vessels, and more on animals, tissue samples, etc. to test procedures for zero-g. These “vomit comet” experiments also have the added concern of someone actually discharging their lunch as the name suggests as opposed to the usual operating procedures on the ISS, but otherwise provide a good simulation of microgravity on Earth in short stints.

In light of planned trips to the Moon and Mars, NASA has been exploring various areas of research to make the mission as safe as possible for astronauts, collectively called the Complement of Integrated Protocols for Human Exploration Research or CIPHER. On these missions, even more dangerous variables can be in place including: 

  • A more significant delay in communications

  • The inability to send an injured party member back to Earth for surgery within a reasonable time frame

  • The limited resources on the mission

  • Blood transfusion: the absence of storage for blood and other blood derivatives for transfusion

  • Risk of trauma: While traumatic injury is more likely to occur during launch, landing, docking, and EVAs, there are additional risks on the Moon, Mars, and other destinations where gravity increases the chance of injuring from falling or crushing

NASA’s Human Research Program has identified over 30 human health risks, prioritizing those categorized as “red” based on their likelihood and severity of impact on human health and mission performance. These include: 

  • Space radiation-related health effects: without the protection of our atmosphere, more of the Sun’s harmful radiation will impact astronauts
  • Spaceflight-Associated Neuro-ocular Syndrome: eye and brain changes due to microgravity
  • Behavioral health and performance decrements: being in isolation can have significant impacts on our physical, mental, and emotional health which can affect all areas of work and life in general
  • Inadequate food and nutrition: while providing a nutritionally adequate diet for astronauts is always a top priority, there is always a concern with deficiencies, especially in the case of an emergency
astronaut holding first aid kit


While any trauma can cause concern, trauma in isolation with limited supplies greatly increases it,  and you can’t get much more isolated with limited supplies than in space. Even from the relatively close proximity of the ISS at an average altitude of 250 miles (400 km), trauma or other medical emergencies in space are a major concern. In addition to those basic concerns, the microgravity environment creates other challenges, affecting how blood behaves and the ability of other astronauts to provide care.

So far, there has been no major injury or surgery on the ISS/ in space that would require first-aid and emergency medical intervention in space, but it is still a concern and procedures are in place for this possibility. Research continues to evaluate potential solutions to these concerns, especially as our space exploration sights expand to the Moon and Mars as these longer-duration flights at further distances amplify these worries.

Continued research with astronauts on the ISS and the astronauts that will be going to the Moon in the next few years will help NASA and other space agencies to better understand and prepare astronauts for longer space exploration missions such as to Mars. ISS astronauts today are testing solutions for going to the Moon and future lunar astronauts will test solutions for going to Mars.

Bleeding in space is a major concern and rightfully so, but this research will help us to continually provide better solutions and procedures for astronauts today and those who will inevitably need them someday.

If you are interested in learning more about the human body in space, the various concerns of keeping astronauts safe, and the history of this research, I highly recommend Mary Roach’s book Packing for Mars. While some of the information is a little outdated as it was published in 2010, it still provides a wonderful foundation for these topics.

Sarah H.

Sarah Hoffschwelle is a freelance writer who covers a combination of topics including astronomy, general science and STEM, self-development, art, and societal commentary. In the past, Sarah worked in educational nonprofits providing free-choice learning experiences for audiences ages 2-99. As a lifelong space nerd, she loves sharing the universe with others through her words. She currently writes on Medium at and authors self-help and children’s books.

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