Physiology of the Cecum and Colon
in carnivores the cecum and colon primarily absorb water and some electrolytes to reduce the volume and fluidity of the digesta in the formation of feces. Feces are also stored in the terminal portions of the colon prior to their movement into the rectum for defecation.
in omnivores (e.g., pig) and some herbivores (e.g., cattle and sheep) the cecum and colon are also sites of some limited fermentation and microbial digestion. In cattle and sheep the cecum and colon are proportionally larger and more complex than in carnivores, but the forestomach is the much more important site of fermentative digestion in these herbivores.Cecum and Colon of the Horse
The extremely large and complex cecum and great colon of the horse are the primary sites of fermentation and microbial digestion of cellulose. Roughage passes relatively quickly through the stomach and small intestine of the horse, but fermentative digestion and passage through the cecum and great colon may take days. Complex movements of the cecum and great colon mix the contents to promote fermentative digestion and to expose the contents to the epithelial surface for absorption of volatile fatty acids. The pelvic flexure of the great colon and the junction between the great and small colons (see Fig. 20-12) are relatively small in diameter, and it appears that the passage of large particles of roughage is restricted. While these sites retain roughage so that it can be subjected to microbial digestion, the potential for impaction at these sites is also increased.
When consumed with roughage, some starches and sugars escape digestion in the equine stomach and small intestine and pass into the cecum and colon. Microbial digestion of these and the cellulose in the roughage produces volatile fatty acids that can be absorbed and used for energy. Microbes in the cecum and colon can also use nonprotein nitrogen sources (urea) for the production of microbial proteins. However, these have limited nutritional value to the horse, because the gastrointestinal mechanisms necessary to digest proteins and absorb the resulting amino acids are not readily available in the cecum or colon. some urea is made available to microbes by diffusing into the cecum and colon from the blood.
Fermentation and microbial digestion produce volatile fatty acids, which could lower the pH of the cecal and colonic contents to potentially harmful levels. The colonic epithelium secretes bicarbonate ions to buffer the pH of the contents. Additional bicarbonate is secreted by the epithelium of the ileum, and this lowers the pH of the cecal contents.