“Lots of people understand that when we eat food, our bodies extract the nutrients and waste gets expelled. But it’s so much more interesting than that!” exclaims Xiao Jing (Iris) Wang, M.D., an assistant professor and general gastroenterologist at the Mayo Clinic in Rochester, MN.
(She’s even written a children’s book, Boo Can’t Poo!—the story of a constipated ghost.) “Different portions of your gastrointestinal tract are receiving input from and giving input to the brain all the time, in a complex back-and-forth. The bowels even have their own nervous system that can function independently.”
After you eat, food and digestive juices move from your stomach to your small intestine, and water and nutrients are absorbed through your intestinal walls into your bloodstream. Anything that remains is pushed into the large intestine, where more water and electrolytes are absorbed. What’s left is poop, a.k.a. stool, which is stored in your rectum until you’re ready to have a bowel movement.
“The presence of stool in the rectum sends a signal to the brain that says, ‘I have to go! Tell the rest of the person it’s time to find a bathroom!’ ” explains Dr. Wang, adding that it also asks the brain to hold off until you get there. When you finally get seated, your pelvic floor coordinates with the brain and the puborectalis muscle to relax the anal sphincter and let it open—and then, as Dr. Wang dramatically puts it, “Ta-da!” “
Posted May 5, 2025, 1:59 AM UTC