The gastrointestinal (GI) tract is a passageway that begins at the lips and ends at the anus (Figure 1). Its purpose is to transport and digest food. Along the way, the passageway changes character because different functions are required at different points.
In the uppermost portion, the teeth begin the process of digestion by grinding food into small fragments. The esophagus delivers the food to the stomach where strong acid further breaks up and degrades the swallowed material.
Small amounts of the liquified food called chyme are then delivered in spurts from the stomach into the duodenum where they are mixed with bile from the liver (via the bile ducts) and pancreatic juice (via the pancreatic duct). Bile aids in the breakdown and digestion of fat, while the pancreatic enzyme amylase fragments starches into smaller molecules. The pancreas also releases a fluid into the duodenum, which neutralizes the acidic stomach contents. This neutral bile/amylase/fragmented food substance passes to the upper small intestine for the next phase of digestion. It is moved along by peristalsis, worm-like contractions of the intestine.
The small intestine is so named because its calibre is small, about one inch in diameter. The term small creates some confusion because, in terms of length, it is not small at all. In fact, it normally measures nearly 23 feet in length! The small intestine's job is absorption of food. The body gains access to the food that we consume by means of absorption of microscopic particles of food through the wall of the small intestine. Vitamins and minerals and large amounts of fluid are also absorbed by the small intestine and pass into the bloodstream for distribution to the rest of the body.
By the time the intestinal contents reach the large intestine, most of its nutritional value has been extracted, leaving a watery waste product. The role of the large intestine is fluid absorption from the remaining waste and compaction and storage of what is left. Expulsion of the waste (feces, stool) is generally under voluntary control and is undertaken when socially convenient.
Large intestine or colon
The large intestine is so named because it is wider than the small intestine, not because it is longer. It is much shorter than the small intestine, measuring about 5 feet. Another name for the large intestine is the colon. It begins in the lower portion of the right side of the abdomen and travels up the right side to turn across the midline and back down the left side much like a square picture frame (Figure 2). As stool moves from the cecum to ascending colon, transverse colon, descending colon, sigmoid colon, and finally rectum, it becomes less watery and more compacted. The rectum is the last foot or so of the large intestine and it lies surrounded by the bones of the pelvis. At the very bottom of the rectum is the anus, where lie the sphincter muscles that control the opening of the bowel.
Michael E. Pezim, MD