illnesses that inflame the lining of the intestinal tract, resulting in nausea, vomiting, crampy abdominal pain, and diarrhea. Gastroenteritis is the most common GI condition in the world. Cancer can affect any part of the GI tract, from the mouth to the anus. The most common cancer of the GI tract is colon cancer, which has an incidence of about 40.1 per 100,000 men and women per year in the US and other developed countries. Appendicitis can be purely inflammatory in nature or can be caused by an infection of the cecum. Various inflammatory diseases can affect the intestinal tract. Crohn’s disease can cause inflammation of any part of the GI tract and ulcerative colitis affects only the colon, although both are autoimmune GI diseases. Celiac disease involves the inability to tolerate gluten in the diet. It mainly affects the small intestinal tract. This, too, is an autoimmune disease. Diverticular disease, like appendicitis, is both inflammatory and infectious. It mainly affects the colon but can be in the small intestine as well. Functional gastrointestinal disorders include irritable bowel syndrome, functional constipation, and chronic functional abdominal pain. These have an underlying problem with the nervous system supply to the GI tract but there are no obvious pathologic findings associated with these diseases. They affect millions of people and are notoriously difficult to treat.
Digestion The main role of the GI tract when it comes to nutrition is digestion. This involves taking food that is unable to be absorbed by the mucosa and breaking it down into smaller subunits that can be easily absorbed into the bloodstream. The actual digestion starts in the mouth and continues to the duodenum. Most of the actual absorption occurs in the small intestinal lumen. There are two types of digestion that happen in the GI tract. The first is called “mechanical digestion”, which is the physical breakdown of food so that digestive enzymes can act on it. Mechanical digestion begins in the mouth with the chewing of food. The second is called “chemical digestion” and involves the action of enzymes. The first enzyme that acts chemically on food is amylase, which is found in the saliva.
Figure 4 shows the salivary glands that provide saliva to the mouth:
Figure 4 8
