Digestion

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.

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

Saliva also contains mucus and hydrogen carbonate, which needs to be present in order to maintain a pH that will help amylase work. After chewing and saliva act on the food, it enters the esophagus as a bolus and travels to the stomach by means of peristaltic action. Gastric juices also take part in chemical digestion but not completely in an enzymatic way. The contents of gastric juice is mainly pepsin and hydrochloric acid, which are protected from actually damaging the mucosa itself by the action of mucus secreted by cells in the stomach lining. Mechanical digestion takes place in the stomach as well because there is peristalsis that further separates the food boluses into a smooth liquid called chyme. Chyme enters the duodenum after the pyloric sphincter in the distal part of the stomach opens to let it into the narrower duodenal lumen. Digestive enzymes from both the liver and pancreas act chemically on the chyme constituents to make the food even more digestible. Bile acids primarily act on the fats in the consumed food, while pancreatic enzymes act on the proteins and carbohydrates. About 95 percent of the digested food is absorbed in the small intestine, with the rest absorbed in the colon. Water and vitamins are mainly absorbed by the large intestine with the help of symbiotic bacteria. In fact, both vitamin K and biotin are made by the symbiotic bacteria in the large intestine and are absorbed there as well. Anything not digested and absorbed is passed as waste through the act of defecation.

There are three main phases of digestion. The cephalic phase is first, followed by the gastric phase and the intestinal phase. The cephalic phase begins before food is even consumed and starts with just thoughts, smells, and the sight of food. This stimulates the medulla oblongata and the hypothalamus in the brain, which send signals by means of the vagus nerve, which releases acetylcholine, triggering the release of gastric acid. The gastric phase of digestion lasts about three to four hours and is stimulated by food and distention of the stomach. The pH of the stomach is partially buffered by food so the pH isn’t as high as it is during the cephalic phase of digestion. More gastric juices are secreted by the release of acetylcholine, which is the main digestive neurotransmitter involved in the action of the stomach on food. The intestinal phase of digestion has two separate parts. The first is the excitatory part and the second is the inhibitory part. It begins in the duodenum when food passes through the pyloric sphincter. Intestinal gastrin is released because of the distention of the duodenum and sympathetic fibers are activated, causing the pyloric sphincter to close so that just the right amount of food leaves the stomach at any given point in time. Protein digestion happens in both the stomach and the duodenum. There are three main enzymes responsible for this digestion: pepsin, trypsin, and chymotrypsin (with pepsin being the only one that acts in the stomach). Exopeptidases and dipeptidases in the duodenum break down small peptides into amino acids, which are then absorbed by the small intestinal lining. Digestive enzymes from the pancreas are not secreted as complete enzymes but are instead secreted as zymogens (pre-enzymes) that are further modified in the duodenal lumen to become active enzymes. Fat digestion starts in the mouth with chemical digestion by lingual lipase in the saliva. It breaks down short-chain fatty acids into diglycerides. The rest of fat digestion happens in the small intestines. Pancreatic lipase helps break down fats and bile acids emulsify fats so they can be better absorbed by the intestinal lining.