What is the difference between ulcerative colitis and Crohn's disease?

In Crohn's disease, the last part of the small intestine, the end part of the ileum, is usually affected, but it can occur in patches in any part of the gastrointestinal tract. In 30% to 50% of cases, the colon or large bowel is also affected. The inflammation occurs in several layers of tissue through to the muscle, causing dilated blood vessels and extensive tissue damage. Sometimes the intestine becomes narrow and blocked, causing muscle spasms and requiring immediate medical attention. Because Crohn's disease affects the small intestine, the damage to the absorptive surface may interfere with nutrient absorption and lead to diarrhea, unabsorbed fat (steatorrhea) and weight loss.

Ulcerative colitis usually involves only the colon and always begins at the anus or rectum and then continues up the colon for varying distances. In contrast to Crohn's disease, the inflammation generally is limited to the inner mucosa of the colon, interfering with normal water reabsorption and frequently causing bloody diarrhea.