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?

Question

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

Answer 1

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.