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Chapter: Medical Physiology: Physiology of Gastrointestinal Disorders

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Disorders of the Large Intestine

Constipation means slow movement of feces through thelarge intestine; it is often associated with large quantities of dry, hard feces in the descending colon that accumu-late because of over-absorption of fluid.

Disorders of the Large Intestine

Constipation

Constipation means slow movement of feces through thelarge intestine; it is often associated with large quantities of dry, hard feces in the descending colon that accumu-late because of over-absorption of fluid. Any pathology of the intestines that obstructs movement of intestinal contents, such as tumors, adhesions that constrict the intestines, or ulcers, can cause constipation. A frequent functional cause of constipation is irregular bowel habits that have developed through a lifetime of inhibi-tion of the normal defecation reflexes.

Infants are seldom constipated, but part of their train-ing in the early years of life requires that they learn to control defecation; this control is effected by inhibiting the natural defecation reflexes. Clinical experience shows that if one does not allow defecation to occur when the defecation reflexes are excited or if one over-uses laxatives to take the place of natural bowel func-tion, the reflexes themselves become progressively less strong over months or years, and the colon becomes atonic. For this reason, if a person establishes regularbowel habits early in life, usually defecating in the morning after breakfast when the gastrocolic and duo-denocolic reflexes cause mass movements in the large intestine, the development of constipation in later life is much less likely.

Constipation can also result from spasm of a small segment of the sigmoid colon. It should be recalled that motility even normally is weak in the large intestine, so that even a slight degree of spasm is often capable of causing serious constipation. After the constipation has continued for several days and excess feces have accu-mulated above a spastic sigmoid colon, excessive colonic secretions often then lead to a day or so of diar-rhea. After this, the cycle begins again, with repeated bouts of alternating constipation and diarrhea.

Megacolon. Occasionally, constipation is so severe thatbowel movements occur only once every several days or sometimes only once a week. This allows tremendous quantities of fecal matter to accumulate in the colon, causing the colon sometimes to distend to a diameter of 3 to 4 inches. The condition is called megacolon, or Hirschsprung’s disease.

A frequent cause of megacolon is lack of or deficiency of ganglion cells in the myenteric plexus in a segment of the sigmoid colon. As a consequence, neither defeca-tion reflexes nor strong peristaltic motility can occur in this area of the large intestine. The sigmoid itself becomes small and almost spastic while feces accumu-late proximal to this area, causing megacolon in the ascending, transverse, and descending colons.

Diarrhea

Diarrhea results from rapid movement of fecal matter through the large intestine. Several causes of diarrhea with important physiologic sequelae are the following.

Enteritis. Enteritis means inflammation usually causedeither by a virus or by bacteria in the intestinal tract. In usual infectious diarrhea, the infection is most extensive in the large intestine and the distal end of the ileum. Everywhere the infection is present, the mucosa becomes extensively irritated, and its rate of secretion becomes greatly enhanced. In addition, motility of the intestinal wall usually increases manyfold. As a result, large quantities of fluid are made available for washing the infectious agent toward the anus, and at the same time strong propulsive movements propel this fluid forward. This is an important mechanism for ridding the intestinal tract of a debilitating infection.

Of special interest is diarrhea caused by cholera (and less often by other bacteria such as some pathogenic colon bacilli). Cholera toxin directly stimulates excessive secretion of electrolytes and fluid from the crypts of Lieberkühn in the distal ileum and colon. The amount can be 10 to 12 liters per day, although the colon can usually reabsorb a maximum of only 6 to 8 liters per day. Therefore, loss of fluid and electrolytes can be so debilitating within several days that death can ensue.

The most important physiologic basis of therapy in cholera is to replace the fluid and electrolytes as rapidly as they are lost, mainly by giving the patient intravenous solutions. With proper therapy, along with the use of antibiotics, almost no cholera patients die, but without therapy, as many as 50 per cent do.

Psychogenic Diarrhea. Everyone is familiar with the diar-rhea that accompanies periods of nervous tension, such as during examination time or when a soldier is about to go into battle. This type of diarrhea, called psy-chogenic emotional diarrhea, is caused by excessivestimulation of the parasympathetic nervous system, which greatly excites both (1) motility and (2) excess secretion of mucus in the distal colon. These two effects added together can cause marked diarrhea.

Ulcerative Colitis. Ulcerative colitis is a disease in whichextensive areas of the walls of the large intestine become inflamed and ulcerated. The motility of the ulcerated colon is often so great that mass movements occur much of the day rather than for the usual 10 to 30 minutes. Also, the colon’s secretions are greatly enhanced.As a result, the patient has repeated diarrheal bowel movements.

The cause of ulcerative colitis is unknown. Some cli-nicians believe that it results from an allergic or immune destructive effect, but it also could result from chronic bacterial infection not yet understood. Whatever the cause, there is a strong hereditary tendency for suscep-tibility to ulcerative colitis. Once the condition has pro-gressed very far, the ulcers seldom will heal until an ileostomy is performed to allow the small intestinal con-tents to drain to the exterior rather than to pass through the colon. Even then the ulcers sometimes fail to heal, and the only solution might be surgical removal of the entire colon.

Paralysis of Defecation in Spinal Cord Injuries

It will be recalled that defecation is normally initiated by accumulating feces in the rectum, which causes a spinal cord–mediated defecation reflex passing from the rectum to the conus medullaris of the spinal cord and then back to the descending colon, sigmoid, rectum, and anus.

When the spinal cord is injured somewhere between the conus medullaris and the brain, the voluntary portion of the defecation act is blocked while the basic cord reflex for defecation is still intact. Nevertheless, loss of the voluntary aid to defecation—that is, loss of the increased abdominal pressure and relaxation of the voluntary anal sphincter—often makes defecation a dif-ficult process in the person with this type of upper cord injury. But, because the cord defecation reflex can still occur, a small enema to excite action of this cord reflex, usually given in the morning shortly after a meal, can often cause adequate defecation. In this way, people with spinal cord injuries that do not destroy the conus medullaris of the spinal cord can usually control their bowel movements each day.

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