Physiological Pharmaceutics

Small intestine 111Figure 6.1 Section of the small intestinec) the epithelium, which is the innermost layer of the mucosa and consists of a singlelayer of columnar epithelial cells or enterocytes, which lines both the crypts and the villi.Organisation of the mucosaThe surface area of the small intestinal mucosa is greatly increased by the folds of Kerckring,villi and microvilli (brush border) and is about 200 m 2 (or roughly the size of a tennis court!)in an adult (Figure 6.2).Folds of KerckringA particularly prominent feature in the small intestine isthe folding of the epithelium, known as the folds ofKerckring. The folds increase the surface area by afactor of 3. These folds extend circularly most of theway around the intestine and are especially welldeveloped in the duodenum and jejunum, where theyprotrude by up to 8 mm into the lumen. They also actas baffles which aid mixing of the chyme in the smallintestine.VilliThe surface of the mucous membrane of the smallintestine possesses about 5 million villi, each about 0.5to 1 mm long. Although the villi are often described as“finger-like”, their shape changes along the gut andduodenal villi are shorter and broader than those foundin the jejunum. Further down the gut the villus heightdecreases. Diet and environment markedly affectmucosal morphology and intestinal biopsiesdemonstrate differences between human populations.There is also a species difference, for example, the villiof the chick are pointed and leaf-like.Figure 6.2 Increases in surfacearea in the small intestine dueto folding