sigmoid colon
Meaning of sigmoid colon: Narrow, muscular and free to swing on its own fold of tissue, this segment collects more than its share of colon problems.
Definition of sigmoid colon
The sigmoid colon is the S-shaped final stretch of the colon, running from the descending colon in the lower left abdomen to the rectum. It is the narrowest part of the large intestine and hangs on a mobile fold of tissue rather than being fixed to the back wall. Stool is held here before it passes onward into the rectum.
Why do so many problems start here?
| Problem | Why this segment |
|---|---|
| Diverticula | Pressure inside is highest where the tube is narrowest, pushing the lining out through weak points in the muscle wall |
| Polyps and cancer | Stool is most concentrated here and stays longest before being passed |
| Volvulus | A long, heavy loop hanging on a narrow base can twist on itself and cut off its own blood supply |
The colon moves stool by squeezing a segment shut behind it, and the pressure that generates rises as the diameter falls. In the sigmoid, the narrowest stretch of all, that pressure eventually forces the inner lining out through the small gaps where blood vessels pierce the muscle layer, creating the pouches called diverticula. Most sit there causing nothing at all; a minority become inflamed or bleed. The pattern is not universal, since right-sided diverticula are more common in some populations, which suggests diet and wall structure both play a part.
What does a sigmoidoscopy actually reach?
A flexible sigmoidoscope is shorter than a colonoscope and reaches the rectum, the sigmoid and usually the descending colon as far as the bend beneath the spleen. It leaves the right side of the colon unseen, so a normal result does not settle what is happening higher up. In return it needs less preparation than a full colonoscopy and is often done without sedation. Anything more than a small polyp usually leads to a full examination anyway, which is why the shorter test is used mainly as a first look rather than a substitute.
Why does bleeding from here look different?
Blood shed in the sigmoid has a short distance to travel and little time to be altered, so it appears bright red and tends to coat the outside of the stool rather than being mixed through it. Blood from the right side of the colon is digested along the way and is often invisible, showing only as falling iron. Neither pattern names a cause on its own, and both are governed mostly by distance, but the visibility is part of why left-sided colorectal cancer tends to be noticed earlier than disease on the right.
Used in a sentence
Colonoscopy identified several small polyps within the sigmoid colon.
Good to know
A flexible sigmoidoscopy reaches the rectum, the sigmoid colon and usually the descending colon, leaving the right side of the colon unexamined, which is what separates it from a full colonoscopy of the entire large intestine.
Word origin
Where “sigmoid colon” comes from
Formed from Greek sigma, the letter S, and -oid, meaning resembling, describing the segment's characteristic curved shape.