PH of the gastrointestinal tract

Unlike fresh-water species, marine species drink appreciable quantities of water to maintain their hydration status and, therefore, can provide drug access to uptake by the gastrointestinal tract. However, regional pH of the gastrointestinal tract may vary considerably between fish species. For example, the pH of the stomach is mucli more alkaline in herbaceous than carnivorous fish. 

Biological factors. The species of animal is a very important factor especially affecting metabolism but also influencing the other phases of disposition. The pH of the gastrointestinal tract, the nature of the skin, and breathing rate all may affect absorption. Plasma proteins and fat can affect distribution. Biliary excretion is affected by species in relation to the molecular weight threshold. 

The pH of the gastrointestinal tract varies, being 1-2 in the stomach, 5-6 in the duodenum, 6-7 in the jejunum, 7-8 in the ileum, and 8-9 in the large intestine. This variation in pH influences the extent to which acidic or basic chemical substances are ionized, which influences the extent of their absorption. The degree of ionization of an acidic substance or a basic substance at a given pH can be expressed by the Henderson-Hasselbalch equations (equations (2) and (3), respectively) ... 

Oral absorption depends partially on the pH of the gastrointestinal tract which is known to vary between species, as shown in table 5,3. Clearly, therefore, considerable differences in the absorption of weak acids from the stomach may occur between species. Similarly, differences might be seen in compounds which are susceptible to the acidic conditions... 

Drugs, such as antacids, H2-receptor antagonists or proton pump inhibitors, that increase the pH of the gastrointestinal tract may reduce the solubility of erlotinib, which the manufacturers state has a decreased solubility above pH 5. The manufacturers therefore recommend caution with concurrent use of these drugs during treatment with erlotinib as they cannot exclude that these drugs will reduce erlotinib efficacy. ... 

It is generally accepted that a normal secretion of hydrochloric acid by the stomach is necessary for optimal absorption of calcium and phosphate, better absorption being due to a lowering of the pH of the gastrointestinal tract. The presence of hypochlorhydria or achlorhydria therefore exerts an adverse influence upon calcium and phosphate absorption. 

The pH of solutions will therefore influence the partitioning and solubility properties of acidic or basic dmgs. The pH of the gastrointestinal tract varies considerably, and will therefore affect significantly the equilibrium between the ionised and un-ionised forms of the species in question, which has important implications for dmg absorption. 

The internal environment of the gastrointestinal tract varies throughout its length, particularly with regard to the pH. Substances taken orally first come into contact with the lining of the mouth (buccal cavity), where the pH is normally around 7 in human, but more alkaline in some other species such as the rat. The next region of importance is the stomach, where the pH is around 2 in human and certain other mammals. 

Apart from influencing the absorption of foreign compounds, the environment of the gastrointestinal tract may also affect the compound itself, making it more or less toxic. For example, gut bacteria may enzymically alter the compound, and the pH of the tract may affect its chemical structure. 

By far the most important physicochemical and physiological parameter determining the functionality of enteric-coated drug delivery systems is the pH of the GI tract. Figure 2 [3] illustrates the variability of pH in the gastrointestinal... 

Most substances are not readily absorbed in the mouth or esophagus one of several exceptions is nitroglycerin, which is administered for certain heart disfunctions and absorbed if left in contact with oral tissue. The stomach is the first part of the gastrointestinal tract where substantial absorption and translocation to other parts of the body may take place. The stomach is unique because of its high content of HC1 and consequent low pH (about 1.0). Therefore, some substances that are ionic at pH values near 7 and above are neutral in the stomach and readily traverse the stomach walls. In some cases, absorption is affected by stomach contents other than HC1. These include food particles, gastric mucin, gastric lipase, and pepsin. 

How does the pH of the fluid vary along the length of the gastrointestinal tract ... 

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