Anyone who has bowel problems will benefit from understanding the anatomy of the large intestine (colon) and how defecation works.
The large intestine
The large intestine is a hollow muscular tube about five feet in length. (See the graphic below) It is divided into the cecum, colon, and rectum. The cecum comprises the first two or three inches of the large intestine. The colon is subdivided into the ascending, transverse, descending, and sigmoid colon. The sigmoid colon bends toward the left as it joins the rectum which allows gravity to aid the flow of water from the rectum into the sigmoid colon. (This is the rationale for lying on the left side when receiving an enema.)
The last portion of the large intestine is the rectum which extends from the sigmoid colon to the anus (about six inches.) The last inch of the rectum is called the anal canal. It contains the internal and external regulating sphincters which play an important role in regulating defecation.
Muscle contractions in the colon push the stool toward the anus. By the time it reaches the rectum, it is solid because most of the water has been absorbed.
The nerve supply to the large intestine contains both parasympathetic and sympathetic nerves. In general, stimulation of the sympathetic fibers inhibits activity in the gastrointestinal (GI) tract. It also excites the ileocecal sphincter and the internal anal sphincter. Thus, stimulation of the sympathetic fibers can totally block movement of food through the GI tract both by inhibition of the wall and closure of two major sphincters. Stimulation of the parasympathetic fibers causes an increase in bowel activity and in the defecation reflexes.
Functions of the large intestine
The large intestine has many functions, all related to the final processing of intestinal contents. Very little, if any, digestion takes place in the large intestine whose most important function is the absorption of water and electrolytes. Approximately 600ml of water is absorbed daily from the intestinal contents. The longer the fecal mass stays in the colon, the more water can be absorbed.
The movement of the intestinal contents in the large intestine is slow. Mass peristalsis, which is a contraction involving a large segment of the colon, moves the fecal mass into the sigmoid colon where it is stored. It occurs two to three times per day, especially after breakfast.
Defecation is a reflex involving the muscles of the anal canal and terminal bowel. Entry of the fecal mass into the rectum distends the rectal walls and stimulates mass peristaltic movements of the bowel which moves the feces toward the anus. As the fecal mass nears the anus, the internal anal sphincter is inhibited and if the external anal sphincter is relaxed (under voluntary control), defecation will occur. The defecation reflex may be halted by voluntary contraction of the external anal sphincter. When this is done, the defecation reflex dies out after a few minutes and usually will not return for several hours. Water continues to be absorbed from the fecal mass causing it to become firmer so that subsequent defecation is more difficult and may lead to constipation.
Defecation is also facilitated by an increase in intra-abdominal pressure brought about by contraction of the chest muscles and simultaneous contraction of the abdominal muscles (Valsalva's maneuver or straining.)
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Posted February 2003
Updated July 2009