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A healthy 25-year-old woman was a subject in an approved research study. Her average urinary urea excretion rate was 12 mg/min, measured over a 24-hour period. Her average plasma urea concentration during the same period was 0.25 mg/mL. What is her calculated urea clearance?

Clearance is the ratio of excretion rate to plasma concentration. In this case, excretion rate is provided (often it must be calculated from the product of urine concentration and urine flow rate): Curea = 12 mg/min / 0.25 mg/mL = 48 mL/min.


Antidiuretic hormone (ADH)-sensitive water channels (aquaporins) are located in the:

collecting duct

In the collecting duct cells, binding of ADH to V2 receptors stimulates the insertion of aquaporins into the apical membranes. This cause sodium-free water absorption.


All of the following factors contribute to establishing or maintaining the medullary interstitial concentration gradient EXCEPT:

distal tubule sodium reabsorption.

Distal sodium reabsorption has no effect on the medullary concentration gradient, whereas the other factors all play significant roles in creating and maintaining the gradient


Increasing which of the following variables would decrease the magnitude of the renal corticopapillary osmotic gradient that allows for urine concentration?

Blood flow through the vasa recta

The corticopapillary osmotic gradient is established by countercurrent multiplication in the loop of Henle. The countercurrent multiplier relies on Na+-K+-2Cl- cotransport by the thick ascending limb, so the gradient collapses when the cotransporter is inhibited by loop diuretics. Increasing flow through the vasa recta washes ions out of the medulla, thereby diminishing the osmotic gradient. Renin is released when arterial pressure falls or when the sympathetic nervous system activates, conditions that signal a likely need to conserve water. Gradient magnitude increases as a result, in part through increased urea reabsorption from the collecting ducts


Genetic evaluation of a 6-year-old boy with growth and mental retardation identified alleles associated with Bartter syndrome. Bartter syndrome mimics loop diuretics by causing thick ascending limb (TAL) dysfunction. Which of the follow best describes the TAL in healthy individuals?

It extracts Na+, K+, and Cl- from the lumen.

The thick ascending limb (TAL) reabsorbs Na+, K+, and Cl- from the tubule lumen via Na+-K+-2Cl- cotransport and transfers these ions to the interstitium, where they help form the corticopapillary osmotic gradient. The TAL has a low water permeability that prevents H2O from following ions into the interstitium, so the tubule fluid becomes relatively dilute ( for this reason. Ca2+ is reabsorbed primarily in the proximal tubule, with regulated reabsorption occurring in the distal tubule.


If the thick ascending limb stopped reabsorbing sodium, then the final urine would be

isosmotic with plasma in all conditions

The tubular fluid entering the medulla is iso-osmotic. If the tubules did not separate salt from water, the medullary interstitium would remain iso-osmotic. The luminal fluid would also remain iso-osmotic because there would be no tendency to either dilute it or concentrate it.


If a healthy young person drinks a large amount of water, which of the following is unlikely to happen?

an decrease in water permeability in the medullary collecting ducts

After drinking a large amount of water there would be a decline in ADH, which would decrease water permeability in the ADH-sensitive regions of the tubule.


In normal kidneys, which of the following is true of the osmolarity of renal tubular fluid that flows through the early distal tubule in the region of the macula dens

Usually hypotonic compared with plasma

As water flows up the ascending limb of the loop of Henle, solutes are reabsorbed, but this segment is relatively impermeable to water; progressive dilution of the tubular fluid occurs so that the osmolarity decreases to approximately 100 mOsm/L by the time the fluid reaches the early distal tubule. Even during maximal antidiuresis, this portion of the renal tubule is relatively impermeable to water and is therefore called the diluting segment of the renal tubule.


Urea is secreted into the tubules

in the thin descending limbs.

Urea is secreted in the deep descending thin limbs where the interstitial concentration is high.


Whether or not ADH is present, the greatest fraction of water is reabsorbed in the

proximal tubule


In the presence of vasopressin (ADH), the greatest fraction of filtered water is absorbed in the:

proximal tubule


A 15-year-old boy was busy with school activities and did not drink any liquids from 8 am to 5 pm. When he urinated late in the day, he noticed that his urine was dark yellow. All of the following events were occurring in his kidney during the day except

increased Na+ reabsorption in the thin descending limb

Dehydration increases plasma osmolarity and stimulates ADH secretion ADH stimulates Na+ and urea reabsorption as part of the countercurrent mechanism. Na+ reabsorption is stimulated in the ascending limb, not the descending limb of the loop of Henle. Aquaporin 2 expression increases to allow water retention by the collecting duct.