Interior of the descending portion of the duodenum, showing the Major duodenal papilla .
They are composed of reduplications of the mucous membrane, the two layers of the fold being bound together by submucous tissue.
Difference from other gastrointestinal folds
The space between circular folds are smaller than the haustra of the colon, and, in contrast to haustra, circular folds reach around the whole circumference of the intestine. These differences can assist in distinguishing the small intestine from the colon on an abdominal x-ray.
The majority extend transversely around the cylinder of the small intestine for about one-half or two-thirds of its circumference, but some form complete circles, and others have a spiral direction; the latter usually extend a little more than once around the bowel, but occasionally two or three times.
The larger folds are about 1 cm. in depth at their broadest part; but the greater number are of smaller size.
The larger and smaller folds alternate with each other.
In the horizontal and ascending portions of the duodenum and upper half of the jejunum they are large and numerous, but from this point, down to the middle of the ileum, they diminish considerably in size.
In the lower part of the ileum they almost entirely disappear; hence the comparative thinness of this portion of the intestine, as compared with the duodenum and jejunum.
The circular folds slow the passage of the food along the intestines, and afford an increased surface for absorption. They are covered with small fingerlike projections called villi (singular, villus). Each villus, in turn, is covered with microvilli. The microvilli absorb fats and nutrients from the chyme.