An inflamed colon looks red, swollen, and raw. During a colonoscopy, the normally smooth, pale-pink lining appears visibly reddened, and the network of tiny blood vessels that’s usually visible through the surface fades or disappears entirely. In more severe cases, the lining bleeds on contact or even spontaneously, and open sores become visible. What you see depends on the cause of inflammation, how severe it is, and where in the colon it’s happening.
What a Healthy Colon Looks Like
A healthy colon has a smooth, glistening lining that’s pale pink or light salmon in color. Small blood vessels are clearly visible through the surface, creating a fine, branching pattern. The tissue looks moist but firm, and when a doctor touches it with an instrument during a colonoscopy, it doesn’t bleed. This is the baseline that doctors compare against when grading inflammation.
Mild Inflammation
In the earliest stages, the colon lining turns noticeably redder than normal, a change doctors call erythema. The vascular pattern starts to fade as the tissue swells with fluid and immune cells. The surface becomes “friable,” meaning it’s fragile enough to bleed slightly when touched by a scope or instrument, though it doesn’t bleed on its own. You might not see dramatic changes at this stage, but the tissue has lost its healthy sheen and looks subtly puffy.
On a CT scan, mild inflammation shows up as thickening of the colon wall. A normal colon wall measures about 3 millimeters or less. Mild thickening falls in the 3 to 6 millimeter range, which may not sound like much but is enough for a radiologist to flag as abnormal.
Moderate to Severe Inflammation
As inflammation progresses, the changes become harder to miss. The redness deepens and spreads, the vascular pattern disappears completely, and the surface develops erosions, which are shallow breaks in the lining. The tissue bleeds easily with minimal contact, and a yellowish-white discharge or pus may coat the surface.
At the most severe end, the colon lining develops open ulcers, which are deeper wounds that can vary from shallow craters to holes that extend well into the wall. The tissue bleeds spontaneously, without any contact at all. In some cases, patches of the lining are stripped away entirely, leaving behind raw, exposed tissue. On CT imaging, severe inflammation can push wall thickness beyond 12 millimeters, four times the normal measurement.
How It Differs by Condition
The visual pattern varies depending on what’s causing the inflammation. In ulcerative colitis, the redness and damage typically start at the rectum and extend upward in a continuous band. There are no “skip areas” of healthy tissue in between. The affected lining looks uniformly angry and raw, and bloody diarrhea with visible mucus or pus is the hallmark symptom that corresponds to this appearance.
Crohn’s disease looks different. Inflammation tends to appear in scattered patches with normal-looking tissue in between. Ulcers may be deeper and more isolated, sometimes creating a “cobblestone” pattern where swollen tissue surrounds deep linear cracks. The damage can affect any part of the digestive tract, not just the colon.
Infections create their own distinct patterns. A Clostridioides difficile infection, one of the most recognizable forms of infectious colitis, produces raised yellow-white plaques that sit on top of the colon lining. These plaques, called pseudomembranes, can start as scattered nodules and eventually merge to cover large stretches of the colon. The tissue between the plaques may look normal or only slightly red, which distinguishes this from the diffuse redness of ulcerative colitis.
What Happens Under a Microscope
A colonoscopy shows the surface, but biopsies reveal what’s happening deeper. Under a microscope, an inflamed colon shows immune cells flooding into the tissue that lines the colon wall. The crypts, which are tiny tube-shaped glands packed into the lining, become distorted, irregular in shape, and sometimes filled with collections of white blood cells called crypt abscesses.
One characteristic microscopic change is the loss of goblet cells, the specialized cells responsible for producing the protective mucus layer. In ulcerative colitis with active crypt abscesses, about 45% of cases show complete goblet cell loss in the affected areas, though this isn’t universal. The remaining cases retain some goblet cells even during active flares. This depletion helps explain the breakdown of the colon’s protective barrier and the raw, vulnerable appearance of the lining seen during a scope.
What You Might See in Your Stool
You can’t see the inside of your own colon, but your stool offers indirect clues about what the lining looks like. Bright red blood mixed into or coating the stool points to inflammation or ulceration in the lower colon or rectum, where blood hasn’t had time to darken. Visible mucus, sometimes streaked with blood or pus, suggests the lining is actively inflamed and leaking protective secretions. Diarrhea that persists over days or weeks, especially with blood, correlates with the kind of widespread redness and erosion seen during a colonoscopy.
Other symptoms map to specific patterns of inflammation. Cramping and abdominal pain reflect swelling in the colon wall. Tenesmus, the urgent feeling that you need to have a bowel movement but can’t, is especially common when the rectum and lower left colon are inflamed. Fatigue, weight loss, and fever suggest the inflammation is extensive or severe enough that the body is mounting a systemic response.
How Doctors Grade What They See
Gastroenterologists don’t just describe inflammation in general terms. They use standardized scoring systems so that the same findings get the same rating regardless of who’s looking. The most widely used is the Mayo Endoscopic Score, which runs from 0 to 3. A score of 0 means the lining looks normal. A score of 1 indicates mild disease: some redness, reduced vascular pattern, slight fragility. A score of 2 means moderate disease with marked redness, no visible blood vessels, erosions, and tissue that bleeds easily. A score of 3 represents severe disease with spontaneous bleeding and ulceration.
This system has limitations. It doesn’t distinguish between shallow and deep ulcers, for example, so two colons that look quite different in severity can both receive a score of 3. It also tends to miss very early inflammatory changes that haven’t yet produced visible redness or erosions. Still, these scores are what appear on your colonoscopy report, and they give you a concrete sense of where your inflammation falls on the spectrum.