The circular folds (also known as valves of Kerckring, valves of Kerchkring, plicae circulares, ''plicae circulae, and'' ''valvulae conniventes'') are large

valvular A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fittings ...

flaps projecting into the lumen of the

small intestine The small intestine or small bowel is an organ in the gastrointestinal tract where most of the absorption of nutrients from food takes place. It lies between the stomach and large intestine, and receives bile and pancreatic juice through the p ...

Structure

The entire

has circular folds of mucous membrane. The majority extend transversely around the cylinder of the small intestine, for about one-half or two-thirds of its circumference. Some form complete circles. 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 smaller. The larger and smaller folds alternate with each other. These can increase the surface area of the small intestine threefold. These larger folds are called ''plica circularis''.

Distribution

They are not found at the commencement of the

duodenum The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine m ...

, but begin to appear about 2.5 or 5 cm beyond the pylorus. In the lower part of the descending portion, below the point where the

bile Bile (from Latin ''bilis''), or gall, is a dark-green-to-yellowish-brown fluid produced by the liver of most vertebrates that aids the digestion of lipids in the small intestine. In humans, bile is produced continuously by the liver (liver bile ...

and pancreatic ducts enter the small intestine, they are very large and closely approximated. In the horizontal and ascending portions of the duodenum and upper half of the jejunum they are large and numerous. 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.

Difference from other gastrointestinal folds

Unlike the gastric folds in the stomach, they are permanent, and are not obliterated when the

intestine The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans ...

is distended. The spaces between circular folds are smaller than the

haustra The haustra (singular haustrum) of the colon are the small pouches caused by sacculation (sac formation), which give the colon its segmented appearance. The teniae coli run the length of the colon. Because the taenia coli are shorter than the col ...

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.

Function

The circular folds slow the passage of the partly digested

food Food is any substance consumed by an organism for nutritional support. Food is usually of plant, animal, or fungal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals. The substance is inge ...

along the intestines, and afford an increased surface for

absorption Absorption may refer to: Chemistry and biology * Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which ...

. They are covered with small finger-like projections called villi (singular, villus). Each villus, in turn, is covered with

microvilli Microvilli (singular: microvillus) are microscopic cellular membrane protrusions that increase the surface area for diffusion and minimize any increase in volume, and are involved in a wide variety of functions, including absorption, secretion, ...

. The microvilli absorb fats and nutrients from the

chyme Chyme or chymus (; from Greek χυμός ''khymos'', "juice") is the semi-fluid mass of partly digested food that is expelled by a person's stomach, through the pyloric valve, into the duodenum valves of Kerchkring, plicae circulares, ''plicae circulae'', and ''valvulae conniventes''.

References

External links

* - "Intestines and Pancreas: The Jejunum and the Ileum" {{Authority control Digestive system