A number of substances within food materials cannot be digested by the human digestive system.
One important material in this group is called cellulose.
Cellulose is a complex carbohydrate found in plants. Cellulose is commonly referred to as "bulk" or "fiber."
Other Undigested Food Materials
When individuals consume great quantities of foods, a portion of it will not be digested.
Passage Out of the Small Intestines
This undigested material will pass out of the small intestines with the non-digestible materials.
The resulting fluid mass enters the large intestines through the ileocecal valve.
Page 3 Large Intestines
Consolidation of Contents
In the large intestines, this fluid mass is gradually consolidated into a semisolid mass called feces.
The major function of the large intestines then is salvage.
Water is the primary salvage item. In addition to water, some previously unabsorbed end products of digestion can be absorbed here.
At the same time certain excretions from the body can be deposited in the fecal mass.
Page 4 Mucus
As the contents increase in solidity, mucus is added to facilitate their movement through the large intestines. (Previously, we have seen the addition of mucus to the bolus in the mouth to facilitate movement.)
This mucus is produced by unicellular glands in the mucosal lining of the large intestines. (Because of their
microscopic appearance, these unicellular glands are called goblet cells.)
Page 5 Microorganisms
Many microorganisms are found within the lumen or cavity of the large intestines.
Certain microorganisms are responsible for the production of vitamin K.
Depending on the type of food present, some species of microorganisms produce various gases (flatulence). On occasion, pathogenic organisms may be present and cause problems for the individual.
Page 6 Storage of Feces
Toward the lower end of the large intestines, the contents (feces) have become relatively consolidated.
This consolidated mass is retained (stored) mainly in the rectum and the lower portion of the sigmoid colon.
Page 7 Elimination
At the appropriate time, the feces is passed out of the body (defecation).
The feces passes through the anal canal and anus.
This is accomplished by the relaxation of the anal sphincter muscles.