The human digestive system

The digestive system – also called the alimentary canal – is a tube passing through the body from mouth to anus. There are a number of glands alongside it, which produce secretions which pass into it in order to assist the process of digestion.

As food substances enter and are processed by the digestive system, it is probably appropriate to consider it as an interface with the environment, in a similar way to the skin and lungs.

Digestion involves the conversion of food into digestion products which consist of smaller and more soluble molecules which can be absorbed into the body. This generally involves a process of hydrolysis, which differs in detail for each food type, because different enzymes are involved.

The main food molecules are polymers or fairly complex molecules, made up from many simpler subunits (repeated monomers or similar building blocks). Hydrolysis is actually the breaking of bonds between the subunits from which they are made, using water. Hydro means water, and lysis means splitting.

The soluble products of digestion enter the bloodstream and are passed on to various parts of the body, to be used for different purposes. The term assimilation refers to the use of digestion products for building new parts of the body, and it refers to the idea that the molecules, cells and tissues that are built up resemble similar structures in the food source.

Digestive system diagram for on-screen labelling

* mentioned in specification

Main parts, in order

Mouth (buccal cavity)
Throat (pharynx)
Gullet (oesophagus*)
Stomach*
Duodenum
Ileum (small intestine*)
Colon (large intestine*)
Rectum*
Anus

Associated glands

Salivary glands*
Liver
Pancreas*

Other parts, sometimes ignored

Caecum
Appendix
Drag and drop the labels below in their correct positions on the diagram
Anus
Colon (large intestine)
Duodenum
Gullet (oesophagus)
Ileum (small intestine)
Mouth (buccal cavity)
Rectum
Stomach
Throat (pharynx)
Liver
Pancreas
Salivary glands
Appendix
Caecum