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 pieces. This process is called mastication, also known as chewing.

Saliva glands in the mouth secrete enzymes that break down large molecules into smaller ones. This helps when food mixes with saliva. The tongue then rolls this mushed-up food into a ball and pushes it toward the opening of the oesophagus to be swallowed.


Food travels down the oesophagus (also known as the gullet), which leads to the stomach.


The stomach contains:

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

Small intestine

This food then passes into the small intestine.

The walls of the small intestine have finger-like structures called villi. These increase the surface area of the intestine, which speeds up digestion. The small intestine absorbs nutrients into the bloodstream, while the remaining food moves into the large intestine.

Large intestine

In the large intestine, water is absorbed into the blood, leaving undigested food that forms faeces. This faeces is stored in the rectum until it is eventually expelled through the anus.

Other Organs Involved in Digestion

Some organs don’t interact directly with the digesting food but are still involved in the process. These include:

  • The liver
  • The gallbladder
  • The pancreas


The liver is crucial for digestion. It produces a liquid called bile, which is stored in the gallbladder and released into the small intestine. Bile helps break down fats in food so 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 needed. It travels through a duct and mixes with the stomach’s contents.

This is important because the small intestine requires a certain level of alkalinity (the opposite of acidity) to properly digest and absorb nutrients. If the stomach acid is not neutralised by the bile, it could damage the small intestine.


The pancreas produces digestive enzymes, such as:

  • Carbohydrase
  • Protease
  • Lipase

These digestive enzymes speed up digestive reactions.