Intestinal villi/microvilli

Minute (visible only through an electron microscope) surface projections that cover the edge of each intestinal villus, called the brush border. The microvilli add a tremendous surface area for absorption. 

The inner surface of the small intestine is not smooth and flat but wrinkled into a large number of finger-like projections called villi, which project into the lumen. If we look at each villus under the microscope (Fig. 1) we find it, in turn, has small fingerlike processes projecting out into the lumen - the microvilli. The result of this is that the surface of the small intestine (which is only 300 cm in length - in the relaxed state after death it may measure 6-7 metres), is estimated to have an area of 250 m. It is obviously designed to absorb, particularly, nutrients and this is also where most of any drug taken by mouth is absorbed. 

The large internal surface area of the small intestine is attributable to its length, folding, and the presence of villi and microvilli within its lumen. The villi contain capillaries and protrude into the lumen of the small intestine. There are approximately four to five million villi in the small intestine. Each villus has many microvilli as its outer surface (Figure 11.3). The microvilli represent the absorptive barrier of the small intestine. The stomach and large intestine do not contain villi and, therefore, have a small absorptive surface area compared with the small intestine. 

The mucosal layer, comprising the lumenal surface of the small intestine, is responsible for the digestive and absorptive functions of the small intestine. The mucosal surface area is much larger than predicted for a simple cylinder. Circular folds account for this amplification. The mucosal surface area is extended further by fingerlike projections called villi and depressions called crypts. The villi are 0.5-1.0 mm in height. Each villus and crypt is lined by epithelial cells that are covered with many closely packed microvilli that project into the intestinal lumen. If the small intestine is viewed as a simple cylinder, its mucosal surface area would be on the order of half of a square meter. However, in reality, the mucosal surface area of the small intestine is approximately 250 square meters, comparable with size of a tennis court. 

The small intestine, which comprises the duodenum, the jejunum and the ileum, is the main absorption site and contains a series of finger-like projections, the vilh, which greatly increase the surface area available for absorption of nutrients. Each villus contains an arteriole and venule, together with a drainage tube of the lymphatic system, a lacteal. The venules ultimately drain into the hepatic portal system, and the lacteals into the thoracic lymphatic duct. The luminal side of each villus is covered with projections, the microvilli, which are often referred to as the brush border. 

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