A colonoscopy follows the large intestine backward, from the rectum up to the point where the small intestine empties in. That is roughly 1.5 meters of looping tube, and the doctor finds their way by landmarks built into the bowel itself. Those same place names end up on your report, so knowing the route turns words like “hepatic flexure” or “sigmoid” into a spot you can picture.
The Route Through the Colon, Segment by Segment
From the scope’s point of view, the trip runs in this order:
- Rectum: the last 12 to 15 cm before the anus, where the scope enters and the first folds appear.
- Sigmoid colon: an S-shaped, loose and very variable stretch in the lower left belly.
- Descending colon: a straighter section running up the left side, just in front of or beside the left kidney.1PubMed Central. Classification of the colonic splenic flexure based on three-dimensional CT analysis
- Splenic flexure: a sharp bend high on the left that sits above the hilum of the spleen in nearly everyone.2PubMed Central. Classification of the colonic splenic flexure based on three-dimensional CT analysis
- Transverse colon: the section crossing the upper belly from left to right.
- Hepatic flexure: the bend where the colon turns about 90 degrees under the right lobe of the liver.3TeachMeAnatomy. The Colon – Ascending – Transverse – Descending – Sigmoid – TeachMeAnatomy
- Ascending colon: the right-side section running down to the start of the colon.4TeachMeAnatomy. The Colon – Ascending – Transverse – Descending – Sigmoid – TeachMeAnatomy
- Cecum: the pouch at the very start of the colon, in the lower right belly, where the small intestine joins.
It helps to see the whole colon as an upside-down U framing the small intestine. It goes up the right side, across the top, down the left side, then curves in an S to the rectum. Doctors call the stretch from the cecum to the transverse colon the right, or proximal, colon. The descending colon, the S-shaped sigmoid and the rectum make up the left, or distal, colon.
Not every part holds still. The ascending and descending colon are attached to the back wall of the belly, so they stay put.5Kenhub. Large intestine The transverse colon and the sigmoid hang loosely and can stretch and shift. Those loose sections are where the scope tends to bow and loop. That looseness is why the sigmoid and transverse colon are the tricky stretches. The sigmoid is the first long, curving stretch the scope meets after the rectum, so how the doctor handles it shapes the rest of the exam. The transverse colon hangs like a hammock across the upper belly, and the scope has to climb down and back up its sag. The doctor often has to pull back and straighten the scope to keep moving forward.
The two flexures are the sharpest turns on the route, so the doctor slows down and steers carefully at each one. If your report says something was found at a flexure, it sits at one of these turns, where folds are tighter and harder to see behind.
Food travels the opposite way from the scope. The ascending colon reabsorbs much of the fluid and salts, gradually forming stool.6Kenhub. Large intestine So the right side holds the loosest contents, which is why thorough bowel prep matters most there.
How the Doctor Knows Where the Scope Is
The doctor puffs in air or gas to open the bowel enough to see its passage.7Basicmedical Key. Colonoscopy Each segment then has a recognizable look. In the rectum, a few large shelf-like folds may be visible.8Basicmedical Key. Colonoscopy The transverse colon has distinctive triangular folds, which help confirm the scope is there.9Abdominal Key. 4: Colon At the bends, a bluish shadow of the liver or spleen can sometimes be seen through the wall.
If stool or fluid blocks the view, the doctor can rinse it with water through the scope and suction it away. Too much air can flatten folds in a misleading way, so the air gets adjusted to show the wall’s real shape.
These landmarks are not perfect location markers. Because loose segments shift, reported positions are estimates; in one large series, 4% of lesions were placed in the wrong segment.10PubMed. Accuracy of colonoscopic localization Almost half of those mistakes involved the sigmoid colon.11PubMed. Accuracy of colonoscopic localization
Distance numbers have the same limit. A report may say “polyp at 30 cm,” which is how much scope had been inserted. Because the colon bunches up on the scope, centimeters don’t map neatly to segments, and the segment name is usually the more meaningful part.
