[Campbell Biology P.1040] In the mammalian kidney, what is the primary process that makes the... | Practice Question

In the mammalian kidney, what is the primary process that makes the filtrate hypoosmotic to body fluids by the time it reaches the distal tubule, and what is the main event contributing to water reabsorption in the collecting duct?

  • A: Active transport of NaCl out of the thick ascending limb; osmosis extracts water from the filtrate as it passes through increasing interstitial osmolarity.
  • B: Passive diffusion of water out of the descending limb; active transport of urea into the collecting duct.
  • C: Filtration of plasma proteins in the glomerulus; secretion of hydrogen ions into the collecting duct.
  • D: Active transport of water into the vasa recta; passive reabsorption of salt in the loop of Henle.

Explanation

The filtrate becomes hypoosmotic in the thick segment of the ascending limb because NaCl is actively transported out of the tubule into the interstitial fluid. The collecting duct is permeable to water but not to salt, so as the filtrate descends toward the medulla, osmosis extracts water from it due to the increasing interstitial osmolarity, concentrating the urine.