MECHANICAL FUNCTIONS OF THE LARGE INTESTINE

1. What are the functions of the large intestine? Is greater time required for these functions?

2. Why is increased colonic activity associated with constipation and decreased activity associated with diarrhea?

3.

4. Note the frequency of defecation in cattle, horses, and carnivores. Note the transit times of food in pigs, horses, and cattle.

The large intestine provides for microbial digestion and for reabsorption of electrolytes and water. Both of these functions take longer than the digestion and absorption that occurs in the small intestine. Fermentation on the magnitude that occurs in the large intestine of the horse requires a large volume of buffered fluid to neutralize the acidic end products of microbial digestion. The motor activity of the large intestine provides for the delay time. Cecal contractions help to mix the contents and remove gas, with controlled emptying into the colon. Haustral contractions are isolated events in the colon and help to mix the contents. The stationary haustral contractions increase resistance to flow in either direction.

Peristaltic movements in the colon occur in either an oral or an aboral direction. In an oral direction they produce retrograde flow, which delays movement of ingesta. Retrograde flow coupled with anatomic narrowing delays filling of various parts of the colon. Anatomic narrowing occurs at the pelvic flexure in the horse, where the ventral colon turns to become the dorsal colon. Accordingly, filling of the dorsal colon is delayed. The frequency of slow wave activity in the small intestine decreases in the aboral direction, but the frequency of slow waves in the colon decreases in the oral direction for.the first half of the colon and accounts for the retrograde movement of contents (see previous text).

Much of the activity of the colon is thus directed toward the delay of transit and filling of its parts (reservoir function). Increased colonic activity is therefore associated with constipation and decreased activity is associated with diarrhea.

Defecation

Defecation is a complex reflex act in which feces are evacuated from the terminal colon and rectum. The frequency of defecation varies among animals but can occur 5 to 10 times daily in vigorous horses, 10 to 20 times daily in cattle, and 2 to 3 times daily in carnivores. The reflex can be assisted or inhibited by certain voluntary muscles.

The time required for food to pass through the digestive tract varies among species. Studies were carried out in various species using dye-stained (marked) food. The average time for food passage was determined because ingested food that is marked is mixed with food ingested at other times. The average time for pigs was found to be 48 hours and for horses it was 24 to 48 hours. Because of the voluminous forestomach in cattle, the dilution of marked food with other food is increased and makes its initial appearance in feces in 12 to 24 hours. About 80% of the initial amount is passed by 3 to 4 days and final evacuation is complete by 7 to 10 days.

Intestinal Transport of Electrolytes and Water

The secretion of water and electrolytes into the digestive tract has many purposes. These secretions are derived from the extracellular fluids. They are particularly voluminous in herbivorous and omnivorous animals. An important function of the intestine is the return of water and electrolytes to the extracellular fluid before they are lost in the feces. The major reabsorption sites for these substances are the distal small intestine and the large intestine. Reabsorption of secretions is compromised in diarrhea and other conditions and, if the problem is not corrected or the secretions are not replenished, an animal can soon die because of blood volume loss and circulatory collapse.

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