The human digestive system

The human digestive system has two functions:

  • breaks down complex food substances
  • provides the very large surface area for maximum absorption of food

The structure of the digestive system

Regions of the digestive system are adapted to the digestion and absorption of food:

Diagram of the human digestive system

Digestion

RegionFunction
MouthBegins the digestion of carbohydrates
StomachBegins the digestion of protein; small molecules such as alcohol absorbed
Small intestine - duodenum Continues the digestion of carbohydrate and protein; begins the digestion of lipids
Small intestine - ileum Completes the digestion of carbohydrates and proteins into single sugars and amino acids; absorption of single sugars, amino acids and fatty acids and glycerol
Large intestineAbsorption of water; egestion of undigested food

Digestive enzymes are used to break down food in the gut into small, soluble molecules that can be absorbed through the gut wall.

Absorption

The surface of the small intestine wall is folded, and has projections called villi. Villi is the plural of villus.

The epithelial cells that cover each villus themselves have projections called microvilli.

Diagram illustrating how lipids pass through the gut wallA close up of the villi in the small intestineDiagram showing how food molecules are absorbedCross-section of a villus

These all increase the surface area over which digested food – now simple molecules – is absorbed.

Most of the digested food passes through the epithelial cells of the gut wall and is carried by blood to the liver.

Digested lipids pass through the gut wall and enter the lacteals.

The lacteals in each villus join together into larger vessels. Then all the digested lipids pass through a duct into the bloodstream.