Most of us don’t think much about our digestive system unless it’s causing problems. An upset stomach, vomiting or even just a growling hungry tummy can draw our attention to this part of our body and make us wonder why it isn’t working quite the way it should. The digestive system is our body’s workhorse—it gets nourishment out of the foods that we eat. You might wonder exactly how that happens.

Digestion simply means the breakdown of food. The process of digestion starts with our mouth and involves a number of other organs and chemicals, including the esophagus, stomach, pancreas, liver, gallbladder and small and large intestines.

The first step in digestion happens when we put food in our mouths. Chewing food starts to break it into smaller pieces, and enzymes in our saliva start to dissolve it further. When we swallow, it travels down the esophagus to the stomach where the stomach acids help turn food into even smaller particles so the body can absorb nutrients into the bloodstream. These acids also help protect us by creating an environment that’s unfriendly to organisms—like bacteria—that might make us sick. However, not all bacteria in our digestive system are bad. Scientists are learning more about the many different kinds of bacteria in our digestive system (called the gut microbiome) that actually help keep us healthy. 

From the stomach, the food—or what’s left of it, now transformed into a substance called chyme—enters the small intestines. Most of the usable parts of food—broken-down fats, carbohydrates and proteins—are absorbed into the bloodstream from the small intestines and then are delivered to other parts of the body. After the small intestine comes the large intestine, where liquid or water is absorbed and our waste product becomes dehydrated. When we have diarrhea, it’s because for whatever reason, the large intestine hasn’t absorbed enough fluid, which leads to watery poop. 

Between the entry of food into our mouth and the excretion of waste in the bathroom, other organs assist in the digestive process. The pancreas makes enzymes that help with digesting proteins, carbohydrates and fats. The liver produces bile, which also helps to break down fats similar to the way in which dish soap dissolves grease on a dirty pan. Bile is stored in the gallbladder, another digestive organ, before it’s released into the small intestine. 

Disruptions in the digestive system that make us feel sick can be an annoyance but, in most cases, are not life-threatening. However, these symptoms do mean the body is having trouble getting the nutrients it needs out of the foods we eat. If you have an upset stomach, nausea, vomiting or diarrhea that doesn’t go away, talk to your doctor.

Anne R. Crecelius, PhD, (1985–2023), was a professor in the Department of Health and Sport Science at the University of Dayton in Dayton, Ohio. She was a longtime APS member, and a tireless leader, mentor, researcher and educator.