Medical Oncologist
CION Cancer Clinics
How the colon works and what it does | CION Cancer Clinics
Your colon is the main part of the large bowel. It takes back water and salt from what is left after digestion, houses the bacteria that ferment fibre, and moves and stores stool until you are ready to pass it. It does very little of the actual digesting. This page explains how a healthy colon works, the words you may see on a report, and why it matters for symptoms and surgery. At CION Cancer Clinics in Hyderabad, our oncologists explain colon cancer in plain words and plan care around you and your family.
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Medically reviewed by Dr. Naresh GunduConsultant Medical Oncologist · MBBS, DNB (Internal Medicine), DM (Medical Oncology, AIIMS) · last reviewed October 2026, next review due October 2027
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The short answer
What does the colon actually do?
Your colon takes the watery leftovers of digestion, draws the water and salt back into your body, and turns what remains into stool. It also houses most of the bacteria in your gut and holds stool until you are ready to pass it. Most of the real digestion of food is already finished before anything reaches it.
Where it sits in your body
The colon is the main part of the large bowel. It starts low on the right side of your abdomen, where the small intestine empties into it. It runs up the right side, crosses under the ribs and comes down the left side, then curves into the rectum, the last straight section before the anus. In an adult it is about one and a half metres long, and much wider than the small intestine. The page on the parts of the colon names each section.
Why a cancer page starts here
A plain picture of a healthy colon explains a lot later on. It explains why a tumour on the right side often causes tiredness instead of a change in the toilet, why a tumour on the left is more likely to block, and why most people live normally after part of the colon is removed.
Losing part of the colon is usually far less disruptive than people fear, because the rest of the bowel takes over much of its work.Four jobs
What are the colon's main jobs?
Each one matters for a different reason once a symptom, a scan or an operation comes into the picture.
Taking back water and salt
What arrives from the small intestine is runny. As it moves along, the colon lining draws water and salts back into the blood, and the contents firm up into stool.
When the balance slips
- Too little water taken back gives loose motions
- Slow movement or too little drinking gives hard stools
Feeding the gut bacteria
Most of your gut bacteria live here. They break down the fibre your body cannot digest, from dal, vegetables, fruit and millets such as ragi and jowar, and make substances that feed the colon lining. Gas is a normal by-product.
Storing and moving stool
The colon wall is muscle. Slow waves mix the contents, and a few times a day a stronger wave sweeps stool towards the rectum, often after a meal. That is why morning tea or breakfast so often brings on the urge.
Renewing its own lining
The inner lining is coated in mucus so stool slides past without damage, and its cells are replaced constantly. Each time a cell copies itself there is a small chance of a mistake, which is where polyps, and occasionally cancer, can begin.
How a polyp becomes a cancer explains that slow change.Not sure whether this applies to you?
Ask an oncologistThe route through
What happens to food on its way through the colon?
Liquid leftovers arrive
The small intestine has already absorbed most of the nutrients. What passes into the caecum, the first part of the colon, is mostly water, salt, fibre and whatever the body could not digest.
The right side does the heavy work
Up the right side and across the top, most of the water is drawn back and the bacteria ferment fibre. The contents are still soft here, which is why a right-sided tumour rarely blocks the bowel.
The left side firms and stores
Down the left side and through the S-shaped sigmoid colon, the stool becomes firmer and the bowel is narrower. A tumour here is more likely to change your bowel habit or cause a blockage.
The rectum waits for you
When stool reaches the rectum, its walls stretch and you feel the urge. The muscles around the anus let you hold on until you reach a toilet. That control sits below the colon, so an operation on the colon usually leaves it intact.
On your report
Which colon words might you see on a report?
- Large bowel
- The colon and rectum together. Large refers to its width; the small intestine is narrower and much longer.
- Caecum
- The pouch at the very start of the colon, low on the right, where the small intestine joins it. The appendix hangs from it.
- Ascending, transverse, descending
- The right side going up, the section across the top, and the left side coming down.
- Hepatic and splenic flexure
- The two bends where the colon turns a corner, one under the liver on the right and one near the spleen on the left.
- Sigmoid colon
- The S-shaped last part of the colon, low on the left, which leads into the rectum.
- Mucosa
- The inner lining of the colon. Most colon cancers start in this layer, usually from a polyp.
