Anatomy of the colon
In short
The colon is a hollow tube about a metre and a half long that frames the abdomen on three sides — up the right, across the top, and down the left — before turning into the sigmoid colon and then the rectum. It is a single continuous organ, but it is described in segments because each segment has its own anatomical landmarks, its own typical pattern of disease, and its own difficulty for the endoscopist. Understanding the segments helps make sense of the words on a procedure note: caecum reached, polyp at the hepatic flexure, diverticulosis throughout the sigmoid.
What this page covers
A walk through the colon in the order the scope follows on a coloscopy — from the rectum back to the caecum, then withdrawn slowly while the bowel wall is examined. Along the way, we note where polyps tend to form, where the natural bends are, why the right side is harder to examine well, and where the small bowel meets the large bowel.
- The major segments and their boundaries
- The two natural sharp turns (flexures)
- Where the appendix attaches and what it looks like from inside
- The valve where the small bowel ends
- Why polyps on the right side are harder to find than those on the left
The shape of the large bowel
The large bowel is, broadly speaking, a frame around the small intestine. If you imagine an inverted U laid against the inside of the abdominal wall, that is the colon: it begins low and on the right, runs up to the underside of the liver, crosses to the left under the stomach and spleen, runs down the left side, then dips into the pelvis as a loop (the sigmoid) before becoming the rectum and ending at the anus.
Two of those corners are sharp enough that the endoscopist has to negotiate them deliberately: the hepatic flexure, where the right-sided ascending colon turns to become the transverse, and the splenic flexure, where the transverse turns to become the descending. These bends are where many of the cramping sensations of an unsedated coloscopy come from. Modern technique — water immersion, careful loop reduction, carbon-dioxide rather than air — has made them considerably more tolerable than they used to be.
The rectum
The rectum is the final straight section of the large bowel, sitting deep in the pelvis. It is roughly the last twelve to fifteen centimetres before the anus. The endoscopist sees the rectum first as the scope is inserted, and again at the very end of the test — usually with the scope turned back on itself in a manoeuvre called retroflexion, so that the lining just inside the anus, otherwise hard to see, can be inspected.
The rectum is a common site for haemorrhoids and small hyperplastic polyps. Cancers here are often picked up earlier than cancers further up the bowel, because they are more likely to cause visible bleeding and changes in bowel habit. The rectum is also the part of the bowel always involved in ulcerative colitis when that condition is present.
The sigmoid colon
The sigmoid is the S-shaped section between the rectum and the descending colon. It is mobile — it is not pinned to the back wall of the abdomen the way the ascending and descending colon are — and it loops freely. For people having an unsedated test, the sigmoid is often the most uncomfortable part: the scope has to follow each curve, and the loop can stretch the bowel and its supporting tissues.
The sigmoid is also where diverticulosis is most often found. Most older adults have at least some diverticula in the sigmoid, often noted as an incidental finding and almost always meaningless on its own. Polyps are common here too. Cancers in this part of the bowel are sometimes diagnosed when they cause partial obstruction, because the lumen is already relatively narrow.
The descending colon
The descending colon runs vertically down the left side of the abdomen, from the splenic flexure to where it joins the sigmoid. It is fixed to the back of the abdomen and is therefore generally a quiet, predictable section to traverse with the scope. Its lining is examined carefully on the way back out.
The splenic flexure
This is the corner under the spleen, where the transverse colon turns down to become the descending colon. It is one of the two natural sharp bends. For some people the splenic flexure is acutely angled and tucked high under the rib cage, which is why a few seconds of cramping at this point during the test is common.
The transverse colon
The transverse colon crosses the upper abdomen. Unlike the ascending and descending sections, the transverse is suspended on a fold of peritoneum that gives it a degree of mobility — in some people it dips low into the pelvis, in others it sits high. This variation contributes to the difficulty of an examination and to the variable position the team may ask you to roll into during the test (often onto your back, sometimes onto the right side) to help the scope round corners.
The hepatic flexure
The hepatic flexure is the corner under the liver, where the ascending colon turns left to become the transverse. The lining here can take on a faint blue-grey discoloration through the bowel wall — the underlying liver showing through — which the endoscopist uses as a landmark.
The ascending colon
The ascending colon runs upwards on the right side of the abdomen, from the caecum at the bottom to the hepatic flexure at the top. Its lining tends to be thinner, its folds (the haustra) are deeper, and its lumen is wider than further down the bowel. These facts matter for two reasons. First, polyps in the right colon are more likely to be flat or sessile rather than mushroom-shaped — that is, harder to see. Second, the deep folds can hide lesions on the far side of a haustral ridge.
The right colon is also where sessile serrated lesions are most often found. These flat polyps were under-recognised in earlier decades; pathology and endoscopic technique have since caught up, and they are now examined for and removed deliberately.
