Basic Anatomy and Physiology of the Gastrointestinal System

The gastrointestinal tract begins at the mouth where mechanical digestion begins. Saliva in the mouth functions to moisten and lubricate food before swallowing, as well as to partially digest food particles. Food moves from the mouth, down the esophagus, and into the stomach. In the stomach, glands secrete hydrochloric acid which helps to digest food and destroy bacteria that enters the body with the food. Other than water, little nutrient absorption occurs in the stomach.

The digested particulate matter, known as chyme, moves from the stomach into the first portion of the small intestine known as the duodenum. It is in the small intestine where most of the digestion and absorption of food occurs. Special enzymes breakdown carbohydrates, fat, and proteins into smaller components that the intestinal cells can absorb and use throughout the body for nutrients. The pancreas and liver also secrete substances into the small intestine to aid in digestion of substances. 

Food then passes through the entirety of the small intestine, through the duodenum, the jejunum, and the ileum. Next, food enters the colon which serves two main roles: acts as a storage chamber for undigested material and functions to reabsorp water. 

When initiated, contractions of the rectum and relaxation of the anal sphincter cause defecation and stool is deposited. The process of digestion and absorption relies on many components of proper contractions, enzymes, and cell function. If any of these functions are impaired, then the process may not proceed as planned, as seen in patients with intestinal failure.