The primary function of the digestive system is to break down food both mechanically and by the use of enzymes, so that it can be used by the body for energy and cell growth and repair. It consists of a large number of organs and processes with the combined functions of breaking down our food into smaller molecules which can be used to produce energy and for other nutritional purposes; and excreting the waste consumed and produced by the system.

Digestive system diagram

The illustration below can be downloaded to colour in as a learning aid or as a blank diagram to print and label.

digestive system diagram

Organs of the digestive system

How does the it work? Digestion essentially occurs in a series of tubes such as the Oesophogus and Intestines as food passes through the body. A number of other organs contribute to digestion by providing enzymes for the breakdown of food.


Mouth: The mouth is the starting point of digestion. Here the process of chewing starts to break down food and enzymes such as salivary lipase and amylase also start to chemically break down the food.

Oesophagus: Once you swallow the food moves into the Oesophagus where continual waves of involuntary contraction push the food into the stomach.

Stomach: The stomach has both a mechanical and a chemical function in digestion. The upper part of the smooth (involuntary) stomach muscle relaxes to allow a large volume of food to be stored. The lower muscle then contracts in a rhythmical manner in order to churn the food inside and mix it together with the gastric acid (mainly hydrochloric acid) and digestive enzymes Pepsin, Gelatinase and Gastric Amylase and Lipase which break it down further. The stomach must then empty its contents into the small intestine.

Small Intestine: Whilst in the small intestine food is subjected to yet more enzymes, those from the Pancreas and from the glands within the intestine walls which break down carbohydrates and proteins. It is also mixed with a product of the liver which is stored and released into the intestine by the gall bladder. This is commonly known as bile. Bile works to dissolve fat so that it can be digested by the other enzymes. Rhythmic smooth muscle contraction continues within the small intestine and pushes the digesting food through its narrow tube.

Once the food is completely broken down into its individual components it is absorbed through the intestinal walls, into the blood flow of the capillaries which surround the intestine. To make this process faster and more efficient the intestinal walls contain numerous folds which are covered in finger-like projections called villi. This vastly increases the surface area of the intestine wall for molecules of digested food to pass through.

Large Intestine: The large intestine continues the foods journey and is the bodies last chance to absorb any water and minerals still remaining. The rest of the contents of the large intestine is waste such as undigestable pieces of food and fiber. This is passed through to the rectum where it is stored until you go to the toilet!

Digestive system facts

I bet you didnt know..

  • At birth, we have about 10,000 taste buds on the surface of the tongue, in the throat, and on the roof of the mouth.
  • It is a myth that the tongue is the strongest muscle in the body. The tongue is made up of 16 different muscles and as yet there is no conclusive evidence that it is the strongest. If all the muscles in the human body had a weight lifting contest then the masseter or jaw muscle would come out top.
  • The appendix has no function in the human body. Charles Darwin thought that it was used for digesting leaves when we were primates.
  • An adult stomach has the capacity to hold around 1 litre of food and liquid. The stomach can expand though and it is believed to be able to increase its capacity to between 2 - 4 litres.
  • About 11 liters of fluid, digestive juices and digested food is circulated around your digestive system.
  • On average 1 to 1.5 litres of Saliva is secreted into the mouth every day.
  • It doesn't matter which way up you are for food to reach the stomach - you can be upside down and the food will still get there and throughout all the parts of the digestive system.
  • The role of the mouth is not only mastication (chewing) but also to get the pulp to a temperature that is acceptable for the rest of the digestive tract.
  • Gastric Acid contains Hydrocholic acid (about 0.5%) which can corrode metal, it has a PH value between 1 and 2. Anything with a PH less than 2 is considered as “strong acid” and can burn the skin.
  • The stomach lining is replaced continuously, and it is completely replaced approximately every 5 -10 days.

Test yourself

Label the diagram - download the diagram and label the various parts.

Colour in the diagram - download the image as a black and whilte outline to colour in.

Related Articles

Anaerobic Respiration & Lactic Acid

Anaerobic respiration is when the body produces energy for exercise without oxygen. The anaerobic energy system kicks in in the first few minutes of all exercise, before there is suffiecient oxygen...

Aerobic System & Krebs Cycle

The aerobic system produces the largest amounts of energy, although at the lowest intensity. At the start of exercise the body cannot deliver oxygen to the muscles fast enough to initiate the...

Effects of Exercise

Exercise affects the circulatory system, respiratory system and the muscles. Short term effects occur immediately as we begin to exercise. Long term effects are more concerned with adaptive changes...

Oxygen Debt & Recovery

What is oxygen debt? When you have a short intense burst of exercise such as sprinting you generate energy for this anaerobically or without oxygen. When you stop exercising you are still breathing...