Alimentary Canal – Large Intestine
The large intestine is so named because its diameter is greater than that of the small intestine. This part of the alimentary canal is about 1.5 meters long. It begins in the lower right side of the abdominal cavity, where the ileum joins the cecum. From there, the large intestine ascends on the right side, crosses obliquely to the left, and descends into the pelvis. At its distal end, it opens to the outside of the body as the anus. The large intestine absorbs water and electrolytes from chyme remaining in the alimentary canal. It also forms and stores feces.
The large intestine consists of the cecum, colon, rectum, and anal canal. The cecum, at the beginning of the large intestine, is a dilated, pouchlike structure that hangs slightly below the ileocecal opening. Projecting downward
from it is a closed end, narrow tube containing lymphatic tissue called the appendix. The human appendix has no
known digestive function.
The colon is divided into four parts—the ascending, transverse, descending, and sigmoid colons. The ascending
colon begins at the cecum and continues upward against the posterior abdominal wall to a point just inferior to the liver. There, it turns sharply to the left and becomes the transverse colon. The transverse colon is the longest and most movable part of the large intestine.
It is suspended by a fold of peritoneum and sags in the middle below the stomach. As the transverse colon approaches the 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 generally follows its curvature. The peritoneum firmly attaches the rectum to the sacrum, and the rectum ends about 5 centimeters below the tip of the coccyx, where it becomes the anal canal. The last 2.5–4.0 centimeters of the large intestine form the anal canal. The mucous membrane in the canal is folded into six to eight longitudinal anal columns. At its distal end, the canal opens to the outside as the anus. Two sphincter muscles guard the anus—an internal anal sphincter muscle, composed of smooth muscle under involuntary control, and an external anal sphincter muscle, composed of skeletal muscle under voluntary control.
Structure of the Large Intestinal Wall
The wall of the large intestine is composed of the same types of tissues as other parts of the alimentary canal but
also has some unique features. The large intestinal wall lacks the villi characteristic of the small intestine, and the
layer of longitudinal muscle fibers is not uniformly distributed throughout the large intestinal wall. Instead, the
fibers form three distinct bands (teniae coli) that extend the entire length of the colon. These bands
exert tension lengthwise on the wall, creating a series of pouches (haustra).
Functions of the Large Intestine
The large intestine has little or no digestive function, in contrast to the small intestine, which secretes digestive enzymes and absorbs the products of digestion. However, the mucous membrane that forms the large intestine’s
inner lining contains many tubular glands. Structurally, these glands are similar to those of the small intestine,
but they are composed almost entirely of goblet cells . Consequently, mucus is the large intestine’s only significant secretion.
Mucus secreted into the large intestine protects the intestinal wall against the abrasive action of the materials
passing through it. Mucus also binds particles of fecal matter, and its alkalinity helps control the pH of the large intestinal contents. Chyme entering the large intestine contains materials that the small intestine did not digest or absorb. It also contains water, electrolytes, mucus, and bacteria. The large intestine normally absorbs water (although most water is absorbed in the small intestine) and electrolytes in the proximal half of the tube. Substances that remain in the tube become feces and are stored in the distal part of the large intestine.
The many bacteria that normally inhabit the large intestine, called intestinal flora, break down some of the molecules that escape the actions of human digestive enzymes. For instance, cellulose, a complex carbohydrate in food of plant origin, passes through the alimentary canal almost unchanged, but colon bacteria can break down cellulose and use it as an energy source. These bacteria, in turn, synthesize certain vitamins, such as K, B12, thiamine, and riboflavin, which the intestinal mucosa absorbs. Bacterial actions in the large intestine may produce intestinal gas (flatus).
Movements of the Large Intestine
The movements of the large intestine—mixing and peristalsis—are similar to those of the small intestine, although usually slower. Also, peristaltic waves of the large intestine happen only two or three times each day. These waves produce mass movements in which a large section of the intestinal wall constricts vigorously, forcing the intestinal contents toward the rectum.
Typically, mass movements follow a meal as a result of the gastrocolic reflex initiated in the small intestine. Irritations of the intestinal mucosa also can trigger such movements. For instance, a person with an inflamed
colon (colitis) may experience frequent mass movements. Clinical Application 15.3 discusses inflammatory
bowel disease. A person can usually initiate a defecation reflex by holding a deep breath and contracting the abdominal wall muscles. This action increases internal abdominal pressure and forces feces into the rectum. As the
rectum fills, its wall distends, triggering the defecation reflex that stimulates peristaltic waves in the descending
colon. The internal anal sphincter relaxes. At the same time, other reflexes involving the sacral region of the spinal cord strengthen the peristaltic waves, lower the diaphragm, close the glottis, and contract the abdominal wall muscles. These actions further increase internal abdominal pressure and squeeze the rectum. The external anal sphincter is signaled to relax, and the feces are forced to the outside. Contracting the external anal sphincter allows voluntary inhibition of defecation.
Feces (fe′se¯z) include materials not digested or absorbed, plus water, electrolytes, mucus, shed intestinal
cells, and bacteria. Usually, feces are about 75% water, and their color derives from bile pigments altered by bacterial action. Feces’ pungent odor results from a variety of compounds that bacteria produce.