10.10: Leaving the Digestive Tract

The slow, kneading contractions in the colon move the fecal matter toward the rectum (the last five inches of the digestive tract). More or less regularly, this leisurely pattern of motion is replaced by a powerful set of waves, accompanied by a series of muscular actions.

This pushes the feces into the normally-empty rectum, and the resulting distention triggers the defecation reflex. The muscles of the abdominal wall contract, and the muscular ring called the anus relaxes. The result is a bowel movement— unless we prevent it. We can voluntarily tighten the anus and begin a pattern of shallow breathing until the rectum accommodates to the mass.

Frequently, eating or drinking triggers this urge to defecate (“gets the intestines moving”). Infants usually defecate after a meal. But adults, often suppress or ignore this defecation reflex.

Constipation

The blocking of the normal defecation reflex is thought to be a common cause of constipation— the chronically constipated person has learned to break the defecation reflex. Such willful breaking of the reflex makes matters worse. As the feces remain overlong in the colon and rectum, more and more water is extracted, and the feces become hard and dry—and harder to pass.

A common American response to habitual constipation is to take a laxative. The person who blocks the defecation reflex (usually unconsciously) and then uses laxatives, is caught in a vicious cycle. The intestines become dependent upon artificial stimulation and doesn’t function as well without it. This is often spoken of as laxative addiction.

People often seek to explain constipation by laying the blame on what they’ve eaten. It’s true that some foods, such as low-fiber foods, tend to slow intestinal motility. It’s also true that modest amounts of fiber can help prevent constipation by holding water, thereby making the stools bulkier, softer, and easier to pass. (Fiber, in this way, can also help prevent diverticulosis, discussed in Chap. 5). But an ordinary mixed diet causes neither constipation nor diarrhea.

It’s a popular belief that a daily bowel movement is essential to health—a belief so strong that many people worry if they miss a day. But there is usually nothing to worry about if one is “irregular.” In fact, it’s quite normal to be irregular. Three times a week—or twice a day—are both well within normal. If given a chance, the bowel is normally self-regulating.

One shouldn’t be unduly concerned with bowel movements, but a physician should be consulted if there’s blood in the stool or a marked change in the usual pattern of bowel movements or in the appearance of the stool. A change to a very light color, for instance, might be a result of fat malabsorption due to inadequate bile production, or a change to a black color might be due to blood from a bleeding ulcer.

Diarrhea

There are many diseases that cause diarrhea. In developing countries, a common cause is disease-causing bacteria (usually from sewage) that have contaminated the drinking water. Because water is lost in diarrhea, severe diarrhea can cause severe dehydration. This is, in fact, a major cause of infant death in developing countries.

When we eat contaminated food, the body’s natural reaction is to get rid of the irritant, often by vomiting or diarrhea. It doesn’t take much vomiting or diarrhea to get rid of the food that causes the problem, but once started, these expulsive reactions may last longer than we’d like.

Though diarrhea, nausea, and vomiting are protective, and perhaps usefully prolonged when one’s digestive state isn’t capable of handling a meal just then, there are limits to how long such reactions are useful. In a matter of hours, vomiting or diarrhea can dehydrate the body and rob it of essential salts. If vomiting or diarrhea continues until it causes weakness, or there’s much pain, call your doctor.

Some diarrheas aren’t related to disease, but caused by emotional upset or, as we have seen, lactose intolerance. Ingesting large amounts of sugar alcohols such as sorbitol and mannitol can also cause diarrhea. Sugar alcohols are commonly used as sweeteners in sugarless gum and dietetic foods, but are only slowly absorbed; some may remain unabsorbed and proceed to the colon and produce diarrhea—and gas.