The aptly named large intestine has a greater diameter than the small intestine, and measures nearly five feet in length. It begins in the lower right of the abdomen where the cecum and the illium are joined. It proceeds up and to the right, indirectly to the left, and then goes down in the direction of the pelvis with the anus at the end, opening to the outside of the body.

The large intestine is the “waste management” portion of the digestive system. It is constantly reabsorbing and recycling water, electrolytes from remaining chyme, as well as digestive residue. This is also where feces is produced and contained.

The four major parts of the large intestine are: the cecum, the colon, the rectum, and the anal canal. As mentioned before, the cecum is located at the beginning of the large intestine. It is an open pouch-like section directly underneath the ileocecal aperture.; Attached to the outside of the cecum, is the appendix, which has no known digestive purpose.

The next section of the large intestine, the colon, is divided into four portions: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. The section immediately following the cecum is the ascending colon. It moves upward against the back of the abdomen to right below the liver where it rotates left and develops into the longest and most mobile section, the transverse colon. The transverse colon is attached to a peritoneal section, and hangs flaccidly in the center of the abdominal cavity below the stomach. When the transverse colon nears the spleen it turns downward and is aptly becomes the descending colon. At the pelvis, it takes on an S-shape and becomes the sigmoid section, followed by the anus.

The rectum is located adjacent and attached (by the peritoneum) to the sacrum (the large, triangular bone at the base of the spine) and follows its shape, ending about 2 inches below the coccyx (tailbone) where it becomes the anal canal – the last section of the large intestine.

The mucous membrane of the anal canal is creased into a sequence of several ridges called anal colums which lead to the anus at the very end of the digestive system. The anus has two sphincter muscles – internal and external. The internal is composed of smooth muscle and is under involuntary control, and the exterior is skeletal muscle and under voluntary control.

While the wall tissue of the large intestine is similar to those present in different sections of the alimentary canal, there are some unique differences. It does not have the villi and plicae circularis like the small intestine, and the layer of muscle fibers (longitudinal) covering the wall are not uniform. These fibers are arranged in three bands, called teniae coli, and extend the entire length of the colon. These bands create tension on the wall, resulting in a series of pouches known as haustra. In addition, the wall features epiploic appendages, which are small collections of fat, in the outer surface serosa (smooth membrane consisting of a thin layer of cells which excrete a fluid, known as serous fluid).

While the function of the small intestine involves the secretion of enzymes and the subsequent absorption of nutrients, the large intestine is not such an active participant in the digestive process. The only significant secretion in the large intestine is mucous, from goblet cells, which does serve important functions. The mucous protects the intestinal from the passing of abrasive material, contributes to the consistency of fecal matter, and its alkaline nature helps control the pH balance, which can be raised by the acidity released by bacterial activity.

Chyme found in the large intestine normally has few nutrients remaining. However, water and electrolytes are absorbed, and the naturally occurring intestinal flora synthesizes and absorbs vitamins and other undigested material such as cellulose.

The movements of the large intestine resemble the peristaltic movements of the small intestine, but in a more sluggish fashion, occurring only two or three times a day and referred to as mass movements - usually occurring following a meal and initiating a defecation reflex. Feces is then forced into the rectum and exits the body through a combination of internal and external anal sphincter control.

This is the final installment of the Digestive Voyage series from Enzymes, Inc. To read the series in it’s entirety, visit www.enzymeexperts.com and click “Health Talk” on the navigation bar. Thanks for reading!