colon
Meaning of colon: Contents can spend a day or more here, which is what makes the colon the place where water is reclaimed and bacteria flourish.
Definition of colon
The colon is the longest part of the large intestine, running from the end of the small intestine to the rectum. It reabsorbs water and salts from the material it receives, turning liquid residue into solid stool, and it houses the bulk of the gut's bacterial population. Its named segments are the cecum, ascending, transverse, descending and sigmoid colon.
What are its segments?
| Segment | Course | Consistency of contents |
|---|---|---|
| Cecum | A pouch in the lower right abdomen where the small intestine empties in | Liquid |
| Ascending colon | Upward along the right side | Liquid to semi-liquid |
| Transverse colon | Across the upper abdomen from right to left | Thickening |
| Descending colon | Downward along the left side | Semi-solid |
| Sigmoid colon | An S-shaped curve dipping into the pelvis | Solid |
The appendix hangs from the cecum. That progressive thickening along the route explains why the position of a problem often follows from what is passing through: a narrowing in the ascending colon can be slipped past by liquid contents for a long time, while the same narrowing in the sigmoid, where stool is already solid, obstructs far sooner.
What work does it do?
Nutrient absorption is essentially finished before material arrives. The colon reclaims water and electrolytes, and it does so slowly, with transit measured in many hours to a couple of days. That unhurried pace explains both its function and its troubles: move contents too fast and water is not recovered, giving loose stool; too slow and too much is recovered, giving hard stool. The lining also secretes mucus to ease passage. Muscle in the wall works in two modes, with slow mixing movements that keep contents against the absorbing surface, and occasional strong sweeping contractions that push material along a long stretch at once.
Is the colon the same as the large intestine?
Almost, though not quite, and the difference is worth keeping straight. The large intestine takes in the colon plus the rectum and the anal canal beyond it. Everyday use treats the two words as equivalent, which rarely causes confusion but does matter in a report specifying which segment is involved. The colon also holds the densest population of bacteria anywhere in the body. These organisms ferment fiber that human enzymes cannot break down, producing short-chain fatty acids that feed the cells lining the colon itself, and they manufacture vitamin K and several B vitamins as a byproduct.
Used in a sentence
A colonoscopy was performed to examine the entire length of the colon.
Good to know
The terms colon and large intestine are often used as though interchangeable, though the large intestine technically extends beyond the colon to include the rectum and the anal canal as well.
Word origin
Where “colon” comes from
From Greek kolon, referring to this portion of the large intestine.