When surgery is planned, doctors often mark the spot with a tattoo of ink, which was found more reliably at surgery than metal clips (about 92% vs 63%).12PubMed Central. Clip or Tattooing: A Comparative Study for Preoperative Colon Cancer Endoscopic Localization So if you need surgery after a colonoscopy, ask whether the area was tattooed.
Many reports also include pictures labeled by segment. Ask your doctor to point out which picture shows the cecum and which shows any polyp. The fold shapes in the photos match the landmarks described here.
Reaching the Cecum and Small Intestine
A complete exam is confirmed by seeing the landmarks of the cecum. The ileocecal valve, where the small intestine opens into the colon, was seen in 98% of exams.13PubMed. Confirmation of cecal intubation during colonoscopy The opening of the appendix was seen in 87%.14PubMed. Confirmation of cecal intubation during colonoscopy A light glowing through the lower right belly wall was seen in 75%.15PubMed. Confirmation of cecal intubation during colonoscopy Nearly all patients (96%) had at least two landmarks documented.16PubMed. Confirmation of cecal intubation during colonoscopy
A single landmark isn’t always enough: the valve can be hard to recognize, and the appendix opening can be hidden by folds or by scarring from a past appendix removal. Doctors therefore look for more than one sign.
The surest proof is passing the scope through the valve into the terminal ileum, the last stretch of small intestine.17PubMed Central. Ilececum: A Comprehensive Review This stretch is also where Crohn’s disease often shows, so it is often checked when that is suspected.18PubMed Central. Endoscopic identification of Peyer’s patches of the terminal ileum in a patient with Crohn’s disease Normal lymph tissue patches there can look like small bumps and need not be a worry.19PubMed Central. Endoscopic identification of Peyer’s patches of the terminal ileum in a patient with Crohn’s disease
To check your own report for completeness, look for words like “cecum reached” or “cecal intubation.” Photos of the appendix opening and the valve are the usual documentation. If the cecum wasn’t reached, the right colon wasn’t examined, so ask whether a repeat or another test is planned. Polyps on the right side can only be found if the scope gets there.
Why Some Parts Hurt More
When the scope pushes into a loose segment, it can bow into a loop that stretches the bowel.20PubMed. Patient pain during colonoscopy: an analysis using real-time magnetic endoscope imaging Looping, especially in the variable S-shaped sigmoid, is the main cause of pain during colonoscopy.21PubMed. Patient pain during colonoscopy: an analysis using real-time magnetic endoscope imaging Certain complex loop shapes, including loops at the bend near the spleen, hurt more than simpler ones.22Wiley Online Library. EVALUATION OF THE LOOPING FORMATION AND PAIN DURING INSERTION INTO THE CECUM IN COLONOSCOPY Pain also tends to be greater in women, partly because their sigmoid is often longer and more curved.23PubMed. Patient pain during colonoscopy: an analysis using real-time magnetic endoscope imaging24Wiley Online Library. EVALUATION OF THE LOOPING FORMATION AND PAIN DURING INSERTION INTO THE CECUM IN COLONOSCOPY A loop usually feels like cramping or pressure, similar to strong gas pain, and it tends to ease quickly once the scope is straightened.
The bends near the spleen and liver can cause a second wave of pressure if the scope has to be pushed around them. Many people feel little once the scope reaches the straighter, fixed right side.
The doctor can straighten a loop by pulling back, twisting the scope, or asking someone to press on your belly. Changing your position, such as rolling onto your back, can also help the scope pass a bend. If pain is sharp, tell the team; it often means a loop they can fix.
Most people are sedated and remember little, though some choose lighter or no sedation. Deeper sedation hides pain signals, which is why doctors still watch for loops.
