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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.
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TRANSEPITHELIAL TRANSPORT
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As should be clear by now, the kidneys are transport machines, moving a large array of substances between the renal tubules and the nearby network of blood vessels. The basic process of moving these substances (secretion and reabsorption) requires that solutes and water cross 2 cell layers: (1) the epithelium that makes up the walls of the tubules and (2) the endothelium that makes up the vascular walls. Substances must also traverse 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 2 consequences. First, the rate of transport 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 membranes of the tubular epithelia, has an osmolality and concentration of small solutes very close to those in plasma. In contrast, 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.
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Crossing 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 transcellular route, a 2-step process through the cells. For reabsorption, this is entrance across the apical membrane facing the tubular lumen, through the cell cytosol, and then exit across the basolateral membrane facing the interstitium. For ...