Absorption

Alcohol, drugs, and some fatty acids are absorbed in the stomach, whereas the products of carbohydrate and protein digestion are not. Partially digested carbohydrate and protein products are not lipid soluble and are too large to cross cell membranes. Unlike in the intestine, no specialized nutrient transporters are present in the stomach mucosa. The continuous mucous gel layer also provides a physical barrier to diffusion of anything other than low molecular weight solutes (mol. wt ~1300). There are also no specialized transport systems for ingested nonlipid soluble electrolytes such as Ca2+.

Gastric lipase activity is largely overlooked. Initial digestion of dietary fat in the stomach is a prerequisite for efficient intestinal lipolysis. In infants gastric lipolysis of milk is extensive and the medium-chain fatty acids released in the stomach are absorbed through the gastric mucosa.

Ethanol and a number of drugs readily pass across the gastric mucosa. Ethanol is partially lipid soluble and can therefore diffuse through the epithelial cell membranes and into the submucosal capillaries.

Nonsteroidal anti-inflammatory drugs, e.g., acetylsalicylic acid (aspirin), oral anticoagulants (e.g., dicoumarol) and sulfonylurea oral antidiabetic agents, can all be absorbed through the gastric mucosa. These weakly acidic drugs are nonionized at gastric pH. In this form they are lipid soluble and can therefore be absorbed quickly by crossing the plasma membrane of the epithelial cells.

Alcohol and nonsteroidal anti-inflammatory drugs can cause gastric mucosal damage and have been used in models of gastroduodenal ulceration. They rapidly diffuse through the protective adherent mucous gel layer (in the case of ethanol causing its dehydration) and in these models cause epithelial damage, cell exfoliation, and, in more severe cases, vascular damage, hemorrhage, and visible lesions. Following acute damage rapid repair occurs by a process of re-epithelialization. The repairing epithelium is protected from the endogenous damaging agents of acid and pepsin by a thick gelatinous coat mainly composed of a fibrin gel formed on the mucous gel template.

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