system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - Why is it if someone puts drugs in their rectum it quickly enters their bloodstream, but people have feces in the same area all the time and it doesn’t go into their blood and make them sick?
gpt
Three things.

A) you have a giant ecosystem of bacteria living in your gut. They protect you by killing foreign bacteria and they help you digest your food.

B) you have a very strong immune presence in your gut and you even have a whole nervous system dedicated to it called the enteric nervous system. These two coordinate to prevent infection.

C) things don't just diffuse through your gut to your blood. There's a very tight barrier. The cells have what you may call a biological seal between them preventing things from passing through. While it can't prevent small things like some ions and water and so on (depends on the region of the gut) it most certainly blocks big things like bacteria. Things that get to your blood usually have to go the transcellular path (as opposed to paracellular). This means the cells lining up your gut take them up actively or passively at the front door (apical or luminal side) and they go out the back door (basolateral side) where the blood vessels are.

Depending on the drug, some passively take the paracellular path as they're small and their properties permit it. Others passively pass through the membrane of the cells due to their chemical properties or they get taken up by the cells like nutrients. So it's a very different thing. Pathogens are big, they can't go about unnoticed. And waste products that the body doesn't want, have no specialized way to cross to the blood, that's why they even stay out in the lumen (inside of the gut tube).