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The digestion process. Foundation. Learning objectives. To understand food is used as a fuel by the body. To recognise the body parts involved in digestion. To know the roles of different body parts in digestion. To understand the four major phases of digestion. Food as a fuel.

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learning objectives
Learning objectives
  • To understand food is used as a fuel by the body.
  • To recognise the body parts involved in digestion.
  • To know the roles of different body parts in digestion.
  • To understand the four major phases of digestion.
food as a fuel
Food as a fuel

The body requires energy from food.

Our bodies act as a converter, releasing energy and nutrients from food.

Sometimes food can take 2 or 3 days to be fully digested and absorbed by the body.

organs of the digestion process
Organs of the digestion process



Small intestine





The teeth mechanically break food down into smaller pieces.

Different shaped teeth tear, chop and grind the food.

The cheeks and tongue help to push the food towards the teeth.

The food is then rolled into a ball and swallowed down the oesophagus.


Saliva will be released into the mouth at the sight, smell, taste or even the thought of food.

Saliva is secreted from glands at the back of the mouth and under the tongue.

An enzyme found in saliva helps breaks down starch into simple sugars.

Saliva also moistens the food making it easier to chew and swallow.


When food is swallowed, the muscles in the oesophagus contract and relax, helping to push the food down into your stomach.

After the food has been swallowed, it is carried down the oesophagus towards the stomach.

Each mouthful of food takes about six seconds to reach the stomach once it is swallowed.

Even, when the body is upside down the food will still pass from the mouth to the stomach.


The stomach is a sack made of muscles that contract and churn food, breaking it down even further.

The acid and enzymes found in the stomach also help to break down the food.

Food can spend up to 2 to 3 hours in the stomach.

When the food has been churned into a creamy mixture known as chyme, it passes gradually into the small intestine.

small intestine
Small intestine

The small intestine is a tube about 6 metres long.

The inner surface of the small intestine is folded into tiny finger-like structures called villi.

The surface area of the villi is about 30m2. This is equivalent to the size of a tennis court.

The first section of the small intestine is called the duodenum.


In the duodenum, food is diluted with pancreatic enzymes and bile, which decrease stomach acidity.

The contents continue to travel through the lower small intestine, becoming more liquid as they mix with water, mucus, bile, and pancreatic enzymes.

pancreas and the gall bladder
Pancreas and the gall bladder

A duct entering the duodenum provide digestive juices to help digestion.

From the pancreas:

- sodium bicarbonate to neutralise acid from the stomach;

-digestive enzymes.

From the gall bladder:

- bile salts to help breakdown the fat.


After the chyme has passed into the duodenum some of the nutrients can pass through the wall of the villi and into the bloodstream.

These nutrients can be used by body cells, for energy and growth.

Insoluble, undigested food moves on inside the small intestine.

Ultimately, the small intestine absorbs most of the nutrients. All but about 1 litre of fluid remains before the chyme moves into the large intestine through peristalsis.


These are the waves of muscular contractions which move food along the digestive system.

Circular muscles in the wall relax in front of the food whilst circular muscles behind the food contract, pushing the food onward from the oesophagus through to the large intestine or colon.


The colon is shorter than the small intestine.

The main function of the colon is to remove water.

Bacteria ferment the remaining food and produce molecules and gases.

The final product formed is faeces, which are is stored in the rectum until these are excreted.

phases of digestion
Phases of digestion

Ingestion - food taken into the mouth.

Digestion - physical and chemical processes that start in the mouth, and continue in the stomach and small intestine.

Absorption – nutrients moving across the gastro-intestinal lining, into the blood.

Elimination – undigested food such as dietary fibre is excreted as faeces.

review of the learning objectives
Review of the learning objectives
  • To recognise the body parts used in digestion.
  • To understand food is used as a fuel for the body.
  • To know the roles of different body parts in digestion.
  • To understand the four major phases of digestion.