BIOLOGY Volume 1 - A Guide to General Biology - 2004

8. HETEROTROPHIC NUTRITION

8.3. The Human Alimentary Canal

8.3.10. The Large Intestine

No further food Digestion takes place in the Large Intestine. The vast majority of fluids (about 90%) and salts are absorbed in the Small Intestine. About 90% of the remaining fluid is absorbed in the colon and cecum. Certain inorganic ions, particularly calcium and iron when present in excess, are excreted as salts, as are some Metabolic waste products. The epithelial Cells of the large intestine secrete mucus, which lubricates the increasingly solid undigested food residues known as feces (stools). A rich community of symbiotic Bacteria resides in the large intestine, synthesizing Amino Acids and certain Vitamins, including vitamin K, which are then absorbed into the bloodstream.

In humans, the vermiform Appendix is a blind-ended pouch extending from the cecum, the function of which remains unclear. In herbivores, however, it plays a vital role (section 8.6.2). Feces consist primarily of dead bacteria, Cellulose and other plant fibers, sloughed epithelial cells, mucus, Cholesterol, Bile pigment derivatives, and Water. They may remain in the colon for up to 36 hours before reaching the rectum, where they are stored briefly before being eliminated via the anus. The anus is encircled by two sphincters: the internal sphincter, composed of smooth Muscle and controlled by the Autonomic Nervous system, and the external sphincter, composed of Skeletal Muscle and controlled by the Central Nervous System.

Table 8.4 summarizes the structural differences among the main Regions of the human alimentary canal.

Class="center">Table 8.4. Comparison of structural differences between the main regions of the human alimentary tract