Crohn’s disease and ulcerative colitis are inflammatory diseases that affect the gastrointestinal (GI) tract, which extends from the mouth to the anus.

The small and large intestine, which have different anotomical features and functions, are the parts of the GI tract most affected by IBD

Anatomy

Small Bowel

The small intestine has three sections:

• the duodenum (at the top, below the stomach)
• the jejunum (in the middle)
• the ileum (at the end, before the colon)

Large Bowel (Colon)

The large intestine is shorter (about 1.5 meters in adults) but wider (about 6 cm) than the small intestine. It has five sections:

• the ascending colon (on the right side of the abdomen)
• the transverse colon (from side to side, below the liver and stomach)
• the descending colon (on the left side of the abdomen)
• the sigmoid colon (between the descending colon and the rectum)
• the rectum (the last section, before the anus)

Function

The three main functions of the GI tract are to:

• ingest (take in) and digest (break down) food
• absorb the food’s nutrients (through the bowel wall)
• eliminate (get rid of) food waste

Food travels down the esophagus and into the stomach, via the esophageal sphincter (valve). In the stomach, special enzymes and acids start a chemical breakdown of the food.

The partly digested food passes from the stomach into the small intestine, via the pyloric sphincter. As the food passes through the small intestine, digestion continues and most of its nutrients are absorbed into the bloodstream.

Finally, the food residue passes from the small intestine, via the ileocecal valve, into the large bowel (colon). There, water is absorbed before the food waste is evacuated (eliminated) as stool (feces), during bowel movements, via the rectum and anus.