A mechanism for transport of water-insoluble vitamins

FIGURE 1.4 A mechanism for transport of water-insoluble vitamins. 

OTHER FACTORS CONTROLLING THE RATE OF ABSORPTION OF DRUGS 

In addition to the lipid-water partition coefficient, other factors that control the rate of absorption of drugs are the degree of ionization, the surface area, blood flow through the region, and the gastric emptying time. 

For example, phenytoin is absorbed primarily from the upper intestinal tract. Phenytoin is an acid with a pK of 8.3 to 9.2 and is insoluble at the pH of gastric juice (2.0). Therefore, it cannot be absorbed significantly from the stomach. On passage into the small intestine, where the pH is less acidic (7 to 7.5), phenytoin is absorbed in a nonionized form. 

At the duodenal pH of 7.0, 96.9% of the phenytoin is in the nonionized form, which favors its absorption. The absorption of phenytoin is greatest in the duodenum and decreases in the lower parts of the small intestine. The absorption from the cecum and the large intestine is severalfold lower than that in the duodenum. 

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