The caecum
The caecum is the pouch where the colon begins. It sits in the right lower abdomen, where the ascending colon turns into a blind end. The endoscopist must reach the caecum for the test to be considered a complete coloscopy. Three landmarks confirm the caecum: the ileocaecal valve (the small-bowel opening), the appendiceal orifice (the small slit-like opening of the appendix), and a Y-shaped pattern of muscle bands on the wall called the tri-radiate fold. A photograph of one or more of these landmarks is usually saved as proof of completion.
The ileocaecal valve
The ileocaecal valve is the muscular gate where the small bowel (the ileum) empties into the large bowel. It usually appears as a fleshy, pursed opening on the wall of the caecum. The endoscopist may pass the scope a short distance into the ileum to examine the terminal small bowel — a procedure called terminal ileum intubation — particularly when investigating possible Crohn's disease, unexplained anaemia, or chronic diarrhoea.
The appendix
The appendix attaches to the caecum below the ileocaecal valve, and from the inside of the bowel it is seen only as a small opening — its orifice. The endoscopist does not enter the appendix during a coloscopy. In a person who has had their appendix removed, the orifice may still be visible as a healed dimple, or may have been excised altogether.
Why right-sided polyps are harder to find
Several features of the right colon — the caecum, ascending colon, and hepatic flexure — make it intrinsically more difficult to examine well than the left side.
- The lumen is wider, so polyps occupy a smaller share of the field of view.
- The folds are deeper, and small lesions can hide on the back of a fold.
- Polyps in the right colon are more often flat or sessile rather than pedunculated, so they have less visible relief against the wall.
- Sessile serrated lesions, more common on the right, can be subtle in colour and shape, and may have a thin mucus cap that disguises them further.
- The right colon is the first part filled by residual stool when the prep is incomplete — what fluid is left tends to pool there.
Endoscopists work against these limitations with deliberate techniques: a careful prep, a slow withdrawal, a second look at the right colon (sometimes by entering the segment again after withdrawing past it), and the use of high-definition white light, narrow-band imaging, and water exchange. These techniques are part of why a coloscopy that finds nothing on the left side is not a less successful test than one that does.
Where polyps tend to form
Polyps can occur anywhere in the colon. That said, certain patterns are reproducible across published series.
- Most adenomas are found in the left colon (descending and sigmoid) and the rectum.
- Sessile serrated lesions are predominantly right-sided.
- Hyperplastic polyps are common in the rectum and sigmoid and are usually small.
- Large pedunculated polyps tend to occur in the sigmoid and descending colon, where the bowel is mobile enough for a stalk to develop.
This is why coloscopy — which inspects the whole length of the large bowel — is a more complete test than flexible sigmoidoscopy, which examines only the rectum and the lower left side.
Layers of the bowel wall
It helps to know that the bowel is built from layers. From the inside out, these are the mucosa (the lining the endoscopist sees), the submucosa (a thin layer carrying blood vessels and lymphatics), the muscularis propria (the muscle that moves the bowel), and the serosa (the smooth outer covering). Most polyps and many cancers begin in the mucosa. Whether disease has reached the deeper layers — and how deeply — is the question pathologists answer when they examine a polyp or biopsy specimen.
Techniques such as endoscopic mucosal resection rely on the submucosal layer: by injecting saline or a similar fluid into the submucosa under a flat polyp, the endoscopist can lift the polyp away from the muscle layer underneath and remove it more safely.
What to ask your clinician
- Where in the colon was the finding noted on my report?
- Did the scope reach the caecum, and was the prep good enough that the right side could be examined well?
- Is there a reason a particular segment was difficult on this examination?
- Were any flat polyps removed, and how confident are you that the margins are clear?
- Was the terminal ileum entered, and if so, what did it show?
Common worries, briefly addressed
The report mentions diverticulosis throughout the sigmoid. Is that bad?
By itself, no. Diverticulosis is very common with age and almost always causes no problem. A separate page covers what it does and does not mean.
Why does the report describe my colon as tortuous or redundant?
Some people have a longer or more looped colon than average. Tortuous and redundant are descriptive terms, not diagnoses. They occasionally explain why a test was harder to complete or why cramping was worse than expected.
Does it matter that the polyp was on the right rather than the left?
Location is one of several pieces of information your clinician uses, alongside size, shape, number, and pathology, to decide when you should return. Right-sided sessile serrated lesions and right-sided adenomas are taken seriously because they are sometimes the type of lesion most easily missed.
If the prep was poor, does the test need to be repeated?
Sometimes yes, sometimes the next coloscopy is brought forward rather than the test redone immediately — the choice depends on what was seen, where, and how poor the prep was. Your clinician will explain.
Sources
- American Society for Gastrointestinal Endoscopy — quality indicators for colonoscopy, including caecal intubation and withdrawal time
- European Society of Gastrointestinal Endoscopy — performance measures for lower gastrointestinal endoscopy
- United States Multi-Society Task Force on Colorectal Cancer — recommendations on adequate examination and surveillance after polypectomy
- World Endoscopy Organization — terminology for colorectal lesions
- Standard reference texts in gastrointestinal anatomy used in clinical training