Gastric pH

In compendial terms, the pH of the stomach contents is mimicked as a hydrochloric acid solution of 1.0, 1.2 or 1.8. At rest, the stomach pH varies: in a large study of

Fig. 2.6 Diagram of the features of the gastrointestinal tract showing location in the abdomen

Duodes

Cecum

Duodes

Cecum

Large bowel

Fig. 2.6 Diagram of the features of the gastrointestinal tract showing location in the abdomen

Large bowel

Fig. 2.7 Key gastric features

685 volunteers, Feldman and Barnett reported that the median basal pH for females was 2.79 ± 0.18 and that for males was 2.18 ± 0.18 [6]. In adults, the population of parietal cells is decreasing which will lead to elevated pH in the elderly. As the drugs encountered for oral medication are often weak electrolytes and many are salts of bases, the pH change between the stomach and the small intestine will exert important effects on systemic exposure. Moreover, differences will emerge when considering absorption from the fasted compared to the fed state. If a medication is taken with water, the pH will be elevated temporarily by dilution, returning to baseline at around 20 min post-imbibing. Intake of the meal will cause acid secretion but meal components dictate the magnitude of the response. Food processing acts as a sustained delivery mechanism regulating the supply of materials by controlling gastric emptying. Backwash of duodenal contents into the stomach will cause decrease in surface tension, which can further aid solubilization, which may either subsequently increase absorption or the rate of compound degradation.

The change in pH environment of the upper gastrointestinal tract is very important in oral drug delivery, although the import of regional variations both within the stomach as an organ and between the lumen and unstirred water layer is sometimes not appreciated. A current research direction is the preservation of the super-saturated state to avoid precipitation, particularly of bases, on change of media from the gastric milieu to intestinal fluid.

The pH gradients within an organ and between the lumen and the unstirred water layer next to epithelium can vary by at least a pH unit (Fig. 2.7). In the stomach, such differences are very large as the parietal cell mass decreases in the fundus raising the pH. In the stomach, such differences are very large as the parietal cell mass decreases in the fundus raising the pH.

The volume of the stomach swells by relaxation of the fundus to accommodate a meal and food layers without significant mixing if the viscosity is high enough. The resting volume is very low - around 50-100 ml but intake of food causes it to relax to accommodate between 1 and 1.5 L. The maximum volumes recorded are around 4 L in man.

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