Digestive System: How It Breaks Down Food, Step by Step

Mouth: the first stop

It all starts with chewing: teeth cut food into smaller pieces while the tongue works it together with saliva. That’s the mechanical side of the job, the first to act on food.

Saliva isn’t just water: it already carries two enzymes. Salivary amylase, sometimes called ptyalin, breaks starch down into maltose and maltotriose, working best at a pH between 6.7 and 7.0 according to StatPearls (2022); lingual lipase, meanwhile, starts working on fat. It’s only a partial job — most of the digestion still lies ahead — but it begins here, before you’ve even swallowed.

Stomach: acid and churning

In the stomach, food gets churned by muscular contractions that StatPearls (2023) describes as a back-and-forth motion, driven by the smooth muscle in the stomach wall — the same tissue type covered in the muscular system Recap.

Gastric juice has a pH of about 1.0 (StatPearls, 2023), among the most acidic fluids the body makes. Hydrochloric acid, secreted by the stomach’s parietal cells — the same basic units described in the cell Recap — switches on pepsinogen, an inactive enzyme form, turning it into pepsin. Pepsin is the enzyme that starts cutting proteins into smaller polypeptides. Stomach emptying into the intestine isn’t constant: according to StatPearls (2023), it slows down when fat and acid show up in the duodenum, and also during stress or exercise.

Real-world example: a freshly chewed steak enters the stomach still as whole proteins and fat; it leaves as an acidic mixture in which the proteins have already been cut into smaller fragments, ready for the small intestine to finish the job.

Liver, pancreas, and gallbladder: the outside suppliers

The liver, pancreas, and gallbladder aren’t technically part of the digestive tract, but they connect to it through ducts and feed in the substances that make much of chemical digestion possible.

Bile’s main job is emulsifying fat; what happens to that fat once it’s absorbed is covered in the body fat Recap. About 80% of pancreatic tissue is exocrine, meaning it’s dedicated to producing the digestive juice that reaches the duodenum through the pancreatic ducts, according to a StatPearls review updated in December 2025; the rest is endocrine tissue that makes insulin and glucagon, released directly into the bloodstream. The pancreas’s protein-digesting enzymes — trypsinogen, chymotrypsinogen, procarboxypeptidase, and proelastase — leave the gland already switched off, as zymogens, and only become active once they reach the duodenum. That’s one reason the pancreas doesn’t digest itself despite making enzymes capable of breaking down protein.

Small intestine: where the work finishes

The duodenum, the first section of the small intestine, receives enzymes from the liver, pancreas, and gallbladder through the ducts that feed into it, according to StatPearls (2024).

Further along, in the jejunum, the intestinal wall contracts in a pattern called segmentation, driven by the myenteric plexus: it mixes food with enzymes rather than simply pushing it forward, according to the same source. Meanwhile, brush-border enzymes — maltase, isomaltase, sucrase, β-galactosidase, and trehalase — finish breaking down sugars, alongside trypsin, chymotrypsin, elastase, and carboxypeptidase A and B, the active forms of the protein-digesting enzymes that arrived from the pancreas. The result: the small intestine, averaging about 7 meters (StatPearls, 2024), absorbs 95% of the nutrients in food, in a process that can take up to 5 hours to run its full course.

Large intestine: water, vitamins, and gut bacteria

By the time undigested material reaches the large intestine, the small intestine has already absorbed most of the nutrients and up to 90% of the water, according to StatPearls (2023).

The large intestine, about 1.5 meters long (StatPearls, 2023) — roughly a fifth of the entire gastrointestinal tract — has three jobs: absorbing water and electrolytes, producing and absorbing vitamins, and forming stool and moving it toward the rectum. The bacteria living there, the colon’s gut microbiome, make vitamin K and B vitamins such as biotin as a byproduct of their own metabolism. The same basic principle — microorganisms transforming organic material into new substances — is what drives fermentation, though in the colon the result stays inside the body instead of ending up in a jar.