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Basic Transport Mechanisms

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?

a. 100 mL

b. 300 mL

c. 333 mL

d. 1000 mL

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

Quantitatively, most sodium enters proximal tubule cells by:

a. paracellular diffusion.

b. transcellular diffusion.

c. the Na-K-ATPase.

d. antiport with hydrogen ions.

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

The tight junctions linking proximal tubule cells permit passive diffusion of:

a. glucose.

b. sodium.

c. all filtered solutes.

d. no filtered solutes.

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.

In the proximal tubule, water can move through

a. apical membranes of proximal tubule cells.

b. basolateral membranes of proximal tubule cells.

c. tight junctions.

d. all of these.

D. In the proximal tubule water is reabsorbed both transcellularly and paracellularly.

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

a. X cannot easily diffuse by the paracellular route.

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

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

d. X is not filtered at the glomerulus.

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

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