Eli5: How does the colon “know” what to absorb. Seeing as it can absorb dissolvable content inserted there. What prevents it from just reabsorbing all the fecal content?
In: Biology
Think of your digestive system as a really large puzzle. When food and nutrients come in that your body needs, they “fit” into the puzzle. All the food and fiber that is not needed to fill out the puzzle is the remaining fecal content. It’s not absorbed because there are no “puzzle pieces” to fit it to, I.e it’s not needed for a biological function.
Poop is what’s left *after* the colon absorbs what it can. It doesn’t “know” how to do anything, it just absorbs what’s absorbable until it reaches equilibrium with whatever that component is in the lining.
Think of it like a colander.
When you boil pasta, you throw the pasta and water into the colander. How does the colander know what to keep and what to let pass through?
The colander doesn’t know anything. It’s all based on the shape of the holes, and the size of the noodles.
Your colon is like that. Kinda. Some things pass through, some things get caught. The stuff left behind is what we call poop.
The intestine is not purely a passive membrane. The cells in the intestinal wall do actively pump substances across and those pumps are directional.
You don’t really need a colon. Your colon just kinda sucks out the last bit of water that’s left from the small intestine.
How do I know this?
Two months ago I had 3/4 of my colon removed because of hundreds of polyps inside of it. Luckily, none of them were cancerous, yet. Eventually one or more would have if not removed.
A week before the surgery, my gastroenterologist did a colonoscopy, and went up the colon until he found the last polyp. He then tattooed that area of the colon to mark where the surgeon had to make his cut. It was done laparoscopically and left me with about 8” of colon. I’ve had to adjust my diet somewhat to cut back on fiber , or else I have diarrhea all the time. Even if they took all the colon, I could still live a somewhat normal life without having a bag. I do have to have yearly colonoscopies to check to see if more polyps start growing in what’s left.
Sadly, the colander theory is correct. The colon does NOT know what it is absorbing, only that the particles have the “right” shape. How do I know? I was diagnosed with the elegantly named condition, “Leaky Gut Syndrome” many years ago. My colon had been damaged (mostly by stress, it turns out) and was actively absorbing particles that should not have been able to cross the intestinal barrier. Those substances were then degrading my health. I took a year off work and did a whole program to recover. If you think you may have this syndrome, see a naturopath for help. Medical Doctors are useless, because there is no drug or surgery that will help, so there’s “nothing they can do”. It requires lifestyle changes (sometimes drastic). Which they don’t exactly believe in as a “cure” for disease.
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