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NBME 18 Answers

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submitted by โˆ—match95(56)
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This would just be the normal response of our kidneys to when we don't drink water enough (aka just normal).

Proximal tubule is isotonic because we are reabsorbing BOTH Na+ and H2O.

Macula densa is hypotonic because it is sensing the distal convoluted tubule -- the most DILUTE part of the nephron. Remember the countercurrent exchange system - thick ascending limb is losing NaCl (urine gets less concentrated as it ascends).

Medullary collecting duct is hypertonic because we are reabsorbing all the water making the urine more concentrated.

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epiglotitties  The collecting ducts are the main site of urine concentration only when in the presence of ADH, right? Normally it wouldn't be hypertonic? +
fatboyslim  @epiglotitties (great name btw) yes that is correct. UW# 1607 illustrates this nicely. In LOW ADH settings (i.e. not dehydrated), fluid in the collecting tubule has the LEAST osmolarity (i.e. dilute), and fluid between the descending and ascending loops of Henle has the HIGHEST osmolarity. +

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