Normal Digestive Tract
The following is a brief, simplified review of the normal digestive system and process.
The digestive system extends from the mouth to the anus. Digestion of food begins immediately in the mouth. Processes such as chewing, the addition of saliva (as a lubricant) and the initial introduction of enzymes all help to start the breakdown of foods. Food is then swallowed and enters the esophagus — a narrow, muscular tube that is about 10 inches long. It begins at the mouth and directs food towards the stomach.
Food then enters the stomach — a muscular pouch where acid is added. The churning of the stomach and the addition of the acid further help break down the food into smaller particles (becoming semi-liquid in consistency) before it enters the small bowel.
The small bowel is about 20 to 25 feet in length and consists of three areas: duodenum, jejunum and ileum. The duodenum (about 1- 1.5 feet in length) is where the food first enters after leaving the stomach. Here, more enzymes and bile (acid) are added to facilitate the digestive process breaking down the food into components that can easily be absorbed. From the duodenum, the food moves into the jejunum (8-10 feet in length) where most of the nutrients (proteins, carbohydrates, fats, vitamins and minerals) of the food are absorbed.
Finally, the food moves through the ileum (about 12-15 feet in length) where final absorption of remaining nutrients occurs. It is important to note that the terminal ileum (the last 3 feet of the ileum) is the only place where vitamin B12 is absorbed. While different foods can impact on the movement of food through the small intestine, generally the movement of food from mouth to terminal ileum takes about 2 ½ to 4 hours to complete.
From the small bowel, the residual material drains into the large intestine (or colon). The colon is about 6 feet in length and begins with the cecum, extends into the ascending colon, the transverse colon, the descending colon, the sigmoid colon and finally the rectum and anus. The function of the colon is to absorb water and salts and bacteria also interacts with the liquid waste matter in the colon assisting with the conversion to feces or stool. This interaction with bacteria contributes to the formation of gas or flatus. Stool is stored primarily in the descending colon and sigmoid colon where it moves into the rectum.
Receptors send a "signal" indicating that stool needs to be passed. Under normal circumstances, these receptors also help to differentiate between formed stool, liquid stool and flatus in the rectum. The anal sphincters are under voluntary control, so individuals can control when and where stool is expelled. Movement of waste matter through the entire large colon is considerably slower than the small intestine and can take from hours to days, depending upon the individual.