The human appendix is a small, finger-shaped pouch that hangs off the beginning of the large intestine. It looks a bit like a short, pinkish worm, typically 5 to 10 centimeters long (roughly 2 to 4 inches) and averaging about 5 to 6 millimeters in diameter, which is slightly thinner than a pencil. Despite its reputation as a useless organ, it has a distinct appearance, a specific location, and a structure that actually serves a purpose.
Size, Shape, and Color
A healthy appendix is a narrow, hollow tube with a closed, rounded end. Its walls are soft and pliable, and its outer surface is smooth, pinkish-red, and glistening with a thin layer of the same membrane that lines most of the abdominal cavity. The color comes from its rich blood supply. In some people the appendix is as short as 2 centimeters; in others it can stretch past 20 centimeters, though those extremes are uncommon.
The diameter stays fairly consistent along its length. On imaging studies, anything wider than about 6 millimeters raises a flag for possible inflammation, though healthy appendixes can occasionally measure up to nearly 9 millimeters without anything being wrong.
Where It Sits in the Body
The appendix attaches to the cecum, a small pouch where the small intestine meets the large intestine, in the lower right side of the abdomen. For over a century, doctors have used a landmark called McBurney’s point to approximate its location: roughly one-third of the way along a line drawn from the right hip bone to the belly button. In practice, though, only about 35% of appendixes actually sit within 5 centimeters of that classic spot. Fifteen percent are more than 10 centimeters away from it.
The base of the appendix stays attached to the cecum, but the tip can point in several directions. The most common position is retrocecal, meaning the appendix curls up behind the cecum, which occurs in roughly 28% to 68% of people depending on the study. The second most common orientation is pelvic, where the tip hangs downward into the pelvis, seen in 27% to 53% of cases. This variability is one reason appendicitis pain doesn’t always show up in the textbook location.
What It Looks Like Inside
If you were to slice an appendix open and look at it under a microscope, the inner lining would resemble the rest of the large intestine in some ways. It has straight, tube-shaped glands (called crypts) and no finger-like projections, giving it a relatively flat interior surface.
What sets the appendix apart is a dense concentration of immune tissue packed into its walls. Clusters of immune cells form organized nodules throughout the lining and deeper layers, far more prominent here than anywhere else in the gut. This immune tissue is especially thick in children and young adults, and it gradually thins with age. It gives the appendix wall a slightly lumpy, textured appearance when viewed under magnification, compared to the smoother walls of the surrounding colon.
Why It Looks the Way It Does
The appendix’s narrow shape and abundance of immune tissue aren’t accidental. Research from Duke University has shown that the appendix functions as a kind of safe house for beneficial gut bacteria. A thin layer of microbes, mucus, and immune molecules coats the interior, and this biological film is more concentrated in the appendix than anywhere else in the intestines.
The organ’s narrow opening and tucked-away position make it difficult for the rushing contents of the bowel to flush out these bacterial colonies. After an illness like food poisoning or a severe bout of diarrhea clears out the intestines, the protected bacteria can emerge from the appendix and recolonize the gut before harmful microbes take hold. The immune cells in the wall appear to actively nourish and protect these friendly microbes rather than attack them.
How It Changes in Children
In children, the appendix looks similar to an adult’s but grows noticeably during the first several years of life. Diameter increases by about 0.4 millimeters per year until around age 6 or 7, then levels off and stays roughly stable into adulthood. A healthy child’s appendix can measure up to about 8.7 millimeters wide, and nearly 40% of normal pediatric appendixes exceed the 6-millimeter threshold that was originally established as a cutoff for adults. Factors like the amount of fat around the cecum, what’s inside the appendix at the time, and the natural activity of immune tissue in childhood all influence how wide a child’s appendix appears on imaging.
What an Inflamed Appendix Looks Like
When appendicitis develops, the organ’s appearance changes dramatically. A normal appendix is slim and pale pink. An inflamed one becomes swollen, angry red, and visibly distended, often ballooning well past the 6-millimeter diameter mark. The outer surface loses its healthy sheen and may become coated in a yellowish-white film of inflammatory material. Surrounding fat and tissue take on a hazy, swollen look as inflammation spreads beyond the organ itself.
In more advanced cases, the wall darkens to a deep red or even greenish-black as the tissue loses blood supply and begins to die. The appendix may become rigid rather than soft, and pockets of pus can form around it. If it ruptures, its contents spill into the abdominal cavity, and the neat, contained shape of the organ gives way to a ragged, disintegrating structure.
What It Looks Like on a CT Scan
Most people will never see their appendix directly, but they may encounter it on a CT scan. On imaging, a healthy appendix appears as a thin, tubular structure in the lower right abdomen, often partially filled with air or fluid. It blends quietly into the surrounding tissue.
An inflamed appendix stands out. Radiologists look for a diameter exceeding 6 millimeters, swelling and haziness in the fat surrounding the organ (a finding called fat stranding), and thickening of the appendix wall. Sometimes a small, bright-white spot appears inside the appendix. This is an appendicolith, a hardened deposit of stool and calcium that looks as dense as bone on the scan. About 87% of patients who show an appendicolith alongside a swollen appendix and surrounding inflammation turn out to have confirmed appendicitis. An appendicolith on its own, without any swelling or inflammatory changes, is usually an incidental finding and not a cause for concern. It shows up in roughly 3% of people who have no appendix problems at all.