The Human Digestive System

Digestion is the process of breaking down the food we eat into nutrients that the body can use. This process takes place in the digestive system, which starts in the mouth and ends in the anus.

Stages of Digestion

The mouth

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

The saliva glands in the mouth secrete enzymes that break down large molecules into smaller 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 the oesophagus to be swallowed.

Food goes down the oesophagus (also known as the gullet), which takes the food into the stomach.


The stomach contains:

  • Enzymes for digestion
  • Hydrochloric acid for killing bacteria
  • Muscles, which contract to churn the food

Small intestine

This food then passes into the small intestine.

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 blood and the remaining food passes into the large intestine.

Large intestine

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

Other Organs Involved in Digestion

There are organs which do not interact with the digesting food but are involved with the digestion process, such as:

  • The liver
  • The gallbladder
  • The pancreas


The liver is an important organ in your body that helps with digestion. It makes a liquid called bile, which is stored in the gallbladder and released into the small intestine. Bile helps your body break down fats in your food so that they can be used for energy.

  • Bile helps to break down lipids (fats and oils) and neutralise stomach acid


The bile produced by the liver is stored in the gallbladder and released into the small intestine when it is needed for digestion. The bile travels through a duct and enters the small intestine in the area where the stomach empties its contents. The acid in the stomach is mixed with hydrochloric acid to help break down food, but it can be harmful to the small intestine if it’s not neutralized.

The bile produced by the liver helps to balance out the acid in the stomach’s contents when they enter the small intestine. This is important because the small intestine needs a certain level of alkalinity (opposite of acidity) to properly digest and absorb the nutrients in the food we eat. If the acid from the stomach was not neutralised by the bile, it could harm the small intestine and make it hard for our body to get the nutrients it needs.


The pancreas creates digestive enzymes, such as:

  • Carbohydrase
  • Protease
  • Lipase

These digestive enzymes speed up digestive reactions.