Section 5: The digestive system

The digestive system is responsible for the intake, breakdown, use and removal of food and drink. An efficient digestive system tells us when we are hungry, full and thirsty by sending messages to and from the brain via the nervous system. It extracts important nutrients for storage and immediate use and removes any waste.

The digestive system has four stages:

Journey through the alimentary canal (also known as the digestive tract/gastrointestinal tract/gut)

Food’s journey through the alimentary canal can take up to 24 hours and covers a distance of around 9m (30 feet) from ingestion through the mouth to excretion through the anus.


This is the entry point of food and where it begins to be broken down through the process of mastication (chewing) into a ball, or bolus.

Oesophagus (gullet)

This is a thick-walled, muscular tube that carries broken down food from the mouth to the stomach.


The stomach is a muscular bag located on the left side of the upper abdomen. It breaks down food further by releasing enzymes, and also kills bacteria.

Small intestine

The small intestine is a small, tightly folded tube that receives food from the stomach. It is the major site of digestion within the alimentary canal. Its role is to absorb important nutrients into the bloodstream to be passed to the body’s tissues and used for energy. The small intestine is divided into three sections: the duodenum, jejunum and ileum.

Large intestine

The large intestine absorbs water and vitamins from food residue, and forms and stores faeces ready for excretion. It is made up of the colon (large tube that surrounds the abdominal cavity) and the rectum (passageway for faeces), which leads to the anus (a valve-like exit from the end of the alimentary canal).

Breakdown and absorption of food

The major food groups (macronutrients) are broken down and used in different ways. Food is broken down at different points of the alimentary canal by digestive enzymes.

Enzymes are biological catalysts, which means they speed up chemical reactions in cells. In the case of digestive enzymes, the role is to speed up the process of nutrients being broken down in the digestive tract and absorbed into the bloodstream for use.




The role of dietary fibre in maintaining effective gut function

Dietary fibre is a type of complex carbohydrate that cannot be digested by the human body. It is found in foods such as fruits, vegetables, wholegrains and cereals and is vital for the efficient function of the digestive system. It is recommended that an average adult should eat at least 30g of fibre per day.

Fibre helps the gut by:

  • Ensuring the smooth and speedy passing of waste.
  • Helping to increase the amount of ‘good bacteria’ in the gut.
  • Helping you feel fuller for longer and reducing the likelihood of snacking on foods high in sugar, thereby reducing the likelihood of obesity and associated conditions, e.g. type 2 diabetes.

For fibre to be effective in supporting gut function, it is also important to maintain adequate hydration levels by drinking lots of water throughout the day. Fluid supports the passing of food through the alimentary canal and helps to facilitate the passing of waste.

The role of the liver in assisting digestion

The liver is the body’s largest internal organ and has many functions that relate specifically to digestion and absorption of food, including:

  • Bile secretion, which is vital for the breakdown of fats in the small intestine.
  • Removal of nutrients from the blood and conversion for storage. For example, it removes glucose (not needed for energy) to be converted back into glycogen.
  • Detoxifying harmful substances in the blood, e.g. alcohol, by converting them into urea to be excreted as urine.
  • Storage of vitamins and minerals that have a number of health benefits.
  • Removal of bacteria from the bloodstream.

The role of the pancreas in assisting digestion

The pancreas is responsible for secreting digestive juices into the small intestine, which contain the aforementioned enzymes, in order to break down nutrients.

Timescales of digestion

The process of digestion, from start to finish, can take anywhere between 12 and 72 hours and is dependent on the individual’s gut function and the type and amount of found ingested. The time frames are broken down as follows: