Most digestive disorders occur in the small intestine and the large intestine.  These can include birth defects, infections, syndromes that affect absorption, and tumors. Although birth defects tend to be somewhat rare, they can cause major problems when they do occur. Some of the birth defects of the small intestine include a failure of the intestinal tube to develop, called artesia, or a narrowing of the tube, called a stenosis. Another birth defect, called a Meckel diverticulum, is notable because it causes a pouch to protrude from the tube. Although the width of this pouch tends to vary, the length is about two to two and a half inches long. The pouches tend to occur in the ileum and the cecum and involve all of the layers of tissue.  Sometimes they can allow too much bacteria to grow within them, which can lead to a lack of vitamin B12. Ulcers can also form in the mucosa next to the pouch, which leads to bleeding or symptoms resembling appendicitis. Major birth defects of the large intestine include malrotation, in which the large intestine fails to assume its proper place in the abdomen, and Hirshprung disease, which leads to an unusually large colon.

Other diseases can be caused by bacterial infections, viral infections, and protozoan infections in the small and large intestine. Bacterial overgrowth in the small intestine can lead to decreased motility of chyme. Damage or destruction of the epithelium by viruses or bacteria can result in diarrhea. Some common diseases include viral gastroenteritis and bacterial enterocolitis. Viral gastroenteritis is usually spread from one person to another or by having the mouth come into contact with a surface that has had contact with feces. Bacterial enterocolitis can result from eating contaminated food or water or from an invasion by bacteria. These bacteria can attach themselves to cells in the epithelium, release toxins that can cause diarrhea without damage to the cell, or damage the cell, and reproduce within the cell, ultimately destroying it. They then invade nearby cells. Protozoans such as amoebas can also be spread by placing contaminated surfaces in the mouth. These amoebas invade the crypt cells in the colon and then burrow down into the submucosa, which creates an ulcer. In some people, these amoebas then travel through the bloodstream to the liver or other organs and create abscesses, which can remain long after the initial infection.

Malabsorption syndromes occur when proteins, carbohydrates, and fats are not adequately broken down by enzymes or when nutrients and electrolytes are absorbed by the small intestine. Conditions such as the decreased ability of the pancreas to secrete pancreatic juice or sodium bicarbonate, decreased ability or inability of the liver and gallbladder to secrete bile can lead to decreased ability to absorb nutrients. Malabsorption is particularly common in Zollinger-Ellison syndrome, in which enzymes secreted by the pancreas are inhibited by too much stomach acid. Malabsorption can occur when the cells in the intestinal mucosa are abnormal. Overgrowth of native bacteria in the small intestine can impair absorption of nutrients and damage the mucosa. Absorption can also be decreased if the surface area of the small intestine is reduced, such as that which occurs as a result of surgery (short gut syndrome), an autoimmune disorder (celiac disease), or an inflammatory disease (Crohn disease).

As in the stomach, tumors can occur in the small and large intestine.  Polyps are formed if the cells of the mucosa mature abnormally, the mucosa is inflamed, or the mucosa structure is not normal. Although some polyps arise from the submucosa or other layers, most arise from the epithelium. They are particularly common in the colon. Some types of polyps include hyperplastic polyps, juvenile polyps, inflammatory polyps, and lymphoid polyps. Although generally benign, some have the potential to become malignant. In particular, hyperplastic polyps that form on the right side of the colon are more likely to become malignant than other polyps.

Most cancerous tumors arise from polyps that can be removed during a colonoscopy. Most malignant cancers of the colon originate in glandular tissue. They generally grow from abnormal cells in the epithelium.  In general, a polyp found upon colonoscopy is likely to be malignant if it is one-and-a-half inches in diameter.

Last Updated: Saturday, July 16, 2011