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  • Identify the major morphological components of an epithelial tissue, including lumen, interstitium, apical and basolateral membranes, and tight junctions.

  • State how transport mechanisms combine to achieve active transcellular reabsorption in epithelial tissues.

  • Define iso-osmotic transport.

  • Define paracellular transport and differentiate between transcellular and paracellular transport.

  • Define the terms channel, transporter, uniporter, multiporter, symporter, and antiporter.

  • Describe qualitatively the forces that determine movement of reabsorbed fluid from the interstitium into peritubular capillaries.

  • Explain why volume reabsorption in the proximal tubule depends on activity of the Na-K-ATPase.

  • Compare the Starling forces governing glomerular filtration with those governing peritubular capillary absorption.

  • Compare and contrast the concepts of Tm and gradient-limited transport.


The kidneys mostly move water and solutes filtered in the glomerulus from the lumen of the tubules back into the renal blood vessels. This movement is tubular transport, and this chapter deals with the mechanisms involved in this transport. The majority of the working cells of the kidneys are epithelial cells that form the walls of the renal tubules, and their task is to move water and a large array of substances between the lumens of the tubules and the nearby network of blood vessels. This requires that solutes and water cross the epithelium of the tubules, the endothelium of the vascular walls, and the thin region of interstitial fluid between them. In the cortex, where the fluxes of many filtered substances are enormous, the vascular endothelium (peritubular capillaries) is fenestrated. The fenestrae and the loose underlying basement membrane offer virtually no resistance to the passive movement of water and small solutes. This facile permeation has two consequences. First, the rate of transport between blood and tubule is governed almost exclusively by events in the tubular epithelium rather than the vascular endothelium; second, the cortical interstitium, which is the medium faced by the basolateral surface of the tubular epithelia, has a composition very much like plasma.

In contrast to the cortex, both blood flow and transport events are less rapid in the medulla. Only some regions of the medullary vasculature are fenestrated, so that (1) overall transport depends on both the properties of the vascular endothelium and tubular epithelium and (2) the medullary interstitium is most definitely not plasma-like in its composition. In the rest of this chapter, we will describe the principles of epithelial transport that apply to all parts of the kidney, with particular emphasis on events in the cortex. We will then see how these principles apply to the medulla in subsequent chapters.

imageCrossing the tubular epithelium can occur either through the cells or around the cells. The paracellular route is when the substance goes around the cells, that is, through the matrix of the tight junctions that link each epithelial cell to its neighbor. In most cases, however, a substance takes the ...

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