At ZOE, we’re fascinated by the gut. We’re colon converts. We’re lovers of the lower intestine. We’re bowel believers, esophageal evangelists, and staunch stomach supporters.
Your digestive system is an incredible network of pipes, glands, nerves, and other structures.
It works in concert with your central nervous system, hormones, and other chemical messengers.
Your gut helps you stay alive and thrive. We don’t give it much thought, but the work it does is almost magical.
Today, we’re going to discuss one of its near-magical skills: How your butt tells the difference between gas and solid.
Fart or poop?
We all need to pass gas. We also need to poop. You’ve likely noticed that these leave your body via the same exit.
Telling the difference between the two is incredibly important, especially in public. Mistakes do happen, but they're uncommon thanks to the competence of your rectum and anal sphincter.
If you think about it, you can nearly always tell what it is. And, by and large, we are confident enough to pass gas without incident. So, how does your butt know?
To get to the bottom of this question, we spoke with Dr. Will Bulsiewicz, ZOE’s U.S. medical director and a board-certified gastroenterologist.
When we asked him how your butt tells the difference between poop and wind, he said: “People don't think or talk about this enough. And they should! I love this shh… shtuff.” So, let’s get into the details.
To fully appreciate what’s going on, we need to get into some anatomy. Thankfully, Dr. B is well-versed.
Your gastrointestinal tract
Your gastrointestinal (GI) tract is the tube that runs from your mouth to your anus.
When you're eating and you swallow, the food enters your esophagus, which takes it down to your stomach. After a few hours, the food moves into your small intestine, then eventually into your large intestine.
Once the food has passed through the length of the large intestine, digestion is done.
The last two sections of your large intestine are the rectum and anal canal. The rectum is like a storage area for poop. And muscles in its walls help you poop when the time is right.
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Below the rectum sits the anal canal, which connects your rectum to your anus — the hole through which poop exits your body.
Your anal canal, the final section of your GI tract, will be the main character in this tale.
Next, we’ll discuss sphincters.
What are sphincters?
A sphincter is a ring of muscle that can open and close.
You have them throughout your body. For instance, there are sphincters at the top and bottom of your stomach to control when food enters and leaves.
The pupil of your eye has a sphincter around it to control how much light enters. And microscopic sphincters help control blood flow in your capillaries.
Two closely related sphincters are involved in pooping. Dr. B explains:
“We have two layers to the sphincter. The external is our squeeze muscle. Like when you decide to squeeze your cheeks to stop a poop from coming out, you are activating the external anal sphincter."
"It's under voluntary control, so we choose whether it squeezes or relaxes.”
“The internal sphincter,” he continues, “is under involuntary control."
"It provides our basal tone, meaning that when we're not thinking about squeezing our cheeks, our internal sphincter is hard at work maintaining the butt barrier so nothing accidentally sneaks out.”
OK, so we’ve got a little anatomy under our belts. Let’s move on to the next phase.
The poop reflex
“When the rectum fills beyond a certain capacity,” Dr. B explains, “the rectal walls are stretched, triggering the defecation cycle. This begins with the rectoanal inhibitory reflex [RAIR], where the internal sphincter relaxes.”
Now we can partially address the question: How do you know if it’s gas or solid? Take it away, Dr. B:
“This RAIR reflex is thought to allow a small amount of rectal contents to descend into the anal canal, where a specialized mucosa samples whether it is gas, liquid, or solid.”
According to research, this sampling happens several times every hour.
Using that information, Dr. B explains, you can decide whether to “initiate passage of that material.”
What’s a mucosa?
Mucosas, or mucous membranes, line the insides of some of your body’s cavities and organs, like your stomach, mouth, and anal canal.
They contain glands that produce mucus, hence the name.
Two of their primary tasks are making sure surfaces don’t dry out and protecting against pathogens.
So, in a nutshell, a specialized mucosa in your anal canal “samples” what’s in your rectum and decides whether it's safe to release.
This brings us closer to answering our initial question. But we still have one more query before the picture is complete: Exactly how does the mucosa differentiate between gas and poop?
How does the mucosa know?
Disappointingly, scientists still can't say exactly how the mucosa differentiates between poop and gas. And there’s been little research into this question, sadly.
Below, we’ll describe some of these nerve endings because this information provides insight into how the final section of your GI tract is so sensitive.
You’ll never need to remember these names, but they’re quite fabulous:
Meissner’s corpuscles: These are incredibly sensitive to fine touch and vibration. They mostly exist on skin without hair that’s sensitive to touch, like your fingertips and the palms of your hand.
Golgi-Mazzoni bodies: These are like smaller versions of Pacinian corpuscles, and they detect changes in tension or pressure. They’re mostly found on your fingertips.
Genital corpuscles: These are similar to the Krause end-bulbs and mostly exist on the penis and clitoris. They likely respond to friction or vibration.
This collection of sensitive detectors helps us understand how our anal canal can differentiate between gas and poop: We would also have no trouble telling the difference if we used our fingers, for instance.
Meanwhile, your anal canal is also sensitive to temperature changes.
Some scientists think that detecting the “thermal capacity of rectal contents” might help discriminate between solids and gas.
To recap, once your anal canal has sampled the contents of the rectum and compared all the sensory signals, it decides whether there's gas or poop in the rectum. This information makes it to your brain.
Then, you can make an informed decision whether to relax your external anal sphincter and release the beast. So, now you know.
Having delved quite deeply into this topic, we have a deeper appreciation for the GI tract’s exit point.
We’re also marveling at how your brain can interpret these signals so well, rarely making a messy error.
Thanks for joining us on this adventure. We hope you’ll agree that it’s pretty amazing.
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