A 9-meter tube with helper organs that breaks food into molecules small enough to absorb.
How it works, step by step
Mouth: chewing and salivary amylase begin breaking starch.
Esophagus: muscular waves move food down.
Stomach: acid and pepsin unfold and cut proteins.
Liver & pancreas: bile emulsifies fat; enzymes digest carbs, protein and fat.
Small intestine: most nutrients are absorbed through villi.
Large intestine: water is absorbed; bacteria ferment fiber.
Timing
Food usually takes several hours to leave the stomach and small intestine; transit through the colon takes much longer and varies widely.
When to get help
Blood in stool, unexplained weight loss, or persistent changes in bowel habits are worth discussing with a clinician.
Evidence & sources
Most nutrient absorption happens in the small intestine. Evidence: established. Known: Foundational physiology. Unknown: — Source: NIH NIDDK, Your Digestive System & How It Works (Current revision).