Plicae circulares and absorption in the small intestine
The plicae circulares are permanent circular folds in the wall of the small intestine that enlarge the surface for absorption and slow chyme (the partly digested food).
Remember
The plicae circulares do two things at once: they provide more surface for absorption, and they slow chyme as it passes, which gives the lining more time to absorb.
Memory aid
plicae circulares circle the whole lumen
The name hints at the shape: the folds circle the whole lumen, and haustra do not.
The plicae circulares are deep ridges that project into the lumen of the small intestine. Each ridge is made of the mucosa (the inner lining) and the submucosa (the layer of supporting tissue beneath the mucosa). The fold therefore has a core of submucosa with lining over it. The plicae circulares are also called valvulae conniventes. They are permanent: unlike the rugae of the stomach (the folds that flatten as the stomach fills), they do not disappear when the intestine stretches.
The folds do two things at once. They fit more absorptive lining into each length of intestine, so the surface area for absorption rises. They also slow the passage of chyme (the partly digested food): the shape of the folds makes it spiral instead of running straight, which gives the lining more time to absorb its contents.
The plicae are the largest of three nested scales of surface enlargement, but the smaller scales add far more area. The villi (finger-like projections of the lining) sit on the folds, and the microvilli (microscopic projections on each absorptive cell) are smaller still. Villi and microvilli enlarge the surface further, but they do not slow chyme.
The plicae also help to tell the small intestine from the colon. They cross the whole width of the intestine, whereas the haustra (the pouches of the large intestine) do not.
Key facts
- Structure
- Permanent folds of mucosa and submucosa that run around the lumen of the small intestine.
- Effect
- A larger surface for absorption, and a slower passage for chyme.
- Where
- They start near the proximal duodenum, end near the middle of the ileum, and are best developed in the jejunum.
- Not rugae
- The rugae of the stomach disappear when the organ is stretched, but the plicae circulares stay in place when the intestine extends.
- Not haustra
- The plicae cross the whole width of the intestine, whereas haustra, the pouches of the large intestine, do not.
- Three levels
- Folds, then villi on the folds, then microvilli on each absorptive cell.
Exam traps
- The plicae circulares are not there to let the intestine stretch rapidly and then collapse: folds that disappear on stretching are the rugae of the stomach, whereas the plicae circulares are permanent and stay in place.
- The plicae circulares do not speed up the flow of the contents: they enlarge the surface but slow their passage, and the slower passage gives the lining more time with the contents.
- Villi do not slow chyme: they greatly increase surface area, but the slowing comes from the larger circular folds on which the villi sit.
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