Colon and Rectum – Tissue and Systems

This article describes the basic anatomy and functions of the human large intestine.  The large intestine is composed of the colon and rectum.  The colon and rectum is distinct from other parts of the intestine in terms of its appearance, functions and location.

The large intestine serves the main purpose of providing an exit route and temporary storage for the waste products of digestion.  During growth and development as a fetus the intestine starts out as a single tube running between the future mouth and anus.  As we grow the gut tube develops into three separate parts but they remain as a continuous connection.  The three separate divisions are termed the foregut, midgut and hindgut.  The foregut runs from the mouth to the first few inches of the small intestine.  The midgut spans the first few inches of the small intestine to the middle of the colon.  The hindgut then continues from the middle of the colon to the anus.  The average colon and rectum is about 5 feet long.

The large intestine begins in the first part of the colon termed the cecum.  The cecum has a small attachment known as the appendix.  The average appendix is about 3-5 inches.  The next part is the ascending colon, transverse colon, descending colon, sigmoid colon and on to the rectum.  These parts are labeled different terms but they are all part of the same continuous tube.

The colon has layers of muscle within it that periodically squeeze in worm like movements to help move things through.  Most of the blood supply of the colon is from the inferior mesenteric artery.  The colon also has nerves that supply it to help regulate its movement and contractions.  The blood and lymph fluid drains in veins that parallel the blood supply.

Many of the key functions of the colon are performed by the microscopic bugs (i.e. bacteria) contained within it.  Most people think that stool (a.k.a. feces or “poop”) is made up of digested food but in fact it is mostly bacteria.  Up to half of the solid portion of stool may be made of bacteria.  The brown color of stool comes from bile with breakdown products of blood cells.  When the liver does not make or cannot excrete bile, the stools will turn white or gray in color.

The main functions of the colon and rectum are:

  • Absorption of water
  • Storage of stool until an acceptable time for release
  • Regulation of bacteria levels in the intestine
  • Vitamin K production
  • Assist with electrolyte balance

Absorption:  Greater than 90% of water that makes it to the colon is absorbed back into the body.  Some water in the stool is necessary to allow easy passage.  About half of the weight of a stool is made up of water.  Stool that is deficient in water becomes very hard and can create the feeling of constipation or difficulty in having a bowel movement.  It is not normal to have to strain and push very hard to have a bowel movement.  The passage of stool should be easy.  When infection or inflammation of the colon occurs, it can interfere with the ability of the colon to absorb water.  When this happens the stool has an increased volume of water and is not formed thus becoming thin.  This is commonly referred to as diarrhea.  Diarrhea actually refers to an increased frequency of bowel movements but in common usage most people would call a single episode of water stool a diarrhea.  The problem with decreased absorption function is that body water can be depleted quickly causing dehydration.

Storage:  The colon also plays a “social” function as it allows storage of stool until a socially acceptable time when it can be released.  The storage of intestinal contents also gives bacteria extra time to help digest or extract key nutrients from the diet.  It is possible to live without a colon but the increased frequency of bowel movements can be uncomfortable.  This is particularly important to someone who develops a colon cancer and has to have part or the entire colon removed with a surgery.  If a tumor grows within the colon, it can interfere with the storage function due to partial or complete blockage of the intestine.  This is often how colon tumors are discovered due to the effects on passage of stool.

Bacteria Regulation:  The bacteria in the colon are carefully regulated and normally consist mostly of anaerobic bacteria (Bacteroides sp.) and aerobic bacteria (E.Coli).  When this delicate balance is upset it can cause illness, diarrhea, blood in the stool and other problems.

Vitamin K: The bacteria in the colon help produce Vitamin K which is very important for many bodily functions including production of clotting factors in the liver.  The bacteria function can be interrupted with colon infection or medications such as antibiotics that disrupt their normal levels.

Electrolyte balance:  The colon can participate in several electrolyte regulations but potassium is perhaps its most important contribution.  The kidneys normally regulate bodily potassium and keep levels in the blood within very narrow parameters.  When the kidneys have difficulty performing this function such as with kidney failure or injury, the colon can help regulate this essential ion.

There is normally no blood found or seen within the stool.  Blood in the stool can occur from a variety of sources.  Many colon or rectal tumors can cause blood in the stool.  This is a red flag for trouble and should be investigated immediately.


  1. Townsend Jr, CM; Beauchamp RD; Evers BM; Mattox KL. (2008) Townsend: Sabiston Textbook of Surgery, 18thed.  Chapter 50.  New York, NY: Saunders.
  2. Goldman L, Schafer, AI;  (2011) Goldman: Goldman’s Cecil Medicine, 24th ed. Chapter 199. New York, NY: Elsevier
  5. This article was originally published on September 3, 2012 and last revision and update was 9/4/2015.