The Coupling of Solute and Water Transport in Epithelia
The problem to be considered in this discussion is the question of how epithelial membranes such as those of the kidney proximal tubule, the intestine, the liver, and the gall bladder can form secretions and absorbates that have the same osmotic pressure as plasma. Traditionally this problem has been approached from two quite different points of view. On the one hand, physiologists have carried out experiments in which they regarded the epithelium as a black box and tried to deduce what solutes were being actively transported, to what extent water transport was coupled to solute transport, and what the coupling mechanism might be. On the other hand, anatomists have been impressed by the intricate and striking subcellular structures that one finds in fluid-transporting epithelia, such as basal labyrinths, tortuous intercellular spaces, brush borders, and systems of intracellular canals, and have speculated what role these structures might play in fluid transport. Within the past several years it has become increasingly clear that although each approach can make partial contributions, neither approach alone would be able to solve the problem, and that further progress could only come from an integrated physiological and anatomical effort. I shall present first some black-box physiological experiments to introduce the problem; then the combined physiological and anatomical experiments that Dr. John Tormey and I have been carrying out; and finally a model of fluid transport in epithelia that may help reconcile the physiological and anatomical observations.
KeywordsProximal Tubule Gall Bladder Fluid Transport Salt Gland Osmotic Gradient
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