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Depending on the number of strands that form the terminal bars, one can discriminate between leaky and tight epithelia. In tight epithelia, the transepithelial electrical resistance is high and most of the transepithelial transport proceeds transcellularly. Tight epithelia are able to create considerable concentration differences because passive backflux, which predominantly occurs paracellularly, is largely prevented. Even with small driving forces, large quantities of solutes and water can be transported paracellularly. This is advantageous where a large mass flow is required, as in (1) the small intestine to reabsorb NaCl, (2) the proximal tubule to reabsorb most of the filtered primary urine, and (3) the acini of glands to form a plasma-like primary secretion. Here, transcellular active transport processes create small osmotic gradients, which are sufficient to promote passive water flux followed by bulk flux of small solutes.
