KS3 · Biology

The human digestive system

Two of the hardest-working organs in your digestive system never touch a single crumb of your food. So how can they be part of it?

Biology · Anatomy

Follow a mouthful, then find the organs it never touches

Showing 3 layers: Body outline, Food's route, Glands beside the tube

Layers

Explore

try switching the glands off ↓

Everything in this layer is a gland: the salivary glands, the liver and the pancreas. Glands make chemicals and secrete (release) them into the tube. The liver and pancreas are organs as well as glands. Food never passes through any of them, which is why the route still works when you switch them off.

Tap each organ to see what it does to food. Then switch the glands off: the food's route is still complete without them.

The food's route

Put the route in order

The order food passes through them

1 · First stop7 · Last stop

  1. Stomach

  2. Anus

  3. Oesophagus

  4. Large intestine

  5. Mouth

  6. Rectum

  7. Small intestine

Why digest at all?

?

Reason it through

Why can't your body just use a sandwich as it is?

Link 1 of 4

First link · your turn

What do we actually need food for?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Where does digestion happen?

Be honest: what do you think?

Your friend says: "Digestion happens in your stomach. Everything after that is just pipes carrying the leftovers away."

Which idea is closest to what you think right now?
How sure are you?

Predict, then check

Here's a puzzle to finish on.

Fibre is never digested and never absorbed: it goes in one end and comes out the other. So does fibre do anything useful in your digestive system?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

One long tube from mouth to anus — plus two busy organs your food never touches.

What you need to know

  • Food is our source of energy and of materials for growth and repair. Digestion breaks large pieces of food into small nutrients that can be absorbed into the blood and used around the body.
  • Food passes through the mouth, oesophagus, stomach, small intestine, large intestine, rectum and anus, in that order.
  • A tissue is a group of one type of cell; an organ is a group of specialised tissues working together; a gland is a tissue or organ that produces chemicals and secretes them.
  • An adaptation is a feature that helps an organ carry out its function: to digest food, absorb nutrients or process waste.
  • The liver and pancreas are organs and glands. They secrete into the small intestine, but food never passes through them.

The big picture

The digestive system breaks large pieces of food into small nutrients that can be absorbed into the blood. Food travels one way through the mouth, oesophagus, stomach, small intestine, large intestine, rectum and anus, while glands such as the liver and pancreas add chemicals to the small intestine without food ever passing through them.

Key points

1Mouth: teeth cut and chew; the tongue rolls and mixes; salivary glands secrete saliva, whose water helps food move and whose chemicals start digestion.
2Oesophagus: rings of muscle contract behind the bolus and push it to the stomach. This is peristalsis, which also moves food and waste through the intestines.
3Stomach: receives food and makes hydrochloric acid, which helps digest food and kills pathogens.
4Small intestine: long and narrow to slow food down so it is fully digested and absorbed; most digestion happens here; villi absorb nutrients into the blood, which carries them to cells.
5Pancreas: secretes chemicals into the small intestine that break food down into nutrients. Liver: secretes bile, which breaks fat into very small globules suspended in water.
6Large intestine: receives what could not be digested and absorbed, including fibre, and absorbs water from it into the blood, making the waste firm.
7Rectum stores faeces; the anus, a ring of muscle, controls when they are egested.

Worked example

Problem

Explain why the pancreas belongs to the digestive system, even though food never passes through it.

⚠ Watch out

Saying that bile digests fat. It doesn't: bile breaks fat into very small globules and surrounds them so they stay suspended in water. It is the pancreas's chemicals that break food down into nutrients.

🧠

Memory hook

My Old Stripy Sock Looks Really Awful: Mouth, Oesophagus, Stomach, Small intestine, Large intestine, Rectum, Anus. And the liver and pancreas? Beside the road, never on it.

✓

Check yourself

Without scrolling up: which part of the tube does most of the digesting, where do nutrients pass into the blood, and what does the large intestine take out of the waste?

Flashcards

(16)
What is digestion?
Breaking large pieces of food down into small nutrients that can be absorbed into the blood and used around the body.
What two things does food give us?
Energy, and materials for growth and repair.
What is a tissue?
A group of one type of cell.
What is an organ?
A group of specialised tissues working together.
What is a gland?
A tissue or organ that produces chemicals and secretes (releases) them.
What do the teeth and tongue do to food?
Teeth cut and chew food into smaller pieces; the tongue rolls and mixes it ready for swallowing.
What is in saliva, and what does it do?
Water, which helps food move, and chemicals that start digestion.
What is peristalsis?
Rings of muscle contracting behind food to push it along the oesophagus and through the small and large intestines.
Give two jobs of the hydrochloric acid made in the stomach.
It helps digest food, and it kills pathogens (microorganisms that cause disease).
Why is the small intestine long and narrow?
It slows the food down, so the food can be fully digested and the nutrients fully absorbed.
What are villi?
Tiny finger-like projections on the surface of the small intestine, where nutrients are absorbed into the blood.
What does the pancreas secrete, and where?
Chemicals that break food down into nutrients, secreted into the small intestine.
What does bile do, and which organ makes it?
The liver makes it. Bile breaks fat into very small globules and surrounds them so they can be suspended in water.
What happens to waste in the large intestine?
Water is absorbed from it into the blood, which makes the waste firm.
Fibre isn't digested or absorbed. Why is it still useful?
It bulks up food so peristalsis can move it through the system, and it forms part of the faeces.
What do the rectum and anus do?
The rectum stores faeces; the anus is a ring of muscle that controls when they are egested.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How this lesson was checked. This KS3 Biologylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 30 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.