Why the Location of a Polyp Matters
An adenoma is a polyp that can slowly turn into cancer. Adenomas were found more often in the right colon (17.1%) than the left (13.5%).25PubMed. Polyp and adenoma detection rates in the proximal and distal colon In another study, three in four right-side polyps were adenomas versus under half on the left.26PubMed Central. Longer observation time increases adenoma detection in the proximal colon – a prospective study Spending longer looking at the right colon finds more of them.27PubMed Central. Longer observation time increases adenoma detection in the proximal colon – a prospective study
Polyps can hide behind folds on the right side, especially near the bend near the liver.28PubMed Central. Is Retroflexion Helpful in Detecting Adenomas in the Right Colon?: A Single Center Interim Analysis So some doctors use retroflexion, turning the scope back on itself, to look behind folds.29PubMed Central. Is Retroflexion Helpful in Detecting Adenomas in the Right Colon?: A Single Center Interim Analysis A second forward look may work nearly as well, and the gain seen in one study may reflect expert care and extra time.30PubMed Central. Impact of Retroflexion Vs. Second Forward View Examination of the Right Colon on Adenoma Detection: A Comparison Study
Some right-side polyps are flat and pale, so they blend into the lining and are easy to overlook. The quality of the right-side exam, not just the polyp itself, affects your follow-up plan.
On your report, right or proximal side findings often get more attention. Location, size and type together set your next colonoscopy date, so ask whether all polyps were removed and sent to the lab. It also helps to ask whether the cecum and right colon were well seen, and whether prep was good enough on that side. Poor right-side prep may mean an earlier repeat.
A left-side finding reads differently. Small polyps in the rectum and sigmoid are common and are often not precancerous. The lab report on the removed tissue, not the location alone, tells you what type of polyp it was. A left-side polyp still needs to be removed and tested, since appearance alone can’t confirm type.
When Anatomy Makes the Exam Harder
Abdominal or pelvic surgery can leave scars and adhesions that bind the bowel.31BULLETIN OF THE TRANSILVANIA UNIVERSITY OF BRASOV SERIES VI – MEDICAL SCIENCES. Impact of Anatomic Factors and Prior Abdominal or Pelvic Surgery on Colonoscopy Outcomes Colonoscopy is often harder after such surgery, though how much depends on the type of operation.32PubMed. Impact of prior abdominal or pelvic surgery on colonoscopy outcomes Adhesions can pin a loop of bowel in a fixed spot, so the scope can’t straighten it by simply pulling back. Pelvic surgery most often affects the sigmoid, because it lies close to the uterus and ovaries. Tell your doctor about any belly or pelvic surgery, including hysterectomy or C-section.
In older people, the colon wall may be weaker, partly from changes in its collagen, which raises tear risk slightly.33BMC Gastroenterology. What are the risk factors of colonoscopic perforation? Doctors take extra care, using gentle technique and limiting air to protect the wall. Older people also often have diverticula, small pouches in the wall that can make the sigmoid narrower and stiffer. Age alone is not a reason to skip a colonoscopy, but it shapes how the team plans it.
In situs inversus, a rare condition where organs are mirror-imaged, the bends are on the opposite side.34International Surgery Journal. Navigating endoscopic challenges in situs inversus totalis: strategies for optimal procedure completion and patient safety If the team doesn’t know about it, orientation and completion can be harder, so share any such diagnosis.35International Surgery Journal. Navigating endoscopic challenges in situs inversus totalis: strategies for optimal procedure completion and patient safety
If the scope can’t get through, the doctor may switch to a thinner scope, change your position, or stop and schedule another test. A CT colonography can image the parts not reached.
How Long Each Part Takes
Most of the inspection happens as the scope is withdrawn, not inserted. A careful withdrawal typically takes at least 6 minutes, a widely used quality standard. The whole exam usually lasts 20 to 45 minutes.
The scope enters the rectum and air is added. It is guided around the sigmoid and the bends to the cecum. The doctor confirms the cecum, then withdraws slowly segment by segment, removing polyps along the way. The rectum is often checked last, sometimes by turning the scope. The slow pullback matters because folds hide small polyps, so the doctor looks behind and between each one.
Afterward, gas and bloating from the air ease within hours. Mild cramping is normal, but severe belly pain or fever needs a call. You may get a preliminary report before lab results.