Intestine small, absorptive surface area

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 plicae circulares, or circular folds, form internal rings around the circumference of the small intestine that are found along the length of the small intestine. They are formed from inward foldings of the mucosal and submucosal layers of the intestinal wall. The plicae circulares are particularly well developed in the duodenum and jejunum and increase the absorptive surface area of the mucosa about threefold. Each plica is covered with millions of smaller projections of mucosa referred to as villi. Two types of epithelial cells cover the villi ... 

The large intestine has a considerably smaller absorptive surface area than the small intestine, but it may still serve as a site of drug absorption, especially for compounds that have not been completely absorbed from the small intestine. However, little absorption occurs... 

Acetylsalicylic acid is readily absorbed from the gastrointestinal tract in dogs, cats, and swine due to its ionization suppression by the stomach acid. In the more alkaline small intestine, the large surface area for absorption makes up for the increased ionization of the drug and rapid absorption continues. In contrast, absorption is slower from the rumen in cattle. 

The gastrointestinal tract is a major site site of absorption primarily because of its very large surface area and extensive blood flow (Table 11.2). Most of the absorptive surface area of the gastrointestinal tract is in the small intestine and thus most absorption from the gastrointestinal tract occurs there. 

Q1 The mucosa is a mucous membrane which forms the innermost layer of the intestine. In the small intestine the mucosal surface area is increased greatly by folds and by villi, finger-like projections containing a core with a lymph capillary (lacteal) and blood vessels. Villi are covered by absorptive columnar epithelial cells whose luminal surface is further increased by microvilli (brush border) on which digestive enzymes and transport mechanisms for inorganic ions are located. 

The epithelium is supported underneath by lamina propria and a layer of smooth muscle called muscularis mucosa (3-10 cells thick). These three layers, i.e., the epithelium, lamina propria, and muscularis mucosa, together constitute the intestinal mucosa.On the apical surface, the epithelium along with lamina propria projects to form villi. The cell membranes of epithelial cells that comprise the villi contain uniform microvilli, which give the cells a fuzzy border, collectively called a brush border. These structures, although greatly increase the absorptive surface area of the small intestine, provide an additional enzymatic barrier since the intestinal digestive enzymes are contained in the brush border. In addition, on the top of the epithelial layer lies another layer, the UWL, as previously described. The metabolic and biochemical components of the epithelial barrier will be discussed. 

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 colonic mucosa does not have the villi and microvilli of the small intestine. Therefore, the surface area available for absorption in the colon is considerably less than that in the small intestine. 

Fig. 2 Effects of mucosal structure on absorptive surface area of the small intestine.

The stomach is the next major portal of entry of substances taken orally and swallowed. The rapid transit through the mouth and esophagus leaves little time for absorption in those regions. The makeup of the pill or capsule and how it is taken also determine the site of absorption. If a tablet is taken with water on an empty stomach, it will likely pass rapidly through the stomach and into the small intestine. On the other hand, if taken with food, the substance will likely spend more time in the stomach. The surface of the stomach, called the gastric mucosa, is made up of a series of infoldings which greatly increases the absorptive surface area. The pH of the stomach is usually quite low (pH < 3 in humans) and therefore substances which are weak acids remain in undissociated form and are easily absorbed. Their rate of absorption will depend on the lipid solubility. 

Diarrhea is a common problem that is usually self-limiting and of short duration. Increased accumulations of small intestinal and colonic contents are known to be responsible for producing diarrhea. The former may be caused by increased intestinal secretion which may be enterotoxin-induced, eg, cholera and E. col] or hormone and dmg-induced, eg, caffeine, prostaglandins, and laxatives decreased intestinal absorption because of decreased mucosal surface area, mucosal disease, eg, tropical spme, or osmotic deficiency, eg, disaccharidase or lactase deficiency and rapid transit of contents. An increased accumulation of colonic content may be linked to increased colonic secretion owing to hydroxy fatty acid or bile acids, and exudation, eg, inflammatory bowel disease or amebiasis decreased colonic absorption caused by decreased surface area, mucosal disease, and osmotic factors and rapid transit, eg, irritable bowel syndrome. 

The small intestine is assumed to be a cylindrical tube with a surface area of 2kRL, where R is the radius and L is the length of the tube (Fig. 4). The rate at which the drug enters the tube is the product of the inlet concentration, C0, and the volumetric flow rate, Q. The rate at which it exits the tube is the product of the outlet concentration, Cout, and the volumetric flow rate, Q. The absorption flux across the small intestinal membrane, , is the product of the effective permeability, Peff, and concentration, C. The total drug loss by absorption from the... 

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