The surface area of the small intestinal mucosa is greatly increased by the folds of Kerckring, villi and microvilli (brush border) and is about 200 m2 (or roughly the size of a tennis court!) in an adult (Figure 6.2).
A particularly prominent feature in the small intestine is the folding of the epithelium, known as the folds of Kerckring. The folds increase the surface area by a factor of 3. These folds extend circularly most of the way around the intestine and are especially well developed in the duodenum and jejunum, where they protrude by up to 8 mm into the lumen. They also act as baffles which aid mixing of the chyme in the small intestine.
The surface of the mucous membrane of the small intestine possesses about 5 million villi, each about 0.5 to 1 mm long. Although the villi are often described as "finger-like", their shape changes along the gut and duodenal villi are shorter and broader than those found in the jejunum. Further down the gut the villus height decreases. Diet and environment markedly affect mucosal morphology and intestinal biopsies demonstrate differences between human populations. There is also a species difference, for example, the villi of the chick are pointed and leaf-like.
The major features of a villus are illustrated in Figure 6.3. Each villus contains an arteriole and a venule, and also a blind-ending lymphatic vessel called a lacteal. The arteriole and venule do not anastomose in the small intestine as they do in the gastric mucosa. Small molecules absorbed through the villus pass into the descending loop of the villus capillary and diffuse into the ascending vessel. This creates a counter-current exchange system in each villus, which is relatively inefficient since it decreases the concentration gradient for passive diffusion. The efficiency of this process has been estimated to be around 15% and has the net effect of slowing the rate of absorption.
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