Understanding Normal Bowel Anatomy

The gastrointestinal tract is one long tube—starting at our mouth and ending at the anus.

Digestion begins in your mouth as you chew food. Swallowed food goes down the esophagus, which then widens into the stomach.

In the stomach, muscles churn and break down food. Strong acids and enzymes (i.e., chemicals) digest and break down food into a thick, liquid substance called chyme.

Small Bowel
Food particles then enter the small intestine or small bowel from the stomach for further processing. Acids and enzymes are released and mixed with food throughout the small intestine. Nutrients are digested and then absorbed into the bloodstream for the body to use.

The small bowel is about 23 feet in length and has many continuous folds that zigzag back and forth in an accordion-like fashion. The small bowel is made up of three sections: the duodenum, jejunum, and ileum.

Digestion of all nutrients begins in the duodenum. Absorption of fats, protein, sugars, and water-soluble vitamins, (i.e., vitamin C, niacin, and the B vitamins—with exception of B-12) takes place in the jejunum. Digestion of these nutrients continues in the ileum, as does the absorption of fats and the fat-soluble vitamins A, D, E, and K.

Vitamin B-12 is absorbed exclusively in the last segment of the ileum called the terminal ileum. That is why patients who have had a large part of the terminal ileum removed cannot absorb oral vitamin B-12 and need to take B-12 in an injectable or nasally administered form.

Colon
After food has been completely digested by the small intestine, what is left is a strong, irritating liquid waste, which enters the 5-foot long large intestine or colon from the ileum. Four sections comprise the colon: the ascending, transverse, descending, and sigmoid colon.

The job of the colon is to absorb and neutralize the liquid, compact the waste, and store it. The final segment is the rectum, which connects to the anus. When compacted waste reaches the rectum, it results in a solid bowel movement.

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