How does the stomach digest food?
Your stomach is a muscular bag of acid strong enough to dissolve meat, yet it doesn't digest itself. Swallow a bolus, cut the stomach open, watch it churn food into chyme three times a minute, and meet the germ a doctor drank to win a Nobel Prize.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- A 25 cm tube and a J-shaped bag. Food goes from the mouth down the oesophagus, about 25 cm long, through the diaphragm into the stomach, which sits under the left ribs. The stomach has a fundus, a body, an antrum and a pylorus, a ring of muscle at the exit. Its wall has a folded lining and three layers of muscle.
- Swallowing is a squeeze, not a slide. Chewing mixes food with saliva, whose amylase starts on starch. When you swallow, the epiglottis seals the windpipe for about a second, then a peristaltic wave carries the bolus down in about 5 to 10 seconds. It even works upside down.
- Acid that doesn't burn its maker. Parietal cells use proton pumps to make hydrochloric acid, keeping the stomach at about pH 1.5 to 3.5. Chief cells make pepsinogen, which acid turns into pepsin to cut proteins. A mucus gel full of bicarbonate keeps the lining nearly neutral; weaken it and an ulcer can form.
- A mill with a gatekeeper. About three times a minute a squeeze sweeps towards the pylorus, grinding food into chyme. The pylorus lets through only bits under about 1 to 2 mm and jets the rest back. Water leaves in minutes; a meal takes about 2 to 4 hours, and fatty food the longest.
- Hungry, full and growling. The empty stomach holds about 75 mL but relaxes to take 1 to 1.5 litres. Stretch receptors signal fullness up the vagus nerve, ghrelin rises before meals, and gastrin calls for more acid. Growls are gas and fluid being squeezed along.
- When things go wrong. Acid slipping past a weak sphincter causes heartburn. Most ulcers are caused by the germ H. pylori, which Barry Marshall proved by drinking it in 1984; spicy food doesn't cause them. Vomiting is a protective reflex run by the brainstem. For your own health, talk to a doctor.
| Term | Meaning |
|---|---|
| Oesophagus | The 25 cm muscular food pipe from the throat to the stomach. |
| Peristalsis | Waves of muscle squeezing that push food along the gut. |
| Lower oesophageal sphincter | The ring of muscle that lets food into the stomach and keeps acid out of the oesophagus. |
| Parietal cell | A stomach cell that makes hydrochloric acid using proton pumps (H⁺/K⁺-ATPase). |
| Pepsin | An enzyme, activated by acid, that breaks proteins into smaller pieces. |
| Mucus barrier | A gel laced with bicarbonate that protects the stomach lining from its own acid. |
| Chyme | The soupy mix of partly digested food and gastric juice that leaves the stomach. |
| Pylorus | The stomach's exit, whose sphincter lets through only small particles. |
| Ghrelin | The hunger hormone, made mostly by the stomach, which rises before meals. |
Two thousand years from a digestive fire and a cooking pot to a hole in a fur trader’s side, a doctor who drank a germ, and the hunger hormone.
- 100 BCE · Agni, the digestive fire (Charaka and the Ayurvedic tradition, Ancient India)
- 170 · Galen’s stomach “concocts” food (Galen of Pergamon, Pergamon and Rome)
- 1752 · A bird of prey swallows metal tubes (René-Antoine Ferchault de Réaumur, Paris)
- 1780 · Spallanzani swallows his own experiments (Lazzaro Spallanzani, Pavia)
- 1822 · A gunshot leaves a window into the stomach (Alexis St. Martin and William Beaumont, Mackinac Island, Michigan)
- 1824 · The acid is hydrochloric acid (William Prout, London)
- 1904 · Nobel Prize for the digestive glands (Ivan Pavlov, St Petersburg)
- 1957 · Light through glass fibres (Basil Hirschowitz, Ann Arbor, Michigan)
The full story, with 29 moments, charts, people and 34 sources: glassbox.how/e/stomachclear/history. The data lives in history.json.
Made with the Glassbox studio from this box's storyboard (window.glassbox.director). Free to reuse under CC BY 4.0.
| File | What | Size |
|---|---|---|
glassbox/reel.mp4 |
Reel / Short, with captions and soundtrack | 1080×1920 |
glassbox/video.mp4 |
YouTube video, with captions and soundtrack | 1920×1080 |
glassbox/slide-1…10.jpg |
Instagram carousel | 1080×1350 |
glassbox/thumb.jpg |
YouTube thumbnail | 1280×720 |
glassbox/cover.jpg |
Share card and repo social preview | 1200×630 |
glassbox/history-reel.mp4 |
“History in 10 moments” Reel / Short | 1080×1920 |
glassbox/history-slide-*.jpg |
History carousel | 1080×1350 |
glassbox/post.json |
Post copy and schedule used by the publish kit |
This box has no accounts and no ads, and it ships its own fonts and libraries. When you run it yourself it sends nothing anywhere. On glassbox.how, the site's /bar.js also loads Glassbox's analytics: Google Analytics to count visits (it asks first in the EU, UK and Switzerland, and stays off when your browser sends Global Privacy Control or Do Not Track) and ClickTrust to detect bots.
It remembers a few things in your own browser only, and never sends them anywhere:
| Browser storage key | What it holds |
|---|---|
stomachclear.v1 |
Which chapters you have opened, your best quiz scores, and sound on or off. |
Exactly what each one sees is at glassbox.how/privacy.
- Code: MIT. Use it, change it, ship it.
- Explanations, text, images and videos (
glassbox.json,glassbox/): CC BY 4.0. Credit “Glassbox, glassbox.how/e/stomachclear”. - Third-party parts keep their own licences: three.js (MIT), Geist, Instrument Serif (SIL OFL 1.1).
- The Glassbox name and logo aren't covered by either licence. See the terms.
Found a mistake? Open an issue. Corrections happen in public.
It's plain HTML, CSS and JavaScript. No build step and no dependencies. Run locally, it contacts no other website.
python3 -m http.server 8000Three.js and the fonts ship in vendor/ and fonts/, so it also works offline.
Then open http://localhost:8000.
| File | What |
|---|---|
index.html, css/app.css |
The page and its styles |
js/app.js, js/stage.js, js/ui.js, js/kit.js |
The shared Glassbox 3D engine: chapters, 3D stage, controls, quiz, video director |
js/stomach.js |
The 3D upper digestive tract (oesophagus, stomach with its regions and dynamic wall, duodenum, diaphragm, liver, ribs), the magnified wall layers and shared helpers |
js/chapters/*.js |
One file per chapter: the 3D model, controls, text, key terms, quiz and video scenes |
glassbox.json |
Title, question, explainer beats, key terms, browser storage and credits shown on glassbox.how |
reel in each chapter |
The storyboard the Glassbox studio records into short videos |
glassbox/ |
The published video, slides, thumbnail and post copy |
fonts/, vendor/three/ |
Self-hosted Geist and Instrument Serif (SIL OFL 1.1) and three.js (MIT) |