The Digestive System

Digestion is the process of breaking down the food we eat into nutrients that the body can use. The three main nutrients in food are:

  • Carbohydrates (e.g. starch)
  • Proteins
  • Lipids (also known as fats)

These food molecules are too large to be directly absorbed into the bloodstream, so they must first be broken down into smaller molecules by enzymes.

Digestion takes place within the digestive system, which starts in the mouth and ends in the anus.

Stages of Digestion

The diagram below shows the different components of the digestive system.

Let’s look at the main functions of some of these components.

The mouth

In our mouth, teeth break down the food we eat into smaller chunks. This process is called mastication, otherwise known as chewing.

The salivary glands in the mouth secrete enzymes that break down starch into smaller sugar molecules. This is useful for breaking down food when it is mixed with saliva. The tongue rolls the mushed-up food into a ball and pushes it towards the opening of your oesophagus to be swallowed.

Food then goes down the oesophagus (also known as the gullet), into the stomach.


The stomach contains protease enzymes that begin the chemical digestion of proteins. It also contains hydrochloric acid, which kills bacteria in the food and provides the optimal pH for protease enzymes to work.

Muscles in the stomach contract to churn the food, mechanically digesting it. Churning the food also mixes it with the digestive juices that are secreted by the stomach lining. So the food turns into a fluid, which increases the surface area for the enzymes to digest.

The fluid then passes into the small intestine.

Small intestine

Chemicals from both the liver and pancreas are released into the small intestine.

  • Liver – Releases bile, which is stored in the gallbladder. Bile speeds up the digestion of lipids and neutralises the acid from the stomach.
  • Pancreas – Releases enzymes that continue the digestion of starch and protein, also starting the digestion of lipids.

The walls of the small intestine are covered in finger-like structures called villi. They give the small intestine a large surface area which speeds up digestion. The small intestine absorbs nutrients into the bloodstream by diffusion or active transport.

Then, the remaining fluid passes into the large intestine.

Large intestine

In the large intestine, water is absorbed into the bloodstream, leaving behind the undigested food. This forms faeces, which are stored in the rectum until it is finally released out of the body, through the anus.

Bacteria in the Digestive System

Bacteria live everywhere around us and inside our bodies. The bacteria that cause disease are called pathogens. However, there are also useful bacteria that live inside our bodies, preventing the multiplication of pathogens. A large number of useful bacteria live in our digestive system, playing a part in the process of digestion.

Useful bacteria that live in the digestive system are mainly located in the large intestine. They also live in some parts of the small intestine. However, neither the oesophagus nor the stomach contains bacteria because they cannot survive in acidic environments. There are trillions of useful bacteria in our gut, including between 300 and 1000 different species.

There are many roles bacteria can have in the digestive system, which include:

  • Controlling the growth of harmful bacteria – Preventing them from multiplying and causing disease
  • Breaking down substances that humans can not digest
  • Producing many vitamins that we require, such as vitamin B and vitamin K

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