Circular folds

The plicae circulares (a.k.a. the circular folds, or valves of Kerckring, or valvulae conniventes) are large, permanent transverse folds of the mucosa and submucosa that project into the lumen of the small intestine.

Gross anatomy of the circular fold

Unlike the temporary folds seen in the stomach, they are not obliterated when the intestine is distended because their core contains submucosal tissue, which provides structural support and maintains their shape. Structurally, each plica circularis consists of a fold of the mucous membrane reinforced by a central core of submucosa. The majority of these circular folds extend transversely around approximately one-half to two-thirds of the intestinal circumference, although some form complete circular rings. Others follow a spiral course, winding around the intestinal wall one or more times. The larger folds may reach approximately 8 mm in height, and larger and smaller folds typically alternate along the intestinal lumen.

Changes in the plicae circulares along the small intestine (regional distribution)

The plicae circulares or circular folds are absent from the proximal few centimetres of the duodenum and begin to appear approximately 2.55 cm distal to the pylorus. They become particularly large and closely spaced in the descending part of the duodenum below the opening of the bile and pancreatic ducts. They are most numerous and best developed in the jejunum, where nutrient absorption is greatest. From the upper jejunum onwards, they gradually decrease in size and number, becoming progressively smaller through the ileum and almost disappearing in the terminal ileum. This reduction contributes to the relatively thinner appearance of the ileum compared with the duodenum and jejunum.

Circular folds, villi, and microvilli

The surface of the plicae circulares is covered by numerous intestinal villi, and the enterocytes lining these villi possess microvilli. Consequently, the absorptive surface area of the small intestine is increased at three structural levels: the plicae circulares, villi, and microvilli. Together, these adaptations enormously expand the surface available for digestion and absorption.

Functionally, the plicae circulares not only increase the absorptive surface area of the small intestine but also slow the passage of chyme through the intestinal lumen. By retarding the movement of intestinal contents and promoting a spiral flow pattern, they allow more time for digestion and absorption while enhancing contact between luminal contents and the absorptive mucosal surface.

Clinical correlate

The plicae circulares contribute to the characteristic feathery appearance of the jejunum on contrast radiographic studies. In disorders associated with mucosal atrophy, such as coeliac disease, these folds may become flattened or reduced, resulting in diminished absorptive capacity and malabsorption.

Summary (key points)

The hierarchy of surface modifications that increase absorptive surface area in the small intestine is:

Plicae circulares → Intestinal villi → Microvilli

• Plicae circulares are folds of mucosa and submucosa. Villi are projections of the mucosa only. Microvilli are microscopic projections of the apical membrane of enterocytes.

Together, these adaptations increase the absorptive surface area of the small intestine manyfold.