I guess you could call this a shower thought but out of the blue I thought about this. How does our stomach acids not float up our throats in zero G? I’m not familiar with how our bodies work in space and would love to hear an explanation
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Your stomach acid isn’t kept in your stomach only by gravity. (You can tell by the fact that you can do a handstand without throwing up.)
Instead, there’s a valve called the [lower esophageal sphincter](https://en.wikipedia.org/wiki/Esophagus#Sphincters) that keeps your stomach contents in your stomach. (“Sphincter” is an anatomical word for “a muscular valve that can open and close”; the common usage of the word to mean the *anal* sphincter is more specific than the medical use.)
Even here on Earth, when this sphincter malfunctions, stomach acid can work its way up your throat, causing minor tissue damage and irritation – you know this condition as *heartburn* (or more precisely as acid reflux, which is the most common cause of heartburn).
A biological process called peristalsis. Although we feel no sensation of it happening, there are smooth muscles along our digestive system that involuntarily contract in a wavelike sequence. These contractions act like our hands on the outside of a tube of toothpaste, pushing the contents of the tube farther down the tube. Thanks to peristalsis, we don’t need gravity to eat or drink. This is important, even on Earth, because our digestive system isn’t always vertical and for many of our animal colleagues, much of it is horizontal.
But, not all animals have peristaltic capability and some thus struggle in zero-G. The Russians learned this the hard way trying to raise quail on the space station Mir. If you observe most birds eating or drinking, you will see that they tilt their beaks upwards to swallow their food or drink, so that gravity will cause the substance to go down their digestive tract. An exception is pigeons, they can eat with their heads down.
Hmmm. Reading the responses in here it seems like you would tend to get heartburn more often in space all things being equal. And you can’t use the trick or sleeping on your left side to help.
For the same reason that you can eat and swallow while upside down.
Food is pushed down your throat and into your stomach by peristalsis (squeezing of the throat to push the food towards the stomach). There are valves at both ends of the stomach that ordinarily keep food in (the one to the intestines opens when the food is ready for its nutrients to be absorbed into the bloodstream). Of course, the top valve can be reopened (stomach pump, vomiting etc.) but gravity on its own won’t cause acid to float up from your stomach.
How does our stomach acids not float up our throats in zero G?
the short answer is…. stomach acid *does* float up into the esophagus in zero G.
Acid reflux and heartburn are a persistent problem for astronauts, for exactly the reasons you’ve described. without gravity, acid is more likely to find it’s way up in to the esophagus. [Burping is also a problem](https://www.cbsnews.com/news/astronauts-share-what-its-really-like-in-space/), because a burp can easily lead to vomiting in zero gravity.
How do astronauts deal with it? They take acid blocking medicine like you’d find in any pharmacy, and they try not to burp.
I feel like it’s worth mentioning… space is chalk full of gravity. Working in space and working in orbit are different things. When orbiting, you experience something very close to zero G because you’re in constant free fall towards the earth but gravity is very much there (causing you to do the free falling). It’s about 9/10ths of the gravity you feel standing on the ground, you just aren’t fighting against it like you do on the ground.
It’s the same experience if you’ve ever been on a free-fall amusement park ride like Disney’s guardians of the galaxy. It’s just falling, albeit for a much longer time.
Your stomach, bladder, etc have nice little doors on them, but other fluids generally get pushed “up” towards you head, and can cause problems with vision. NASA uses compression cuffs and a lot of monitoring to keep fluids where they are supposed to be.
From NASA, “human exposure to microgravity results in changes to… renal function, fluid redistribution, bone loss, and muscle atrophy, all of which contribute to an altered urinary environment and the potential for renal stone formation.”
Surprisingly, one of the least worried about things is stomach acid.
This is useful here:
https://www.nasa.gov/content/gravity-who-needs-it/
The movement of stomach acids and other digestive fluids in the human body is primarily controlled by muscular contractions and valves rather than gravity. Therefore, even in zero gravity or microgravity environments, the digestive process can still occur effectively without the acids rising up.
In a normal gravity environment, such as on Earth, gravity helps in keeping the stomach acids and contents in the stomach by pulling them downward. The lower part of the esophagus, called the lower esophageal sphincter, acts as a valve that prevents the backward flow of stomach acids into the esophagus. This sphincter remains closed most of the time, except when we swallow food or liquid.
In zero gravity, the absence of gravitational force doesn’t affect the functioning of the esophageal sphincter or the muscular contractions responsible for moving food and digestive fluids through the digestive system. The rhythmic contractions, known as peristalsis, help push food downward through the esophagus and into the stomach. Similarly, the stomach muscles contract to mix the food with digestive enzymes and acids.
While some astronauts experience temporary effects like space sickness or changes in digestion during their initial adaptation to microgravity, the digestive process continues to function relatively normally. The body’s natural mechanisms, including the muscle movements and sphincter functions, work to keep the stomach acids in place and prevent them from rising up into the esophagus.
However, it’s worth noting that prolonged exposure to microgravity or space travel can have various effects on the human body, including changes in bone density, muscle mass, and other physiological functions. These changes are areas of active research for scientists studying the effects of space travel on human health.
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