The large intestine (colon)
The large intestine receives the liquid residue after digestion and absorption are complete. This residue consists mostly of water as well as materials (e.g. cellulose) that were not digested. While the contents of the small intestine are normally sterile, the colon contains an enormous (~1014) population of microorganisms. (Our bodies consist of only ~1013 cells!)
Most of the species live there perfectly harmlessly; that is, they are commensals. Some are actually beneficial, e.g.,
- by synthesizing vitamins and
- by digesting polysaccharides for which we have no enzymes (providing an estimated 10% of the calories we acquire from our food).
Most of the bacteria belong to the Firmicutes and Bacteroidetes (although used as an indicator of fecal pollution, E. coli is actually a minor component). In both obese mice (ob/ob) and humans, the relative proportion of Bacteroidetes declines and, in mice at least, the efficiency with which residual food is absorbed increases. Putting humans on a diet causes them to regain the normal proportion of Bacteroidetes. Why this relationships exists remains to be discovered.
Bacteria flourish to such an extent that as much as 50% of the dry weight of the feces may consist of bacterial cells.
Reabsorption of water is the chief function of the large intestine. The large amounts of water secreted into the stomach and small intestine by the various digestive glands must be reclaimed to avoid dehydration. If the large intestine becomes irritated, it may discharge its contents before water reabsorption is complete causing diarrhea. On the other hand, if the colon retains its contents too long, the fecal matter becomes dried out and compressed into hard masses causing constipation.