A bite of a sandwich is made of huge molecules: long chains of starch in the bread, folded proteins and fats in the cheese. Your cells cannot use them in that form, and they are far too big to pass through the wall of your intestine. The digestive system solves this by breaking food down into small molecules (sugars such as glucose, amino acids, fatty acids and glycerol) and then absorbing them into the blood, which carries them to every cell. Digestion is a perfect example of an organ system: a long tube of organs, with helper organs adding chemicals along the way.
Two kinds of digestion
- Physically breaks food into smaller pieces
- Does not change the molecules
- Examples: chewing, churning by the stomach muscles, bile breaking fat into tiny droplets
- Purpose: increases surface area so enzymes can work faster
- Breaks large molecules into smaller, different molecules
- Uses enzymes, helped by acid in the stomach
- Examples: amylase turning starch into sugar, pepsin cutting proteins
- Purpose: makes molecules small enough to absorb
| Organ | What happens there |
|---|---|
| Mouth | Teeth chew food (mechanical). Saliva moistens it and contains amylase, which starts breaking starch into sugar (chemical). |
| Esophagus | Waves of smooth muscle contraction called peristalsis push food down to the stomach. No new digestive juices are added here. |
| Stomach | Muscular walls churn food. Gastric juice contains hydrochloric acid (pH about 2), which kills many microbes, and pepsin, which begins protein digestion. Mucus protects the lining from its own acid. |
| Small intestine | Several metres long. Most chemical digestion is finished here with enzymes from the pancreas and the intestinal lining, and nearly all nutrients are absorbed through the villi. |
| Liver and gallbladder | The liver makes bile; the gallbladder stores it and releases it into the small intestine, where it breaks fat into small droplets. |
| Pancreas | Sends a juice of digestive enzymes (for starch, protein and fat) into the small intestine, plus a base that neutralizes stomach acid. |
| Large intestine | Absorbs much of the remaining water and some minerals, forming solid waste (feces). Helpful bacteria here make some vitamins, such as vitamin K. |
| Rectum and anus | The rectum stores feces until they leave the body through the anus. |
Enzymes do the chemical work
An enzyme is a protein that speeds up one particular chemical reaction without being used up. Each digestive enzyme fits one kind of food molecule, the way a key fits one lock: amylase breaks down starch, pepsin and other proteases break down proteins, and lipase breaks down fats. Enzymes work best at a particular temperature and pH. Human enzymes work best near body temperature (37 °C); pepsin needs the strong acid of the stomach, while the pancreatic enzymes need the nearly neutral conditions of the small intestine.
Absorption: the job of the villi
The inner wall of the small intestine is folded and covered with millions of finger-like bumps called villi (one is a villus), and the cells on each villus have even tinier projections called microvilli. Together they give the small intestine an enormous surface area for absorption. Each villus has a wall only one cell thick and a dense net of capillaries, the smallest blood vessels. After a meal, the concentration of digested nutrients is higher inside the intestine than in the blood, so many of them move into the blood by diffusion; others are actively pumped in by the lining cells. Sugars and amino acids enter the capillaries, while fats enter small lymph vessels inside each villus. The blood then carries the nutrients to the liver and on to every cell in the body: this is where the digestive and circulatory systems meet.
| Disorder | What goes wrong |
|---|---|
| Heartburn (acid reflux) | Stomach acid splashes up into the esophagus, which lacks the stomach's thick protective mucus layer |
| Stomach ulcer | A sore in the stomach lining; most are caused by infection with the bacterium Helicobacter pylori or by regular use of some pain medicines |
| Celiac disease | The immune system reacts to gluten (a protein in wheat, barley and rye) and damages the villi, so fewer nutrients are absorbed |
| Lactose intolerance | Too little of the enzyme lactase, so milk sugar is not digested and is fermented by gut bacteria, causing gas and cramps |
| Colorectal cancer | A malignant tumour of the large intestine or rectum; screening with a stool test (FIT) can catch it early |