Gastrointestinal tract surface area

Chemicals have to pass through either the skin or mucous membranes lining the respiratory airways and gastrointestinal tract to enter the circulation and reach their site of action. This process is called absorption. Different mechanisms of entry into the body also greatly affect the absorption of a compound. Passive diffusion is the most important transfer mechanism. According to Pick s law, diffusion velocity v depends on the diffusion constant (D), the surface area of the membrane (A), concentration difference across the membrane (Ac), and thickness of the membrane (L)... 

A summary of how physiological factors affect the dissolution rate is given in Table 21.2. The effective surface area will be affected by the wetting properties of the bile acids and other surface-active agents in the gastrointestinal tract. The dif-fusivity of a drug molecule in the intestinal juice will be altered by changes in viscosity that are induced, for instance, by meal components. An increased dissolution rate could be obtained at more intense intestinal motility patterns or increased... 

Despite the gastrointestinal absorption characteristics discussed above, it is common for absorption from the alimentary tract to be facilitated by dilution of the toxicant. Borowitz et al. (1971) have suggested that the concentration effects they observed in atropine sulfate, aminopyrine, sodium salicylate, and sodium pentopar-bital were due to a combination of rapid stomach emptying and the large surface area for absorption of the drugs. 

GIT gastrointestinal tract PSA polar surface area log P octanol/water partition coefficient. 

After oral administration, griseoftilvin is absorbed from the gastrointestinal tract and deposited in new epithelial cells that make up skin, hair, claws, and nails. The drug has a greater affinity for diseased skin than for normal skin. Increasing tlie surface area of the griseoftilvin particles and the dietary fat intake also increases drug absorption. 

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. 

Figure 3.12 The mammalian gastrointestinal tract showing important features of the small intestine, the major site of absorption for orally administered compounds (A) liver (B) stomach (C) duodenum (D) ileum (E) colon (F) longitudinal section of the ileum showing folding, which increases surface area (G) detail of fold showing villi with circular and longitudinal muscles, (H) and (I) respectively, bounded by (J) the serosal membrane (K) detail of villi showing network of (L) epithelial cells, (M) capillaries, and (N) lacteals (O) detail of epithelial cells showing brush border or (P) microvilli. The folding, vascularization, and microvilli all facilitate absorption of substances from the lumen. Source From Ref. 1.

There are three main sites of absorption skin (large surface area poorly vascularized not readily permeable) gastrointestinal tract (major site well vascularized variable pH large surface area transport processes food gut bacteria) lungs (very large surface area well vascularized readily permeable). Compounds may be administered by direct injection (i.p., i.m., s.c., i.v.). 

Surface area skin, lungs, and gastrointestinal tract all have a large surface area. 

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. 

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