Mucosal Structure

The lining of the small intestine is remarkably adapted for the function of absorption by increasing the surface area for transmucosal transport at three levels (Figure 1):

• The inner surface has circular folds, which increases the area by approximately 3-fold.

• The mucosa projects from the folds into the lumen with finger-like structures called villi approximately 1 mm in length. Villi increase the surface area by an additional 10-fold. The surface of each villus is covered with epithelial cells known as enterocytes. Absorption takes place across the enterocyte barrier.

• Small hair-like filaments known as microvilli project from the luminal surface of each enterocyte into the lumen. Microvilli increase the surface area for absorption by an additional 20-fold.

The three structures combine to increase the surface area by approximately 600-fold.

Each villus is supplied with its own connective tissue support known as the lamina propria, its own arteriolar and venous microcirculation with capillaries draining the basolateral regions of all the enterocytes, and its own lymphatic system.

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