Diffusion barriers intestinal mucosa

Passive transcellular transport across the intestinal epithelium involves three discrete steps (1) uptake across the apical membrane, (2) diffusion through the cytoplasm, and (3) efflux across the basolateral membrane. Occasionally, drug molecules without favorable physicochemical properties traverse the intestinal epithelium using endogenous membrane transporters.6-8 In addition, the intestinal mucosa, with its numerous drug-metabolizing enzymes and efflux transporters, such as P-glycoprotein (Pgp), functions as a biochemical barrier.9... 

The intestinal mucosa serves two purposes being constantly exposed to exogenous substances and pathogens, it represents a formidable barrier against the diffusion of toxins and against invasion and infection by pathogenic... 

Passive secretion of water into the lumen of the intestine depends on the semipermeable diffusion characteristics of the mucosa of the intestine and the osmotic and oncotic differences exerted across this semipermeable barrier. The permeability of capillaries in the submucosa and mucosa and the density of intercellular tight junctions in the mucosa determine the permeability of the mucosa. The density of tight junctions is very high in the stomach, relatively low in the small intestine and moderate in the large intestine and small colon. 

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