Small intestine active surface area

Differences in the absorptive areas and volumes of gut contents in different animals are important when comparing experimental results on dmg absorption in various species. The human small intestine has a calculated active surface area of approximately 100 m. No analogous calculations are available for the most commonly used laboratory animals, although the surface area of the small intestine of the rat is estimated to be 700 cm. ... 

The use of a bioadhesive, polymeric dosage form for sustained dehvery raises questions about swallowing or aspirating the device. The surface area is small, and patient comfort should be addressed by designing a small (less than 2 cm ), thin (less than 0.1 mm (4 mil) thick) device that conforms to the mucosal surface. The buccal route may prove useful for peptide or protein dehvery because of the absence of protease activity in the sahva. However, the epithelium is relatively tight, based on its electrophysiological properties. An average conductance in the dog is 1 mS/cm (57) as compared to conductances of about 27 and 10 mS/cm in the small intestine and nasal mucosa, respectively (58,59) these may be classified as leaky epitheha. 

Although transfer of drugs across the intestinal wall can occur by facilitated transport, active transport, en-docytosis, and filtration, the predominant process for most drugs is diffusion. Thus, the pK of the drug and the pH of the intestinal fluid (pH 5) will strongly influence the rate of drug absorption. While weak acids like phenobarbital (pK 7.4) can be absorbed from the stomach, they are more readily absorbed from the small intestine because of the latter s extensive surface area. 

Q4 The water-soluble vitamins (B and C) and essential minerals, such as iron, are absorbed in the small intestine. The reabsorption of iron in the duodenum and proximal jejunum involves a complex active transport process. When there is a large reduction in the surface area of this part of the gut, there is a marked reduction in reabsorption of iron (and B vitamins). Haemoglobin synthesis is decreased, leading to development of anaemia, which is a common symptom in celiac disease. 

The presence of physiological surface-active agents in the stomach and small intestine will influence the solubility and the dissolution of sparingly soluble drugs by improved wetting of solid particle surface areas and by micellar solubilisation. This has been reviewed in more detail by Gibaldi and Feldman (1970) and Charman et al. (1997). 

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