Nutrition and digestion · System
The digestive system
A nine-metre tube with seven jobs along it. Food spends four hours in the organ everybody names, and sixteen in the one that actually does the work.
Start here
Blend a meal to a smooth liquid. Is it digested?
A cheese sandwich goes into a blender for two minutes on high. What comes out is a smooth grey liquid with no lumps in it at all — smoother than anything your teeth or stomach could manage.
Could that liquid pass straight into your blood?
No — and not because it is not smooth enough. The starch molecules in that liquid are still starch molecules, thousands of glucose units long and far too large to cross a cell membrane. A blender makes pieces smaller. Digestion makes molecules smaller, which is a completely different operation and needs enzymes rather than blades.
Follow the meal · seven stops
Where the sandwich actually goes
1 of 7 stops visited
Mouth
Mechanical and chemical
Teeth cut and grind the food into a soft mass, and the tongue mixes it with saliva. Saliva does two jobs: it lubricates the mass so it can be swallowed, and it contains amylase, which starts cutting starch chains into shorter ones.
Time spent here
about 1 minute
Molecules broken here
Starch, a little
Absorbed here
Nothing
Worth knowing: Hold a plain cracker on your tongue without chewing and it turns faintly sweet in about ninety seconds. That is amylase producing sugar from starch, and it is the only digestion you can taste happening.
Oesophagus
Transport only
A muscular tube from throat to stomach. Rings of muscle contract behind the food and relax in front of it, squeezing it along. This wave is called peristalsis and it happens the whole length of the gut.
Time spent here
about 8 seconds
Molecules broken here
Nothing
Absorbed here
Nothing
Worth knowing: Peristalsis does not need gravity. An astronaut can swallow upside down in orbit, and so can you — the muscle wave, not the fall, moves the food.
Stomach
Mechanical and chemical
A muscular bag that churns the food, adds hydrochloric acid at about pH 2, and adds the enzyme protease to begin breaking protein into shorter chains. The acid also kills most bacteria swallowed with the meal.
Time spent here
about 4 hours
Molecules broken here
Protein
Absorbed here
Almost nothing
Worth knowing: The acid is strong enough to dissolve the stomach itself, which is why the lining secretes a layer of mucus and replaces its own cells every few days. Ulcers are what happens when that defence fails — usually because of a bacterium, not because of stress or spicy food.
Small intestine
Chemical, then absorption
Six or seven metres of narrow tube, and the organ that does most of the work. Enzymes from the pancreas and from the intestine wall finish breaking every nutrient down to molecules small enough to cross a membrane, and those molecules are then absorbed into the blood through the intestine wall.
Time spent here
about 16 hours
Molecules broken here
Starch, protein and lipid — all of them, to completion
Absorbed here
Almost everything
Worth knowing: Its surface is folded, then covered in villi, then each villus cell is covered in microvilli — three levels of folding that bring the absorbing surface to around 30 m². That is the next lesson’s subject and it is the single best piece of engineering in the body.
Pancreas, liver and gall bladder
Secretion — no food passes through
These three feed into the small intestine without ever holding food. The pancreas supplies amylase, protease and lipase together with an alkali that neutralises the stomach acid. The liver makes bile; the gall bladder stores it and releases it onto fatty food.
Time spent here
no food enters these
Molecules broken here
They supply what does the breaking
Absorbed here
Nothing
Worth knowing: Bile is not an enzyme and digests nothing. It breaks large fat droplets into many small ones — emulsification — which is mechanical digestion happening chemically, and it multiplies the surface available to lipase enormously.
Large intestine
Absorption of water
Wider and shorter than the small intestine. By the time material arrives, the nutrients have gone; what is left is water, fibre and bacteria. The wall absorbs most of the water back, leaving a solid mass.
Time spent here
12 to 30 hours
Molecules broken here
Nothing your enzymes can break
Absorbed here
Water, some minerals and some vitamins
Worth knowing: This is where the trillions of gut bacteria live, and they digest some of the fibre your own enzymes cannot touch — releasing fatty acids and making vitamin K and some B vitamins as by-products. That is lesson eight.
Rectum and anus
Storage and egestion
The remaining solid material is stored in the rectum and leaves through the anus. This is egestion — the removal of material that was never absorbed.
