Anatomy and Function of the Gastrointestinal Tract

We want anyone considering gastric bypass surgery to have a basic understanding of gastrointestinal (GI) anatomy and physiology. In this chapter we explain the structure and function of various parts of the gastrointestinal tract involved in the specialized surgery for the treatment of clinically severe obesity.

The GI tract is the pathway food takes from the mouth, through the esophagus, stomach, small and large intestine within where the nutrients are extracted for the needs of the body. The residue then passes to the rectum where it is evacuated.

The first part of the pathway is the esophagus, which is a conduit that guides food from the mouth, where it is prepared by chewing, down to the stomach where it is stored. The stomach is both a storage space, holding as much as a quart and a half of ingested food, and a secretory organ that produces the gastric, acid necessary for digestion. However, the stomach does not absorb food. When food enters the stomach from the esophagus it remains for a short period while it is mixed wfth gastric. acid.  The stomach then by involuntary muscle contractions (peristalsis) empties the food gradually into the duodenum, the first part of the small  intestine.

The small intestine consists of three parts: the duodenum, the jejunum and the ileum. In these three parts, certain digestive secretions are mixed with food, and the nutrients are absorbed into the blood stream.

The duodenum treats the food it receives with bile from the liver and enzymes from the pancreas. It also adds liquid duodenal fluid that comes from the wall of the duodenum itself. The food, bile, enzymes and liquids brought together in the duodenum are then passed into the jejunum.

The jejunum or second portion of the small intestine, is approximately 10 feet long. It lies immediately behind the duodenum and continues the process of digestion, breaking down food into essential elements.

The ileum, or third portion of the small Intestine, like the jejunum, is about 10 feet long. It is here that a major part of the absorption of food products and liquids occurs. Waste products of the digestive process are passed from the small intestine or terminal ileum, into the large Intestine, also known as the colon. The beginning of the colon is in the right lower quadrant of the abdomen, near the appendix The colon moves waste products through about four feet by the continuing process of undulating motions or peristalsis, which is common to all parts of the gastrointestinal tract. The primary function of the colon is to store waste products of digestion prior to evacuation. The colon absorbs small amounts of water and electrolytes.