Stomach drug absorption

Table 6 Influence of pH on Drug Absorption from the Stomach and Intestine of the Rata...

Usually, an increase in Cg that would affect the dissolution rate would occur only when another process, such as membrane transport or stomach emptying, becomes the rate-limiting step in drug absorption. As a general rule pharmacists should advise patients to take their oral medications with a full glass of water to ensure that dissolution occurs under optimal conditions. 

Another compartmental transit and absorption model, the GITA model, has been described by Sawamoto et al. [44] and reviewed by Kimura and Higaki [30], In this model the GI tract is divided into eight different well-stirred compartments and similarly to the CAT model presented by Yu et al. ([60], [63]), the transit of drug is described by a first-order transit rate constant (Ki for the intestine and Ks for the stomach). The absorption in each segment is assumed to be a first-order process described by an absorption rate constant (Ka). The amounts of compound in the different compartments are described by Eq. 18 for the stomach (Xs) and Eq. 19 for the intestinal compartments (Xi + i) [44],... 

In reality, there is more than just passive diffusion at work for drugs to traverse the cell membrane. Most drugs are absorbed in the intestine. Often, if an oral drug is taken and a fast response is desired, the drug is taken on an empty stomach to ensure a quick passage through the stomach for absorption in the intestine to take place. 

Transport of the benzodiazepines into the brain is rapid, the rate of uptake being determined by the physicochemical properties of the drug. Absorption from the gastrointestinal tract, or from an injection site, is the rate-limiting step governing the speed of onset of the therapeutic response. Oral absorption is more rapid when the drug is taken on an empty stomach. 

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. 

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