The small intestine is the longest section of the digestive tube and consists of three segments forming a passage from the pylorus to the large intestine:

• Duodenum (25 cm) : a short section that receives secretions from pancreas and liver.
  
  
• Jejunum (2.5 m) : considered to be roughly 40% of the small gut in man.
  
  
• Ileum (3.5 m) : empties into the large intestine; considered to be about 60% of the intestine in man.

It is in the small intestine that the final stages of digestion occur. The small intestine is the sole site in the digestive tube for absorption of amino acids and monosaccharides. Most lipids are also absorbed in this organ. All of this absorption and much of the digestion takes place on the small intestinal epithelial cells, and to accommodate these processes there is a huge mucosal surface area.

The structure of the small intestine is similar to other regions of the digestive tube, but the small intestine incorporates three features which account for its large surface area:

• Mucosal folds: the inner surface of the small intestine is not flat, but circular folds which increase surface area.
  
  
• Villi: the mucous forms multitudes of projections which protrude into the lumen and are covered with epithelial cells.
  
  
• Microvilli: the lumenal plasma membrane of absorptive epithelial cells is studded with densely-packed microvilli.

The epithelium of the intestine is a confluent cell layer consisting of a number of cell subtypes each with specialized functions that help it to perform its two physiological functions:

1. To provide an effective barrier between the lumenal contents of the intestinal tract and underlying sterile mucous.
2. To efficiently absorb nutrients and water from ingested food.

Loss of either of these functions is associated with varying degrees of morbidity. The body has evolved an efficient mechanism of maintaining homeostasis within the epithelium by ensuring a steady supply of cells are available to repopulate the mucosa. Mucosal epithelium undergoes a continuous rapid turnover throughout life. However the potential capacity of this mechanism is only shown during pathological conditions when major damage has occurred to the intestine. The ability of the multipotent intestinal
epithelial stem cells to repopulate insufficient areas of intestine is now recognized as a potential therapeutic strategy for conditions where the function of the intestine are insufficient. The function of epithelial stem cells is explained here.