Time spent here
a few hours
Molecules broken here
Nothing
Absorbed here
Nothing
Worth knowing: Egestion is not excretion, and examiners care about the difference. Excretion is getting rid of waste your own cells produced, such as urea and carbon dioxide. Egestion is getting rid of what you never took in.
Hours spent in each organ, to scale
- Mouth<1 h
- Oesophagus<1 h
- Stomach4 h
- Small intestine16 h
- Pancreas—
- Large intestine20 h
- Rectum and anus6 h
The stomach is the organ everyone names first and it holds the meal for about four hours. The small intestine holds it for four times as long, and that is where nearly all digestion is completed and nearly all absorption happens.
Two kinds of breaking
Smaller pieces is not smaller molecules.
Mechanical digestion · Making pieces smaller
Teeth cutting and grinding, the stomach churning, bile splitting fat droplets. The chemical nature of the food does not change at all — starch is still starch afterwards. Does it let food into the blood? No. A crumb of bread a thousand times smaller is still made of the same molecules, and they are still too large to cross a membrane.
Chemical digestion · Making molecules smaller
Enzymes cut the long chains at each link: starch to glucose, protein to amino acids, lipid to fatty acids and glycerol. This is a chemical change and the products are different substances from what went in. Does it let food into the blood? Yes. This is the only process that produces molecules small enough and soluble enough to be absorbed — which is why enzymes, not teeth, are the point of the system.
Key fact
Digestion is the breaking of large insoluble molecules into small soluble ones. Chewing and churning only increase surface area so the enzymes can work faster — they digest nothing on their own.
Think again
“Digestion is food being squashed into smaller and smaller pieces.”
If that were true, the blender in the hook would have done the whole job. Follow the sizes instead. A starch molecule in bread is a chain of a few thousand glucose units; a glucose unit is about 0.0000009 mm across, and that is about the size limit for a molecule crossing a cell membrane. Cutting the bread into a million crumbs leaves every starch molecule exactly as long as it was — you have made more, smaller pieces of the same molecule. What has to happen is that the chain is cut chemically at each link, and only an enzyme does that. The word for what your teeth do is mechanical digestion, and its entire purpose is to expose more surface for the chemical kind.
“Food sits in your stomach until it is digested, then goes to the intestine.”
The stomach is not the destination — it is a holding tank with an acid bath and a strong set of muscles. It holds a meal for roughly four hours, kills most of the bacteria that came in with it, starts protein digestion, and releases the result into the small intestine a little at a time. Almost nothing is absorbed through the stomach wall: no glucose, no amino acids, no fatty acids. If your stomach were removed — and people do live without one — you could still digest and absorb a meal, because the organ that does the real work is the next one along.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Where does the work happen
In which organ is most food digested and most food absorbed?
Rung 2 · The one that catches people
A patient has their gall bladder removed and is advised to avoid very fatty meals. Bile contains no enzymes. Why does losing bile storage make fat harder to digest?
Rung 3 · Explain the blender
A student argues that if you blended a meal finely enough, you would not need a digestive system. Explain what is right and what is wrong in their reasoning, using the difference between mechanical and chemical digestion.
Rung 4 · Take it somewhere new
A person has a section of small intestine removed after an injury. Predict what will change about their digestion and absorption, say why the stomach cannot compensate, and name one thing that would not change at all.
Key note
Food passes through the mouth, oesophagus, stomach, small intestine and large intestine. The pancreas, liver and gall bladder add secretions without food passing through them. Mechanical breakdown increases surface area; chemical breakdown by enzymes turns large insoluble molecules into small soluble ones. Almost all absorption happens in the small intestine.
Going further
There is a real sense in which the inside of your gut is outside your body. The tube runs from mouth to anus without interruption, so its contents are topologically outside you in the same way that the hole in a doughnut is outside the doughnut — nothing has entered you until it has crossed the gut wall into the blood. This is not a word game; it is why the gut can hold trillions of bacteria without your immune system treating it as an infection, and why swallowing a small coin is usually harmless while the same coin in your bloodstream would not be. Absorption, not swallowing, is the moment food enters you.
Before this lesson
- This follows on from When diet goes wrong.
Connects to
At GCSE this becomes
- The digestive system as an organ system, with named enzymes, bile, and the role of the liver in metabolism.
Where to next
Ask Mr Badmus AI
Unclear why chewing is not digestion on its own?
Transit times are typical figures for a mixed meal in a healthy adult and vary widely between people and between meals.
Lesson content © MrBadmusAI.