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Basic Transport Mechanisms

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A healthy patient has a normal plasma osmolality (close to 300 mOsm/kg). If 100 mmol of solutes are reabsorbed iso-osmotically from the proximal tubule, approximately how much water is reabsorbed with the solute?

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a. 100 mL

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b. 300 mL

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c. 333 mL

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d. 1000 mL

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(c) 100 mmol is the amount of solute in one third of a kg of filtrate (333 mL), so this much water accompanies the reabsorbed solute.

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Quantitatively, most sodium gains entrance to proximal tubule cells by

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a. paracellular diffusion.

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b. transcellular diffusion.

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c. the Na-K-ATPase.

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d. antiport with hydrogen ions.

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(d) Sodium enters the cells across the apical membrane by several pathways, the major one being the NHE3 antiporter.

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The tight junctions linking proximal tubule cells permit passive diffusion of

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a. glucose.

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b. sodium.

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c. all filtered solutes.

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d. no filtered solutes.

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(b) Tight junctions exhibit selectivity just as membrane transporters do. The proximal tubule tight junctions are leaky to sodium and a number of other solutes, but not glucose.

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In the proximal tubule, water can move through

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a. apical membranes of proximal tubule cells.

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b. basolateral membranes of proximal tubule cells.

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c. tight junctions.

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d. all of these.

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(d) In the proximal tubule, water is reabsorbed both transcellularly and paracellularly.

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A drug X is secreted into the proximal tubule by a Tm-limited system. This implies that

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a. X cannot easily diffuse by the paracellular route.

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b. all the X that enters the renal vasculature will be secreted.

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c. the rate of X secretion is independent of the plasma concentration.

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d. X is not filtered at the glomerulus.

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(a) A substance moving by a Tm-limited system cannot move paracellularly. It moves transcellularly via transporters that have an upper limit ...

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