Water Transport by Epithelia


Water transport in epithelia can occur down an established concentration gradient, in which case it is indisputably osmotic. In other epithelia, water transport occurs in the absence of or even against differences in the osmolalities of the adjacent solutions. In the latter case, water flow is in the same direction as the net solute flux and in near‐isosmotic proportions. The pathways and mechanisms of water transport in this class of epithelia remain controversial, but the prevailing views are that water flow is largely transcellular, via both water pores (aquaporins, AQP) and the phospholipid bilayer, with little contribution of other membrane proteins. In addition, solute–solvent flux coupling is not a molecular, but a thermodynamic phenomenon, i.e. water transport is driven by small differences in osmolality between epithelial fluid compartments created by solute transport. Paracellular water flow is likely to be small relative to transcellular water flow.

Keywords: epithelium; water transport; osmosis; aquaporin

Figure 1.

Functional architecture of epithelia. This figure summarizes the structural elements of transporting epithelial cells that influence the physiology of water transport. A section of an epithelial monolayer is depicted. The top faces the lumen, from which it is separated by the apical membrane (am), which generally has microvilli. Tight junctions (tj) separate the apical regions of adjacent cells, and are followed by convoluted lateral intercellular spaces. The nuclei (n) are generally basally located and surrounded by mitochondria (not shown). The bottom is the basal pole of the cell, separated from the extracellular fluid by the basolateral membrane (blm), which has lateral and basal regions and infoldings. Under the epithelium there is a basement membrane (bm), which is a barrier to water and solute fluxes. As discussed in the text, these structural elements create small fluid compartments that do not exchange efficiently with the adjacent solutions, principally the continuous space formed by the lateral intercellular spaces and the fluid adjacent to the basal poles of the cells.

Figure 2.

The presence of unstirred‐layer causes underestimation of membrane Pos. The figure depicts a membrane impermeable to solute (blue dots) separating two stirred solutions. The concentration is higher on the right side, causing a water flux from left to right (black arrow). The initial solute concentrations are depicted by the red lines. The water flow tends to ‘concentrate’ the solute on the left side and ‘dilute’ the solute on the right side because the solutions adjacent to the membranes are not mixed by convection (unstirred layers). The changes in solute concentration in the vicinity of the membrane are denoted by the blue lines. The concentration difference at the membrane surfaces (distance between the blue lines) is less than the concentration difference between the bulk solutions (distance between the red lines). Therefore, Pos, which is directly proportional to the fluid flow (Jv) and inversely proportional to the concentration difference (ΔCs), is underestimated.

Figure 3.

Three‐compartment model (Curran and MacIntosh, ). The diagram shows a simplified view of an epithelium; darker blue denotes hyperosmolality. am, apical membrane; tj, tight junction; blm, basolateral membrane and bm, basement membrane. Full arrows, solute transport; segmented arrows, water transport. The middle compartment is the lateral‐basal space, which is hyperosmotic to the cell and the apical bathing fluid. The semipermeable membrane is the basolateral cell membrane, and the porous membrane is the basement membrane. Salt is transported to the lateral intercellular spaces by active mechanisms, elevating the local concentration and causing osmotic water flow across blm, elevating the hydrostatic pressure in the space. Since the basolateral membrane is porous (permeable to both water and solute), solution flows from the space across bm. In this model, the fluid emerging from the space is hyperosmotic to that in the lumen (see text).

Figure 4.

Standing‐gradient hypothesis (Diamond and Bossert, ). Solute transport from the cells into the lateral intercellular spaces (full arrows) increases the space fluid osmolality thereby causing osmotic water flow (segmented arrows), which progressively dilutes the space's solution and creates a longitudinal gradient in osmolality. Solute transport along the lateral intercellular spaces is by both bulk flow and diffusion. Depending on model parameters, the emerging fluid can be almost isosmotic to the lumen fluid (see text).

Figure 5.

Near‐isosmotic transport model. The high cell membrane Pos implies that the osmotic driving forces necessary to drive water transport are very small, and localized at the interfaces between the epithelial cell and the adjacent solutions. Solute transport dilutes the cis solution and concentrates the trans solution, thus establishing the small, localized osmotic gradients that drive osmotic water flow. These gradients are minimal (see text).



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Further Reading

Boone M and Deen PM (2008) Physiology and pathophysiology of the vasopressin‐regulated renal water reabsorption. Pflügers Archiv 456: 1005–1024.

Hill AE (2008) Fluid transport: a guide for the perplexed. Journal of Membrane Biology 223: 1–11.

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Reuss, Luis(Mar 2009) Water Transport by Epithelia. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020622]