Most of us only think about digestion when something goes wrong: heartburn after a late dinner, a few days of constipation, a meal that seems to sit in the stomach all afternoon. The rest of the time it runs quietly in the background, a chain of muscle contractions, digestive juices and nerve signals that turns a plate of food into molecules small enough to pass into the blood.
The organs involved
Digestion relies on two groups of organs. The first is the gastrointestinal (GI) tract, a hollow tube that runs from the mouth through the esophagus, stomach, small intestine and large intestine to the anus. Stretched out, it would measure about 30 feet, or 9 meters, according to Cleveland Clinic. The second group is the solid organs that feed digestive juices into that tube: the liver, the gallbladder and the pancreas, linked to the intestine by bile ducts. Together these are sometimes called the biliary tract.
The point of all this machinery is to break food down far enough for the body to absorb it. As the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) explains, proteins are broken into amino acids, fats into fatty acids and glycerol, and carbohydrates into simple sugars. The body uses those pieces for energy, growth and cell repair.
The mouth
Digestion begins before the first bite. When you see or smell food, the brain signals the salivary glands, and your mouth waters. Chewing breaks food into smaller pieces while saliva moistens it. Saliva also carries an enzyme that starts breaking down starches, so carbohydrate digestion is already under way by the time you swallow.
Swallowing starts as a deliberate act, but once it begins, the process becomes automatic. A small flap of tissue called the epiglottis folds over the windpipe to keep food out of the airway.
The esophagus and stomach
Food travels down the esophagus by peristalsis. The muscle behind each mouthful contracts and squeezes it forward, and the same wave-like movement carries food through the rest of the gut. At the bottom of the esophagus sits a ring of muscle called the lower esophageal sphincter. It relaxes to let food into the stomach and otherwise stays closed, which keeps stomach contents from flowing back up. When acid does get into the esophagus, the result is the indigestion and heartburn of acid reflux.
In the stomach, glands in the lining release acid and enzymes, and the stomach muscles churn food together with these juices. This is where protein digestion gets going. The stomach then passes its contents, now a thick mixture called chyme, into the small intestine a little at a time.
The small intestine
The small intestine is the longest part of the GI tract, a muscular tube about 22 feet long, coiled in the lower abdomen. It has three sections: the duodenum, the jejunum and the ileum. This is where digestion is finished and where most nutrients are absorbed.
Several digestive juices meet here. The pancreas supplies one with enzymes that break down carbohydrates, fats and proteins. The liver makes bile, which helps digest fats and some vitamins. Between meals the gallbladder stores bile, and when you eat it squeezes bile through the bile ducts into the intestine. The small intestine adds a digestive juice of its own, and even some of the bacteria living there contribute, making enzymes needed to digest carbohydrates.
Water moves in both directions. The small intestine draws water from the bloodstream into the gut to help break food down, then absorbs water back along with the digested nutrients. Specialized cells help nutrients cross the intestinal lining. From there, the blood carries simple sugars, amino acids, glycerol and some vitamins and salts to the liver, which stores and processes them and sends them on to the rest of the body as needed. Fatty acids and some vitamins take a separate route, through the lymph system.
The large intestine
By the time material reaches the large intestine, most of the useful nutrients have been taken up. What's left is mainly undigested food, fluid and old cells shed from the lining of the gut. The large intestine, which includes the cecum, appendix, colon and rectum, absorbs more water and turns this liquid waste into stool. Peristalsis pushes the stool into the rectum, which holds it until a bowel movement.
Bacteria in the large intestine also break down some of the remaining nutrients and make vitamin K. That community of microbes, the gut microbiome, gets its own explainer: what the gut microbiome is and what it does.
How nerves and hormones run the process
Hormones and nerves work together to control digestion. Cells lining the stomach and small intestine release hormones that tell the body when to make digestive juices, and that send signals to the brain about hunger and fullness. The pancreas makes hormones that matter for digestion as well.
The nerve supply has two parts. Some nerves link the digestive system to the brain and spinal cord, and that connection is how the sight or smell of food sets off saliva. The other part is the enteric nervous system, a web of nerves inside the walls of the GI tract itself. These nerves release substances that speed up or slow down the movement of food, and they signal the gut muscles when to contract and when to relax.
Everyday habits that support digestion
Cleveland Clinic's advice starts with water, which helps food move more easily through the digestive system. How much you need depends on your size, your metabolism and how active you are, so there isn't one target that suits everyone.
Cleveland Clinic also suggests building fiber into balanced meals alongside lean protein, fruits and vegetables to keep the system working as it should. Fiber matters to the bacteria in the colon too. Our piece on what shapes your gut microbiome covers which foods supply it and why it's worth adding slowly if you aren't used to much.
Mindful eating, which simply means paying attention to how and why you're eating, helps people slow down and chew food thoroughly. Chewing is the first step in breaking food down, and it's a step that is entirely under your control.
Alcohol can increase the amount of acid in the stomach and can cause heartburn, acid reflux and stomach ulcers. Smoking almost doubles the risk of acid reflux.
Stress is harder to pin down. Cleveland Clinic describes it as associated with constipation, diarrhea and irritable bowel syndrome (IBS), which describes a link rather than a proven cause. Still, if your gut reliably acts up during hard weeks, that pattern is worth mentioning to a clinician.
Probiotics appear on Cleveland Clinic's list as well. They're a bigger subject than they look, and the piece on what shapes your gut microbiome goes through what is and isn't known about them.
When to have symptoms checked
Everyone's digestion has off days. Cleveland Clinic's guidance is to give an upset system a few days to recover, but to contact a healthcare provider about any symptom that lasts more than a few days, gets worse, or goes away and comes back. Severe pain, or anything that frightens you, is a reason not to wait.
One reason for that advice is that abdominal pain, constipation, diarrhea, nausea and vomiting are shared by most digestive disorders, so the symptom alone rarely points to the cause. Cleveland Clinic's list of common conditions runs from acid reflux, stomach flu and hemorrhoids to celiac disease, inflammatory bowel disease, stomach ulcers and diverticulitis, and it also includes cancers of the colon, esophagus and liver. Sorting these out takes a clinician, and sometimes blood tests or a biopsy. Changes to what and how you eat can ease some symptoms, but they're no substitute for a diagnosis when something keeps coming back.
This article is for general education only and isn't medical advice, so please talk to a doctor or other qualified clinician about any symptoms or before making changes to your diet or medicines.
Sources
- Your Digestive System & How It Works, National Institute of Diabetes and Digestive and Kidney Diseases
- Digestive System, Cleveland Clinic