Alec Titan
Circular folds, also known as valves of Kerckring, valves of Kerchkring, plicae circulares, plicae circulae, and valvulae conniventes, are permanent folds that slow the passage of partly digested food along the intestines. They are covered with small finger-like projections called villi, which in turn are covered with microvilli that absorb fats and nutrients from the chyme. The duodenal mucosa, for example, forms circular folds. The intestinal glands, or crypts of Lieberkühn, are simple tubular glands that open at the base of the intestinal villi and occupy the entire thickness of the lamina propria mucosae, spanning from the muscularis mucosae, which is composed of inner circular and outer longitudinal layers of smooth muscle, to the surface epithelium.
| Characteristics | Values |
|---|---|
| Definition | Circular folds are permanent folds that are not obliterated when the intestine is distended. |
| Location | Circular folds are found in the duodenum, jejunum, ileum, and the small intestine. |
| Composition | Circular folds are composed of circular muscle layers. |
| Function | Circular folds slow the passage of partly digested food along the intestines, providing an increased surface for absorption. |
| Appearance | Circular folds are transverse folds that lie perpendicular to the long axis of the bowel. They are covered with small finger-like projections called villi. |
| Other Names | Valves of Kerckring, plicae circulares, valvulae conniventes. |
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What You'll Learn
Circular folds in the intestine
Circular folds, also known as plicae circulares, valves of Kerckring, plicae circulae, or valvulae conniventes, are permanent large valvular flaps projecting into the lumen of the small intestine. The entire small intestine has these mucous membrane circular folds, which increase the surface area for absorption. They are most prominent in the jejunum.
The majority of these folds extend transversely around the cylinder of the small intestine, for about half or two-thirds of its circumference. Some form complete circles, while others have a spiral direction, extending more than once around the bowel. The larger folds are about 1 cm deep at their broadest part, while most folds are smaller. There is usually an alternating pattern between larger and smaller folds.
The duodenum, a 25-cm-long cylindrical tube with a 4 cm diameter, has large circular folds in addition to small villi and microvilli. The duodenal mucosa forms circular folds called valvulae conniventes. The duodenum is an exception, as it does not have Brunner's glands in its submucosa, which secrete a bicarbonate-rich mucus that protects the small intestine from the stomach's acidic contents.
The inner surface of the small intestine has circular folds, which increase the area by approximately threefold. The mucosa projects from the folds into the lumen with finger-like structures called villi, which increase the surface area by an additional tenfold. The surface of each villus is covered with epithelial cells called enterocytes, and absorption occurs across the enterocyte barrier. Microvilli, small hair-like filaments, project from the luminal surface of each enterocyte into the lumen, increasing the surface area for absorption by another 20 times. Together, these structures increase the surface area by about 600 times.
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Duodenal mucosa and its folds
The duodenum is the first part of the small intestine and is about 25 to 30 cm long. It is C-shaped and is located in the upper abdomen. The duodenum is an important metabolic signalling centre and is responsible for the breakdown of food in the small intestine, using enzymes. It also regulates the rate of stomach emptying via hormonal pathways. The duodenal mucosa forms circular folds called valvulae conniventes. The duodenal mucosa looks very different compared to the gastric mucosa, which is lined with villi surrounded by crypts and submucosal Brunner's glands, as well as absorptive cells, endocrine cells, and Paneth cells. The Brunner's glands secrete mucus and bicarbonate to neutralise stomach acids.
The duodenal mucosa has a unique structure that is distinct from other parts of the gastrointestinal tract. It consists of simple columnar epithelium (lamina epithelialis), a connective tissue layer (lamina propria), and a smooth muscle layer (lamina muscularis). The intestinal epithelial cells (enterocytes) are covered by a layer of glycoproteins and mucin. The duodenal mucosa is a leaky barrier against gastric acid, which diffuses into duodenocytes but does not damage them due to the release of protective prostaglandins and nitric oxide.
The duodenal mucosa plays a crucial role in protecting the duodenal epithelium from gastric acid. It achieves this through the enhanced expression of cyclooxygenase and the release of nitric oxide synthase, which stimulate duodenal bicarbonate secretion. Additionally, the release of neurotransmitters such as vasoactive intestinal peptide, pituitary adenylate cyclase-activating polypeptide, acetylcholine, and melatonin further stimulate bicarbonate secretion.
The duodenal mucosa is susceptible to infections by the bacteria Helicobacter pylori, which can lead to duodenal ulcers. These ulcers can cause recurrent abdominal pain and dyspepsia. Management of duodenal ulcers involves the use of antibiotics to eradicate the bacteria and proton-pump inhibitors and antacids to reduce gastric acidity. Furthermore, duodenal cancer, although rare, can occur in the first section of the small intestine. It is often observed as adenocarcinoma, arising from glandular cells in the epithelial tissue lining the duodenum.