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Did you know
The colon does very little of the work of digesting food. Its jobs are mainly water, salt and bacteria, and the rest of the bowel can take over much of that. It is why most people go back to eating normally after part of the colon is removed, once the bowel has settled.
Being straight with you
What this page cannot tell you
It cannot tell you whether your own bowel is working normally. Normal varies widely between healthy people. What matters far more is a change from your own pattern that does not settle, and only a doctor who has asked about it and examined you can judge that.
When to stop reading and see a doctor
Blood in or on the stool, a change in bowel habit lasting more than a few weeks, weight loss you cannot explain, or new tiredness from anaemia, meaning a low haemoglobin, all need a doctor this week, even if you feel otherwise well. Severe abdominal pain with vomiting, or being unable to pass stool or wind, needs the emergency department today. The page on changes in bowel habit explains which changes matter.
Who this page does not describe
If you have already had part of your colon removed, or you live with a colostomy or an ileostomy, your bowel now works differently from the picture above. The same is true for people with long-standing ulcerative colitis or Crohn's disease. Your own surgeon, oncologist or stoma nurse can explain how your bowel behaves now.
If a bowel symptom is worrying you and you do not know where to begin, call the helpline. Someone will tell you which doctor to see first.Questions we are asked
Common questions about how the colon works
How long is the colon?
In an adult it is about one and a half metres long, though this varies from person to person. The small intestine before it is several times longer and much narrower. The colon gets the name large bowel from its width, and part of it can be removed without the rest losing its function.
Is the colon the same as the large intestine?
Nearly. The colon is the longest part of the large intestine, which also includes the rectum and the short anal canal at the very end. Doctors often say colorectal when they mean the colon and rectum together. Rectal and anal cancers are treated differently, so the exact word on a report matters.
Does the colon absorb food?
Very little. The small intestine absorbs almost all the protein, fat and carbohydrate and most vitamins and minerals. The colon absorbs water and salts, along with some substances made by its bacteria. That is why people do not usually become malnourished after part of the colon is removed.
What counts as a normal bowel habit?
There is a wide range. Anything from a few times a day to every two or three days can be normal, as long as it is your usual pattern. A lasting change from your own normal, especially with blood or weight loss, is what deserves a doctor's attention.
Why do I need the toilet soon after eating?
Eating sets off a reflex that makes the colon squeeze more strongly and push stool towards the rectum. It is usually strongest in the morning, which is why tea or breakfast so often brings on the urge. It is normal, and only worth mentioning to a doctor if it is new for you.
Is a lot of gas a sign that something is wrong?
Usually not. Gas is made when the colon's bacteria ferment fibre, so foods such as rajma, chana, dal, cabbage and onions naturally produce more. Gas that comes with a lasting change in bowel habit, weight loss, pain or blood is a different matter and needs a doctor.
Can you live without your colon?
Yes. People live full lives after part of the colon is removed, and even after all of it. When a section is taken out the ends are usually joined, and stools may be looser for a while. If the whole colon is removed, the small bowel is joined to the rectum or brought out as an ileostomy.
Where in the colon does cancer start?
Most colon cancers start in the inner lining, usually from a polyp that has grown slowly over years. It can happen anywhere along the colon. The side it starts on can change the symptoms it causes and the way it is treated, which is why your report names the exact section.
Your Specialists
Meet CION's colon cancer specialists. Bring your colonoscopy, scan or biopsy report.
Our surgical and medical oncologists plan colon cancer care together: which operation, whether chemotherapy is needed, and the follow-up that comes after.
Medical Oncologist
Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
Medical Oncologist
Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
Medical Oncologist
Dr. Owais Mohammed
MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
Medical Oncologist
Medical Oncologist
Surgical Oncologist
Dr. Muralidhar Muddusetty
MBBS (AIIMS), MS (Surgery) (AIIMS), DNB (Surgical Oncology), MRCS (Edinburgh)
Surgical Oncologist
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Sources
- American Cancer Society — What Is Colorectal Cancer?
- National Cancer Institute — Colon Cancer Treatment
- NHS — Constipation
- NHS — Symptoms of bowel cancer
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
Parts of the colon and why location mattersEach section named, and what a tumour there tends to doWhat is colon cancer?The plain-language starting point for the whole clusterRight-sided vs left-sided colon cancerWhy the side a tumour starts on changes symptoms and treatmentChange in bowel habit: when it mattersWhich changes deserve a doctor, and how soon
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