Fluid Mechanics of the Duodenum

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Abstract

Throughout the duodenum (the first 30 em or so of the small intestine), the smooth muscle lies in two layers. In the outer layer, the muscle cells lie with their long axis parallel to the axis of the intestinal tube while the long axis of the cells of the inner layer lies along the circumference of the tube. The two layers are of nearly equal thickness and, together, are called the muscularis propria, or simply, the duodenal musculature. Although bound together, the longitudinal and the circular layers seem to be able to contract independently. These contractions produce quite different deformations of the tube: contractions of the outer longitudimil muscle shorten the tube locally, while contractions of the inner circular layer of muscle reduce its diameter. The muscularis propria is lined on the outside by the serosa and on the inside by the mucosa. This mucosa contains another muscle layer, the muscularis mucosae. This muscle layer is separated from the inner circular layer of the muscularis propria by a layer of loose fibrous tissue so that, presumably, considerable movement may occur between the mucosa and the muscularis propria. It is not known what kind of movement actually occurs here. The mucosa is not uniformly applied to the inner surface of the muscularis propria. Instead, it lies in folds, called the folds of Kerkring or the valvulae conniventes. The innermost lining of the duodenum, the epithelium, constitutes finger­ like projections called villi. Each villus contains, at its core, a bundle of muscle connected to the muscularis mucosae. The cells of the intestinal