You might have noticed that if you have watched a space flight landing, the astronauts don’t climb out of the space capsule they land in. Usually, the ground crew will assist them or even carry the astronauts out of the spaceship. Is this because their muscles are too atrophied to walk, or is something else going on?
After returning to earth, astronauts are more likely to faint than usual due to low blood pressure from plasma loss in microgravity. Microgravity also disrupts the vestibular system, relying on gravity-induced inner ear signals to the brain for balance, vision, and motor control. The vestibular system is responsible for space sickness until the brain adapts to space, and upon the return to earth, it needs to adapt again to the change in gravity.
Muscle atrophy and loss have minimal impact on the need for assistance when landing. NASA has a good and comprehensive exercise program that minimizes this and keeps the astronauts in good condition.
I did a little research into this because I always thought astronauts have trained ahead of time and had to be medically qualified to minimize these types of problems. I’ll go over some of the main things I learned in the rest of this article. If you want to go into a deep dive into this information, I found a lot of useful information in this article(NASA PDF)
Why Does the Vestibular System Cause so Many Problems?
I am not a medical professional and got most of the information in this article from NASA research papers scattered around the internet(Like the one in this link), so please excuse my simplifying the subject. My wife has vertigo problems, so I am way more familiar with the inner ear than I want to be. This video gives an excellent description of how it works.
The Vestibular system is how your body knows which way is up or down, detects motion, acceleration, and helps maintain balance and coordination. If you close your eyes, you know where the ground is. If you spin around in a circle, kind of like my four-year-old is right now; you’ll be disoriented, and if you close your eyes, you might even fall.
Your inner ear has three semicircular canals that are at right angles to each other and are filled with liquid. When you move your head(nodding up and down, for example), the fluid in the appropriate canal flows past stereocilia. Stereocilia are basically a set of hairs with a stand connecting them. The liquid flow stimulates the nerve attached to the hair, telling your brain that you moved your head.
Another part of your inner ear has crystals of calcium carbonate that shift around due to head movement, again moving hairs that sent signals to your brain.
This all combines into the vestibular-ocular reflex, which keeps your eyes focused on an object even when you move your head.
Microgravity screws everything up in this process that your brain uses to figure out which way is what. When you look down, for example, while in microgravity, your eyes see the movement, but your brain does not get the expected response from your inner ear. This conflicting signal is probably the primary cause of space sickness for astronauts.
The brain takes a couple of days to adapt to the sensory input disaster, and the space sickness goes away. Returning astronauts are much less likely to suffer from space sickness because their brain has already run across this before and adapts faster.
Returning to earth has the same effect. The astronaut’s brain is used to the new way of doing things in microgravity, with a different response from the vestibular system. Once back at normal gravity, it causes the same conflict with expected inputs, and the astronaut needs a couple of days to return to normal.
Quick head motion, standing with your eyes closed, catching a ball, and especially driving a vehicle all need to be done with care or not at all until they readjust to gravity. It’s a lot like a sailor returning from an ocean voyage getting their land legs back.
The flat ground seems to be rolling a bit because your brain adapted to the motion of the ocean. My longest record for getting back to normal after a long trip at sea was about two days.
Astronauts are assisted in exiting the spacecraft to avoid injuries from the changes in motor control and balance. Our government spends enough money on them to want to avoid injury after landing.
Typically astronauts should avoid driving, sports, or even standing on the sand for a couple of days after returning to earth. The vestibular system returns to normal quickly, and the more times the brain is exposed to the different conditions, the faster it can adapt.
The vestibular effects on balance and motor control are not as severe as the effects of blood volume loss in space.
How can Blood Plasma Possibly be a Problem?
One surprising fact that I learned is the response of your body’s vascular system due to microgravity. The vascular system is very basically your heart, blood vessels, and lungs working together to keep enough blood flowing to your brain to read this article.
Changes in body position, walking, stress all require different blood flow and pressure, which your body regulates automatically.
In space, a change in position doesn’t require a change in your vascular response to get more blood to your head because blood tends to pool in your upper body in microgravity.
This causes your heart to have to work less hard during normal rest conditions. NASA thinks that this tricks the body into thinking it has too much blood leading to an average loss of 15% of blood plasma.
This plasma loss caused a big problem during landing back on earth. Since the astronaut has 15% less blood volume than before, blood pressure is much lower, and astronauts are in danger of fainting. This is referred to as orthostatic intolerance, with a quarter of astronauts feeling light-headed after landing when they stood up.
The good news is that we have many highly competent scientists, researchers, and doctors working for NASA studying and trying to mitigate the problems with orthostatic intolerance since the beginning of the space program.
Right before landing, astronauts drink a lot of salty liquids to boost blood volume. NASA has experimented a lot to determine the right mix to use. This might also contribute to the need for diapers when landing. (read my article on Do Astronauts Wear Diapers?)
Compression suits also are used to keep blood from pooling in the legs and lower body of astronauts during landing. This basically has air bladders that compress the legs to direct more blood towards the brain.
Women seem to be more seriously affected than men for lightheadedness. NASA continues to investigate and, in the meantime, will protect its astronauts by, if possible, assisting them when exiting the spacecraft.
Resources
https://www.neuroscientificallychallenged.com/blog/know-your-brain-vestibular-system