Introduction

The intestinal tract is the point of entry of all metabolic en­ergy into the animal. The processes of digestion and absorp­tion undertaken in the intestinal tract need to be carried out over a large surface area.

The intestinal tract has numerous functions, all of which are necessary for normal digestion. The five main digestive func­tions of the intestinal tract are:

■ Motility - The movement of ingesta through the intestinal tract from the stomach to the colon /rectum and eventual expulsion as feces.

■ Secretion - Secretion of fluid, enzymes, and electrolytes into the GI lumen.

■ Digestion - Degradation of ingesta occurs in the cranial small intestine, mediated largely by pancreatic digestive en­zymes. The small intestinal mucosa also has a role in the further digestion of food material at the brush border (see below).

■ Absorption - Following degradation of ingested food­stuffs, the resulting nutrients need to be assimilated and introduced into the animal’s circulation. Fluid and electro­lytes secreted during the processes of digestion are reab­sorbed during the formation of feces.

■ Barrier Function - Prevention of the translocation of in­testinal bacteria and digestive enzymes into the circulation as well as the prevention of loss of plasma proteins.

In addition to the digestive functions listed above, the intesti­nal tract has an important role in normal immune function, antigen processing and presentation, and a role in the endo­crine regulation of digestive enzyme synthesis and release by other digestive organs, such as the pancreas. In this section, the anatomical structure of the intestinal tract is described, par­ticularly with reference to the anatomical features contribut­ing to the large surface area of the small intestine. The endo­crine and immunological features of intestinal function are described in 5.2.

5.1.2