How Digestion Works
Every meal takes a one-way trip through a muscular tube about 30 feet long, and the chemistry shifts dramatically along the way. Press 🍽️ Eat a meal and follow the bite: salivary amylase starts on starch in the mouth, waves of muscle called peristalsis push it down the esophagus, the stomach churns it in acid near pH 2 with pepsin, then bile from the gallbladder and pancreatic enzymes take over in the small intestine, where villi absorb the freed glucose, amino acids, and fatty acids into the blood. Watch the live pH graph swing from acidic to neutral as the meal moves — and switch the meal type to see fat pull in more bile.
What's happening
The Science in Plain Language
1. Digestion is two jobs at once. Mechanical digestion physically breaks food into smaller pieces — teeth grinding, the stomach churning, the intestines squeezing. Chemical digestion uses enzymes and acid to split large molecules into small ones your body can absorb. The three macronutrients each have a target end-product: carbohydrates → glucose (and other simple sugars), proteins → amino acids, and fats → fatty acids + glycerol. Only those small molecules are small enough to cross the gut wall.
2. Mouth and esophagus (pH ~6.7). Chewing grinds food and mixes it with saliva, which carries the enzyme salivary amylase that begins breaking starch into maltose and shorter sugars. When you swallow, the food ball (a “bolus”) is pushed down the esophagus by peristalsis — traveling rings of muscle contraction. It is not gravity doing the work: peristalsis is strong enough to move food even if you are upside down.
3. The stomach churns in acid (pH ~2). The stomach is both mechanical and chemical. Muscular walls churn the food while hydrochloric acid drops the contents to about pH 2 and the enzyme pepsin (which only works in that acid) starts dismantling proteins into peptides. The acid also denatures proteins and kills many swallowed microbes. After a few hours the meal becomes a soupy mix called chyme, released a little at a time through the pyloric sphincter into the small intestine.
4. The small intestine — neutralize, then absorb (pH ~6→7.4). As acidic chyme enters the duodenum, the pancreas pours in bicarbonate that neutralizes the acid (raising the pH from ~2 toward ~6), so its enzymes can work. The gallbladder squirts bile (made by the liver) that emulsifies fat — breaking large fat globules into tiny droplets so enzymes can reach them. Pancreatic enzymes finish the job: amylase on carbohydrates, proteases (trypsin, chymotrypsin) on proteins, and lipase on fats. The inner wall is carpeted with millions of finger-like villi (and microscopic microvilli), giving a surface area the size of a tennis court. Absorbed glucose and amino acids pass into the bloodstream; most fatty acids are packaged into chylomicrons and enter the lymph first. Regional specialists matter too: iron is taken up in the duodenum, while vitamin B12 and bile salts are reclaimed in the ileum.
5. The large intestine and the microbiome (pH ~6.7). What is left is mostly water, electrolytes, and indigestible fiber. The large intestine reabsorbs the water and salts. It is also home to trillions of gut bacteria — your microbiome — which ferment fiber into short-chain fatty acids that feed the cells lining the colon, and which make vitamin K and several B vitamins. What remains is compacted into stool and eventually eliminated. The whole journey takes roughly one to three days, most of it spent in this final stretch.