The large intestine is so named because its diameter is greater than that of the small intestine. This portion of the alimentary canal is about 1.5 meters long, and it begins in the lower right side of the abdominal cavity, where the ileum joins the cecum. From there, the large intestine travels upward on the right side, crosses obliquely to the left, and descends into the pelvis. At its distal end, it opens to the outside as the anus.
The large intestine re-absorbs water and electrolytes
from the chyme remaining in the alimentary canal. It also
forms and stores the feces until defecation occurs.
Parts of the Large Intestine
The large intestine, consists of the cecum , colon, rectum, and anal canal.
The cecum, which represents the beginning of the large intestine, is dilated, pouchlike structure that hangs slightly below the ileocecal opening. Projecting downward from it is a narrow tube with a closed end called the vermiform appendix. Although the human appendix has no digestive function, it does contain lymphatic tissue that can provide immune functions.
The colon is divided into four portion - the ascending, transverse, descending, and sigmoid colons. The ascending colon begins at the cecum and travels upward against the posterior abdominal wall to a point just below the liver. There it turns sharply to the left and becomes the transverse colon. The transverse colon is the longest and the most mobile part of the large intestine. It is suspended by a fold of peritoneum and tends to sag in the middle below the stomach. As the transverse colon approaches spleen, it turns abruptly downward and becomes the descending colon. At the brim of the pelvis, the descending colon makes an S-shaped curve, called the sigmoid colon, and then becomes the rectum.
The rectum lies next to the sacrum and follows its curvature. It is firmly attached to the sacrum by connective tissue, and it ends about 5 centimeters below the tip of the coccyx, where it becomes the anal canal.
The anal canal is formed by the last
2.5 to 4.0 centimeters of the large intestine. The mucous membrane
in the anal canal is folded into a series of six to eight longitudinal
anal columns. At its distal end, the canal opens to the outside
as the anus. This opening is guarded by two sphincter muscles,
an internal anal sphincter, composed of smooth muscle, under involuntary
control, and an external anal sphincter, composed of skeletal
muscle, under voluntary control.
Structure of the Large Intestinal Wall
Although the wall of the large intestine includes
the same types of tissues found in other parts of the alimentary
canal, it has some unique features. For example, it lacks the
villi that are characteristic of the small intestine. Also,
the layer of longitudinal muscle fibers does not cover its wall
uniformly. Instead, the fibers are arranged in three distinct
bands (teniae coli) that extend the entire length of the
colon. These bands exert tension on the wall, creating a series
of pouches (haustra).
Functions of the Large Intestine
Unlike the small intestine, which secretes digestive enzymes and absorbs the products of the digestion, the large intestine has little or no digestive function. On the other hand, the mucous membrane that forms the inner lining of the large intestine contains many tubular glands. Structurally these glands are similar to those in the small intestine, but they are composed almost entirely of goblet cells. Consequently, mucus is the only significant secretion of the large intestine.
The mucus secreted into the large intestine protects the intestinal wall against the abrasive action of the material passing through. It also aids in holding particles of fecal matter together, and because it is alkaline, mucus helps to control the pH of the large intestinal contents.
The chyme entering the large intestine contains
materials that were not digested or absorbed by the small intestine.
It also contains water, various electrolytes, and bacteria. The
proximal half of the large intestine functions to re-absorb
some of the water and electrolytes. The substances that remain
in the tube become feces, which are stored for a time in the distal
portion of the large intestine.
Movements of the Large Intestine
The movements of the large intestine - mixing and peristalsis - are similar to those of the small intestine, although they are usually more sluggish. Also, instead of occurring frequently, peristaltic waves in the large intestine come only two or three times each day. These waves produce mass movements in which a relatively large section of the colon constricts vigorously, forcing its contents to move toward the rectum. Typically, mass movements occur following a meal, as a result of a reflex that is initiated in the small intestine. Abnormal irritations of the mucosa also can trigger such movements. For instance, a person suffering from an inflamed colon (colitis) may experience frequent mass movements.
When it is appropriate to defecate, a person
usually can initiate a defecation reflex by holding a deep breath
and contracting the abdominal wall muscles. This action increases
the internal abdominal pressure and forces the feces into the
rectum. As the rectum fills, its wall is distended and the defecation
reflex is triggered. As a result, peristaltic waves in the
descending colon are stimulated, and the internal sphincter relaxes.
At the same time, other reflexes involving the sacral region of
the spinal cord cause the peristaltic waves to strengthen, the
diaphragm to lower, the glottis to close, and the abdominal wall
muscles to contract. These actions cause an additional increase
in the internal abdominal pressure and assist in squeezing the
rectum The external sphincter is signaled to relax, and the feces
are forced to the outside. A person usually can inhibit defecation
voluntarily by keeping the external anal sphincter contracted.
As was mentioned, the feces are composed largely of materials that were not digested or absorbed, together with water, electrolytes, mucus, and bacteria. Usually the feces are about 75 percent water, and their color is normally due to the presence of bile pigments that have been altered somewhat by bacterial actions.
The pungent odor of the feces result from a variety of compounds produced by the bacteria acting upon the residues.