The gastrointestinal system or the digestive system as it is sometimes referred to consists of the mouth, the esophagus, the stomach and the small and large intestine. The food is digested partially in the mouth by chewing and by the action of the enzyme amylase.
This enzyme is responsible for digesting carbohydrates to sacharides. The major site of digestion is the small intestine. In the stomach proteins are degraded to amino acids by the action of the enzyme pepsin.
Between the stomach and the esophagus there is a valve that under normal conditions prevents the food from backflowing from the stomach to the esophagus.
Under pathological conditions such as when administering drugs that function by relaxing smooth muscles food can regurgitate into the esophagus, a condition that is called reflux.
The normal epithelium of the esophagus is stratified squamous epithelium. After repeated reflux, this epithelium can change to columnar epithelium, a condition that is called metaplasia. This condition in which the epithelium changes from stratified squamous to colmunar epithelium is called Barret esophagus.
Metaplasia is in general not a malignant condition but can become malignant under certain conditions. Therefore, Barret esophagus can predispose to malignancy.
Between the stomach and the small intestine there is a valve that is called gastroduodenal valve. It makes sure that food flow in one direction from the stomach to the duodenum. As was mentioned, the major site of digestion is the small intestine.
Secretions from the gallbladder and the pancreas contribute significantly to the digestive process. Both the pancreatic and the bile secretions are basic secretions that contain bicarbonate anion.
Three enzymes are secreted by the pancreas and which facilitate the digestion process. These are amylase, lipase and peptidase. These three enzymes help digest the carbohydrates and the lipids and the proteins in the diet.
In the case of pancreatic insufficiency there is a lack of secretion of these enzymes. This leads to maldigestion and constipation. In the case of lack of bile acids such as occurs in gallbladder blockage by a stone, also maldigestion occurs because bile salts help in the digestion process.
The large intestine or the colon does not have an apparent role in digestion although it can absorb water. It secretes mucus that help lubricating the lining of the large intestine. Thus facilitating the movement of the feces.
Tho movement of food in the gastrointestinal tract is mediated by a process that is called peristalsis. Peristalsis is governed by the enteric nervous system and by the vagus nerve which is a cranial nerve. Anti persitalsis is a movement of food in the upward direction. It can happen in pathological cases and which emesis occurs.
Cancer of the colon or the large intestine can occur due to chemicals that are carcinogenic and which are food residues that are located in the colon. Another cause to colon cancer are forms of bacteria that reside in the large intestine and which secrete carcinogenic compounds. These bacteria usually feed on lipids in the intestine. This is the reason why people who live on fats are predisposed to develop malignancy more than other people.
Polyps are nonmalignant protrusions in the colon which protrude inward due to the peristaltic movement of the colon. They are usually considered nonmalignant. They can predispose to malignancy only if they are more than 2 cm in diameter.
Diverticula is another condition of the colon. It is considered nonmalignant. However it can become inflamed and cause a condition that is called diverticulitis.