What digestion does
Food contains large molecules such as starch, proteins and fats. Digestion breaks them into small, soluble molecules that can be absorbed into the blood: glucose, amino acids,fatty acids and glycerol. It happens in two ways. Mechanical digestion (chewing and the stomach’s churning) breaks food into smaller pieces, giving enzymes more surface to work on.Chemical digestion uses enzymes to break chemical bonds.
The journey, organ by organ
- Mouth: teeth chew; saliva moistens food into a soft ball called a bolus and starts digesting starch.
- Oesophagus: peristalsis carries the bolus to the stomach in about 5–10 seconds.
- Stomach: muscular churning mixes food with acid and pepsin, turning it into a soupy liquid called chyme. Food stays here about 2–4 hours, longer for fatty meals.
- Small intestine: about 6 m long. Most digestion and nearly all absorption of nutrients happen here, over about 3–5 hours.
- Large intestine: about 1.5 m long. It absorbs water and salts over 10–40 hours, and its bacteria ferment some fibre.
- Rectum and anus: the remains (faeces) are stored and then egested.
Digestive enzymes
| Enzyme | Made in | Works in | Breaks down → into |
|---|---|---|---|
| Amylase | Salivary glands, pancreas | Mouth, small intestine | Starch → maltose (then glucose) |
| Pepsin | Stomach | Stomach (acidic) | Proteins → peptides |
| Trypsin and peptidases | Pancreas, intestine wall | Small intestine | Peptides → amino acids |
| Lipase | Pancreas | Small intestine | Fats → fatty acids + glycerol |
Each enzyme has an optimum pH: pepsin works best in the strongly acidic stomach (about pH 2), while pancreatic enzymes need the slightly alkaline conditions of the small intestine, created by bicarbonate from the pancreas.
Absorption and villi
The inner wall of the small intestine is folded and covered in millions of finger-like villi, each covered in even smaller microvilli. Together they give a surface area of about 30 m². Each villus has a thin wall and a rich network of capillaries, which take in glucose and amino acids, and a lymph vessel called alacteal, which takes in most digested fats. Absorbed nutrients travel first to the liver.
Liver, gallbladder and pancreas
Food never passes through these accessory organs, but the tract could not work without them. Theliver makes bile, which the gallbladder stores and releases after a meal. Bileemulsifies fats into tiny droplets. The pancreas releases a juice full of enzymes into the small intestine, and separately makes the hormones insulin and glucagon that control blood sugar.
Key terms
- Bolus / chyme
- A swallowed ball of chewed food / the acidic liquid that leaves the stomach.
- Peristalsis
- Waves of muscle contraction that push food along the gut.
- Absorption / egestion
- Taking digested molecules into the blood / removing undigested waste from the body.
Common misconceptions
- “Most digestion happens in the stomach.” The stomach starts protein digestion, but most digestion and absorption happen in the small intestine.
- “Bile is an enzyme.” Bile contains no enzymes; it physically breaks fat into droplets.
- “Fibre is useless.” It is not digested, but it feeds gut bacteria and keeps food moving.
Educational model only. Times and percentages are typical textbook values; real digestion varies a great deal between people and meals. This simulator is not medical or dietary advice. If you have symptoms or questions about your digestion or diet, speak to a qualified healthcare professional. See oureducational and medical disclaimer.
Key takeaways
- Digestion turns large food molecules into small soluble ones the body can absorb.
- Amylase digests starch, proteases (pepsin, trypsin) digest proteins, and lipase digests fats.
- Most absorption happens through the villi of the small intestine.
- The large intestine absorbs water; fibre and waste leave as faeces.
Model note: the diagram is simplified, and the meal is shown as a single lump moving through. In reality food leaves the stomach gradually in small squirts, and time in the large intestine varies widely.