■ Paracellular route
FIGURE 2 Functional anatomy of the small intestinal absorptive cells, illustrating the transcellular and paracellular routes for absorption.
peptides; these enzymes are not present in the basolat-eral membranes. At the same time, the basolateral membranes of all intestinal epithelial cells contain the Na + ,K + -ATPase (adenosine triphosphatase), or Na + -K+ pump; this enzyme pump is not present in their apical membranes. It appears that, in addition to binding these epithelial cells, the junctional complexes prevent intermingling of these membrane-bound proteins and thus preserve the asymmetric properties (or polarity) of these cells, which, as we will see, are essential for absorption and secretion.
The name zonulae occludens, or occluding zones, stems from the fact that these intercellular barriers are essentially impermeable to macromolecules such as proteins and polysaccharides. On the other hand, their permeabilities to water and small ions are variable. Some epithelia are characterized by junctional complexes that are highly permeable to water and small ions and for this reason are often referred to as leaky or low-resistance epithelia. Examples of leaky epithelia in the gastrointestinal (GI) tract are the gall bladder and the small intestine. Other epithelia are characterized by junctional complexes that are essentially impermeable to water and electrolytes and they are referred to as tight or high-resistance epithelia. Between these two extremes are epithelia such as mammalian colon, whose junc-tional complexes are moderately leaky to water and small ions.
The leakiness or tightness of the junctional complexes determines the magnitude of paracellular movements of water and small ions. Because paracellular movements are the results of diffusion or, in the case of water movements, osmosis, they tend to dissipate differences in ion concentrations and osmolarity across the epithelium. The more leaky the epithelium, the lower the differences in ionic concentrations and total osmolarity between the luminal contents and the plasma. Thus, the ionic composition and total osmolarity of the luminal contents in the very leaky small intestine do not differ markedly from those of the plasma; like the renal proximal tubule (see Chapter 29), the small intestine carries out isosmotic absorption of fluid and electrolytes. On the other hand, the colon, being a much tighter epithelium than the small intestine, can sustain relatively large transepithe-lial gradients. Thus, the 100-200 mL of fluid contained in the daily excreta may be hypertonic with respect to plasma and have a Na+ concentration of only 25-50 mmol/L and a K+ concentration as high as 90 mmol/L; these asymmetries would not be possible if the junc-tional complexes were highly permeable to water and ions.
In short, there are in general two routes for the transepithelial transport of water and small ions. One is transcellular, involving movements across both the apical and basolateral membranes. Substances absorbed via this route must enter the cell across the apical membrane and then leave across the basolateral membrane, whereas the opposite sequence pertains to substances that are secreted. This double membrane model of transcellular transport is universally accepted. The other route is paracellular, in which movements of water and small ions circumvent the two limiting membranes of the epithelial cell and are driven solely by passive forces. The rate-limiting step that determines the magnitudes of these paracellular flows is the permeability of the tight junctions.
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