A healthy colon has a smooth inner lining that helps absorb water and move stool through the large intestine efficiently. The lining appears intact, with no active inflammation, ulceration, or bleeding. The rectum and colon work together to store and pass stool normally, without urgency, pain, or frequent bowel movements.

In mild ulcerative colitis, inflammation usually begins in the rectum and may extend into part of the colon. The lining may appear mildly reddened and irritated, with early swelling and small superficial ulcers. These changes can lead to symptoms such as intermittent diarrhea, mild rectal bleeding, urgency, or abdominal discomfort. Mild UC often involves 4 or fewer stools per day, and symptoms usually occur without fever, weight loss, or signs of more serious illness.

Moderate ulcerative colitis is marked by more visible inflammation and a broader area of involvement in the colon, while rectal inflammation may remain persistently active. The lining becomes more swollen, friable, and ulcerated, and bleeding may be more apparent. Symptoms often become more frequent and disruptive, including more than 4 loose or bloody stools per day, increased rectal bleeding, abdominal cramping, urgency, and fatigue. Fever and weight loss are usually absent or limited.

In severe ulcerative colitis, inflammation becomes extensive, and the lining of the rectum and colon may appear markedly swollen, ulcerated, and prone to bleeding. The disease may involve a much larger portion of the colon and can significantly impair normal bowel function. Severe UC often causes 6 or more bloody stools per day, severe urgency, abdominal pain or cramping, fatigue, and weight loss. Fever, rapid heartbeat, anemia, or abnormal inflammatory blood tests may signal a more serious flare that needs urgent treatment.