|Anatomy & Physiology 5/e Seeley/Stephens/Tate|
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Disorders of the Large Intestine
The large intestine is the source of many medical problems, from such familiar digestive discomforts as gas to more serious disorders.
People do not often talk about intestinal gas, but this common evidence of digestion is a source of pain and sometimes embarrassment to many. What exactly is in intestinal gas? Most of it is nitrogen and oxygen gulped in while breathing and eating. Undigested food fermented by bacteria contributes methane (CH4), carbon dioxide (CO2), and hydrogen. These gases account for 99% of intestinal gas. The other 1% comes from compounds also produced by intestinal bacteria, and these impart foul odors. Intestinal gas can be minimized by eating slowly, avoiding milk if you are lactose intolerant, and not eating gas-inducing foods--beans, bagels, bran, broccoli, brussels sprouts, cabbage, cauliflower, and onions.
Because the large intestine absorbs water from material within it, the rate of movement through it determines the consistency of feces. Diarrhea, the frequent and too-rapid passage of lose feces, results when material moves along so quickly that too little water is absorbed. The condition may reflect poisoning, infection, a diet too high in fiber, or nervousness. When the large intestine is the site of an infection or chemical irritation, diarrhea protects by flushing toxins out of the body.
Constipation, the infrequent passage of hard feces, is caused by abnormally slow movement of fecal matter through the large intestine. Because the feces remain in the large intestine longer than usual, excess water is absorbed. Constipation can be caused by a failure of the sensory cells in the rectum to signal the spinal cord to defecate or by the conscious suppression of defecation, both of which can be a result of emotional stress. A diet low in fiber can also cause constipation by slowing fecal movement through the large intestine. Eating foods high in fiber, drinking at least eight 8-ounce glasses of water a day, and regular exercise can prevent constipation.
More Serious Disorders
In diverticulosis, parts of the intestinal wall weaken, and the inner mucous membrane protrudes through. Many times there are no symptoms, but if the outpouching becomes blocked with chyme and then infected (a condition called diverticulitis), antibiotics or surgery may be required. More than half of all Americans develop diverticulosis by age 60. It is not known exactly what causes diverticulosis, but the fact that it is nonexistent in populations whose diets are high in fiber and that it was not known in the United States before refined foods were introduced earlier this century suggests that lack of dietary fiber may be a cause. Fiber speeds the movement of material through the digestive system, hastening exit of toxic substances.
In ulcerative colitis, inflammation of the inner lining of the colon and rectum produces abdominal and rectal pain, bloody diarrhea, and weight loss. Drugs can often treat the symptoms, but sometimes removal of the colon is necessary. Again, the cause of this disorder is not known.
Cancer of the large intestine and rectum, known as colorectal cancer, is the second most prevalent cancer in the United States, with 152,000 new cases and nearly 60,000 deaths yearly. Symptoms include a change in the frequency or consistency of bowel movements, bloody feces, and abdominal pain. A home test kit called a hemoccult (hidden blood) test can detect intestinal bleeding that may signal the presence of cancer. Blood in feces is often black and not visible. Follow-up at a doctor's office entails use of a fiber-optic colonoscope that searches and samples colorectal tissue for cancer. Soon, physicians will be able to diagnose colon cancer by performing genetic tests on cells in the feces.
Cancer or other severe maladies may require that the large intestine be removed. A new opening for feces to exit the body is then needed. The free end of the intestine can be surgically attached to an opening created through the skin of the abdomen, and a bag attached to the opening to collect the fecal matter. This procedure is called a colostomy.
Some colon cancers may take many years to develop. A sequence of genetic changes cause the cells lining the large intestine first to divide more frequently than normal, then to enter a precancerous stage. Next, the growths form benign polyps, then, perhaps years later, cancer. Still other genetic changes control the cancer's spread. The entire process may be triggered by a susceptibility gene. By understanding the steps that lead to inherited types of colon cancer, researchers may learn how other colon malignancies form and progress as well.
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