The duodenal mucosa is also implicated in the pathogenesis of type 2 diabetes (T2D). Duodenal mucosal hyperplasia, induced by a western diet, contributes to insulin resistance, T2D, and obesity. This hyperplasia is characterised by increased mucosal mass, increased villus length, decreased crypt density, proliferation of enteroendocrine cells, increased enterocyte mass, and lipid droplet accumulation in the mucosa. The DMR procedure is a novel endoscopic treatment for T2D that targets the abnormally thickened duodenal mucosa, inducing regeneration of the duodenal surface to mitigate excessive insulin resistance.
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The jejunum and ileum
The jejunum has a richer vascular supply, a thicker wall, and a wider lumen than the ileum. The jejunal mucosa is extensively folded, and these transverse ("circular") folds lie perpendicular to the long axis of the bowel. The mucosal surface of the jejunum is increased by prominent villi, which are finger-like projections of mucosa. The villi, in turn, are covered with microvilli, which absorb fats and nutrients from partly digested food. The submucosa contains a network of capillaries, lymphatics, and a nerve plexus (of Meissner) within loose areolar tissue. The jejunum has few and small, discrete lymphoid nodules.
The ileum, on the other hand, has a thinner and less vascular wall, with less prominent transverse folds and villi. The circular folds in the ileum are less pronounced than in the jejunum. The ileum also has the same five layers as the jejunum, but its mucosal surface is less extensive due to the reduced number of villi.
The fold pattern of the jejunum and ileum can be altered or even reversed by various disease processes, as seen in some patients with celiac-sprue.
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The role of circular folds in absorption
Circular folds, also called plicae circulares, are deep ridges in the mucosa and submucosa of the small intestine. They play a crucial role in absorption by increasing the surface area of the intestinal wall. This increased surface area facilitates the absorption of nutrients into the body.
The small intestine is the primary site of digestion and absorption. Structural adaptations such as circular folds, villi, and microvilli increase the mucosal surface area of the small intestine, enhancing absorption. Circular folds, in particular, contribute to absorption by intensifying mixing in the small intestine. The folds create prominent and long-lasting swirls or vortices, which enhance both radial and axial mixing. This mixing process helps to distribute nutrients more effectively, making it easier for the absorptive cells in the mucosa to take up the nutrients.
The mucosal surface of the small intestine is covered by absorptive cells called enterocytes. These cells line the villi, which are small hairlike projections found within the circular folds. The enterocytes absorb water, salts, vitamins, and the breakdown products of carbohydrates and proteins. The increased surface area provided by the circular folds enhances the absorptive capacity of the small intestine, allowing more efficient uptake of nutrients into the bloodstream.
In addition to their role in absorption, circular folds may also contribute to other functions in the small intestine. For example, the folds may help to protect the intestinal wall or regulate the flow of contents through the intestine. However, the specific functions of circular folds beyond absorption have not yet been fully elucidated and require further investigation.
It is worth noting that the large intestine lacks circular folds, villi, and enzyme-secreting cells, reflecting its different digestive and absorptive functions compared to the small intestine. The mucosa of the large intestine is primarily composed of absorptive enterocytes and goblet cells, which secrete mucus to facilitate the passage of feces and protect the intestine from acids and gases produced by bacteria.
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Circular folds and the pyloric sphincter
The pyloric sphincter is a crucial component of the gastrointestinal system, located at the distal end of the stomach and marking the transition to the duodenum, the first segment of the small intestine. This positioning is vital for its role in regulating the passage of food from the stomach to the small intestine, maintaining proper digestion and nutrient absorption.
The pyloric sphincter is composed of three distinct muscle layers: circular, longitudinal, and oblique. The circular muscle layer is the most prominent and functionally significant. It encircles the sphincter, allowing it to constrict and relax, thereby controlling the passageway. The contraction of these circular muscles maintains a tight seal, preventing the backflow of intestinal contents. This layer's mechanics are essential for understanding and treating gastrointestinal disorders.
The longitudinal muscle layer runs parallel to the pyloric sphincter, supporting its function. While not directly involved in constriction, these muscles contribute to overall stomach motility by aiding in the shortening and widening of the pyloric canal, facilitating the movement of chyme. Research highlights the importance of coordination between longitudinal and circular muscles for efficient gastric motility, ensuring stomach contents are adequately mixed and propelled.
The oblique muscle layer, though less prominent, plays a unique role in providing additional support and stability. The diagonal arrangement of these fibres assists in the complex torsional movements required for mechanical digestion within the stomach. Together, the three muscle layers of the pyloric sphincter work in harmony to ensure the efficient and controlled passage of food from the stomach to the small intestine, contributing to overall digestive health.
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Frequently asked questions
Circular folds are permanent folds that are found in the small intestine. They are also called valves of Kerckring, plicae circulares, plicae circulae, and valvulae conniventes.
Circular folds slow down the passage of partly digested food along the intestines, providing an increased surface for absorption.
Circular muscles produce circular folds. These muscles can be found in the pyloric sphincter, duodenum, and small bowel wall.
The duodenum has three layers: an inner layer of circular muscle, a middle layer of longitudinal muscle, and an outer layer of longitudinal muscle.
The small bowel wall has five layers: mucosa, submucosa, circular muscle, longitudinal muscle, and serosa.
